Roundabouts and Access Management March 2014 Final Report

RoundaboutsandAccessManagement
FDOTProjectBDK77977‐22
FinalReport
March2014
Preparedfor:
FloridaDepartmentofTransportation
605SuwanneeStreet,MS19
Tallahassee,FL32399
ProjectManager:
GinaBonyani
SystemsPlanningOffice
Preparedby:
PrincipalInvestigator
Dr.RuthL.Steiner
DepartmentofUrban&RegionalPlanning
UniversityofFlorida
431ArchitectureBuilding
Gainesville,FL32611
Dr.ScottWashburn,Dr.LilyElefteriadou
EngineeringSchoolofSustainableInfrastructureandtheEnvironment
UniversityofFlorida
365WeilHall
Gainesville,FL32611
Dr.AlbertGan
DepartmentofCivilandEnvironmentalEngineering
FloridaInternationalUniversity
10555WestFlaglerStreet,EC3603
Miami,FL33174
Disclaimer
Theopinions,findings,andconclusionsexpressedinthispublicationarethoseoftheauthorsandnot
necessarilythoseoftheStateofFloridaDepartmentofTransportation.
Roundabouts and Access Management
Page ii
MetricConversionTable
SYMBOL
WHENYOUKNOW
in.
ft.
yd.
mi
inches
feet
yards
miles
Roundabouts and Access Management
Page iii
MULTIPLYBY
LENGTH
25.4
0.305
0.914
1.61
TOFIND
SYMBOL
millimeters
meters
meters
kilometers
mm
m
m
km
TechnicalReportDocumentationPage
1.ReportNo.
2.GovernmentAccessionNo.
4.TitleandSubtitle
RoundaboutsandAccessManagement
7.Author(s)
RuthL.Steiner,ScottWashburn,LilyElefteriadou,AlbertGan,Priyanka
Alluri,DimitraMichalaka,RuoyingXu,ShantyRachmat,BenjaminLytle,
AmyCavaretta
9.PerformingOrganizationNameandAddress
DepartmentofUrbanandRegionalPlanning
UniversityofFlorida
P.O.Box115706
Gainesville,FL32611‐5706
12.SponsoringAgencyNameandAddress
FloridaDepartmentofTransportation
605SuwanneeStreet,MS30
Tallahassee,FL32399
3.Recipient'sCatalogNo.
5.ReportDate
March2014
6.PerformingOrganizationCode
8.PerformingOrganizationReportNo.
10.WorkUnitNo.(TRAIS)
11.ContractorGrantNo.
BDK77‐977‐22
13.TypeofReportandPeriodCovered
FinalReport
September2012–March2014
14.SponsoringAgencyCode
15.SupplementaryNotes
GinaBonyani,FDOTProjectManager
16.Abstract
Transportationengineersandplannersarebecomingmoreinterestedinusingroundaboutstoaddressaccess
managementandsafetyconcernsinthetransportationsystem.Whileroundaboutsarebeingusedincreasinglyina
varietyofcontexts,existingresearchdoesnotprovidedetailedguidanceonhowtoevaluatetheuseofroundabouts
asaformofaccessmanagement.ThisFloridaDepartmentofTransportation(FDOT)researchprojecthasthree
primarycomponents:areviewandassessmentofnationalandstateguidancerelatedtoroundaboutsandaccess
management,asafetyanalysisofall283roundaboutsinFlorida,andanoperationalanalysisofselectedroundabouts.
Literaturerelatedtosafety,accessmanagement,andmultimodaltransportation(especiallyforbicyclistsand
pedestrians,androadwaycapacityassociatedwiththeuseofroundabouts)isreviewed,andgapsinknowledge
regardingtheuseofroundaboutsareidentified,particularlyastheyapplytosafety,access,andcapacity.Oneofthe
findingsoftheliteraturereviewisthatlittleresearchhasbeencompletedonaccessmanagementnearroundabouts.
AreviewofnationalandstateguidanceidentifiesmajorstudiesincludingNCHRP672andguidanceinKansas,
WisconsinandVirginiathatrecommendintersectionanddrivewayspacingsimilartothatrecommendedforun‐
signalizedintersections.Thesafetyandoperationalanalysisidentifiesfourareasofconcern:cornerclearance,
includingstoppingsitedistance(SSD)andintersectionsightdistance(ISD);theneedforguidanceonthefunctional
areanearroundaboutsincludingdrivewayandintersectionspacing,andtheuseofmedians;accesstomajoractivity
centers;andsafetyofvulnerableroadusers,especiallybicyclistsandpedestrians.Theoperationalanalysisconfirms
previousresearchthatshowsthatroundaboutsaresimilartoun‐signalizedintersections,butthedifferencesmay
influencetheoperationsandsafetywithinthefunctionalareaoftheroundabout.AnassessmentoftheprimaryFDOT
utilizedsoftwaretoolsfocusesonthecurrentsuitabilityofthesesoftwaretoolstoassistpractitionersinassessingthe
suitabilityofincorporatingroundaboutsintoexistingandproposedroadwayconfigurations.Recommendationsare
madeforadditionalnationalresearchonguidanceondrivewayandintersectionspacing,medians,andSSDandISD
inthedifferentcontextsinwhichroundaboutsareinstalled.ChangestotheFDOT’sAccessManagementTools,Median
HandbookandDrivewayInformationGuidearealsorecommendedalongwiththedevelopmentofFlorida‐specific
parametersforcapacityandsafetyanalysis.Modificationstoroundaboutdesignguidelinesandhandbooksforaccess
managementwillleadtosafer,moreeffective,andultimately,betterperformingroundaboutsforallusersofFlorida’s
transportationsystemandthroughouttheUnitedStates.
17.KeyWords:Roundabout,accessmanagement,safety, 18.DistributionStatement
capacity,operationalanalysis
Norestrictions.
19.SecurityClassif.(ofthisreport)
20.SecurityClassif.(ofthispage)
21.No.of
22.Price
Unclassified.
Unclassified.
Pages177
FormDOTF1700.7(8‐72)
Reproductionofcompletedpageauthorized
Roundabouts and Access Management
Page iv
Acknowledgements
TheprojectteamwouldliketothankGinaBonyaniandGarySokolow,oftheFloridaDepartmentof
Transportation(FDOT)SystemsPlanningOffice,fortheirassistanceinunderstandingthescopeofthe
projectandfortheirfeedback.Theresearchteamwishestoacknowledgetheeffortsofnationalexpertson
roundabouts,includingAndreaBill,PhilDemosthenes,PatrickFlemming,HillaryIsebrands,MarkJohnson,
HowardMcCullough,LeeRodegerdts,EugeneRussell,JeffShaw,KenSides,MichaelWallwork,andBrian
Walsh,fortheirwillingnesstoparticipateinthisresearchinavarietyofways,includingdiscussingdesign,
safety,policy,andothertopicsrelatedtoaccessmanagementnearroundabouts,providingtimeinmeetings
oftheITERoundaboutsandtheTransportationResearchBoard(TRB)RoundaboutsCommittee,andto
reviewingthedraftsofthisfinalreport.
Roundabouts and Access Management
Page v
ExecutiveSummary
SummaryofFinalReport,BDK77977‐22
March2014
Background
Overthelasttwentyyears,engineersandplannershavebecomeincreasinglyinterestedintheuseof
roundaboutsbecausetheyofferseveraladvantagesoverothertrafficcontrols;theymaycostlesstoinstall,
havegreatersafetypotentialbyreducingthenumberofconflictpoints,canaccommodateaseriesofU‐
turnsandleft‐turnlanesandreducedelayinacorridor,and,mayhaveloweroperationsandmaintenance
costs.Floridahasrecentlybeguntoencouragetheuseofroundaboutsonthestatehighwaysystemandis
systematicallyupdatingitsguidancedocuments(e.g.,PlansPreparationManual,IntersectionDesign
Manual,andManualonUniformTrafficStudies)butneedsguidanceonwhattoincludeintheMedian
Handbook,andDrivewayInformationGuideandotheraccessmanagementdocuments.
Objectives
Thepurposeofthisstudyistounderstandpreviousresearchandstateandnationalguidanceon
roundaboutsandaccessmanagement,andtoconductempiricalresearchonthesafetyandoperationof
roundaboutsinFlorida.Adviceonimplementingroundaboutsandaccessmanagementintostateguidance
documentswillbeprovided.Theresearchobjectiveswereachievedbycompletingthefollowingtasks:
1. Literatureandbackgroundreviewofnationalandstateguidance;
2. Safetyanalysisofall283roundaboutsinFlorida;
3. OperationalanalysisofthirteenselectedroundaboutsitesinFlorida;and
4. Softwaretoolsreviewforroundaboutsimulationandevaluation.
FindingsandConclusions
Thereviewofnationalguidanceonroundaboutandaccessmanagementshowsthatonlyfivefederalaccess
managementreportsrefertoroundabouts:AASHTOGreenBook,NCHRPReport672–Roundabouts:An
InformationalGuide,SecondEdition,NCHRPReport572–RoundaboutsintheUnitedStates,NCHRPReport
674–CrossingSolutionsatRoundaboutsandChannelizedTurnLanesforPedestrianswithVision
Disabilities,andNCHRPSynthesis264–CrossingSolutionsatRoundaboutsandChannelizedTurnLanes
forPedestrianswithVisionDisabilities.NCHRPReport672,whichisthemostrelevanttothisreport,
referstotheaccessmanagementinthecontextofroundaboutsandreinforcestheideathatmanyofthe
accessmanagementprinciplesthatapplytoconventionalintersectionscanbeappliedtoroundabouts.
Stateguidanceonroundaboutsandaccessmanagementprovidesvaryinglevelsofspecificity,withmost
statesadoptingnationalguidancefromNCHRPReport672–Roundabouts,AnInformationalGuide;afew
statesprovidestate‐specificparametersandguidance.Whileseveralstatesadoptlocalparametersfor
roundabouts,onlythreestates–Wisconsin,Virginia,andKansas–addresstheuseofaccessmanagement
withinthebroadercontextofthedesignofroundabouts.
ThesafetyandoperationalanalysesofexistingroundaboutsinFloridaidentifythreeareasofconcern
aboutaccessmanagementnearroundabouts:(1)conflictswithinthefunctionalareaofroundabouts;(2)
safetyofvulnerableroadusers,includingpedestriansandbicyclists;and(3)roundaboutsthatprovide
directaccesstoactivitycenters.Ofatotalof2,941crashesthatoccurredfrom2007–2011within500ft.
ofthe283roundaboutsinthestate,1,882crashesweredirectlyrelatedtoaroundabout;thisisanaverage
of6.65crashesperroundaboutwithanaverageof8.10and5.4crasheseacharoundcommercialand
residentiallanduses,respectively.Consistentwiththepreviousfindings,thesafetyandoperational
analysisofroundaboutsshowedarelativelowrateofcrashes,butsomeareasofconcern.Theoperational
analysisidentifiedsituationsinwhichaleft‐turningvehicleorpedestrianscouldcausedelaysinvehicles
movingthroughtheroundabout.Thesafetyanalysisshowedthatcrashesinvolvingvehiclesturningleftat
medianopeningswererelativelyrare.Whilethesafetyanalysisshowedthatthedownstreamdriveway
cornerclearancehasagreatersafetyimpactthantheupstreamdrivewaycornerclearance,theoperational
analysisdidnotidentifysuchconflicts.Highpedestrianandbicyclevolumescanaffectthecapacityandthe
effectiveoperationofroundabouts.Crashesinvolvingpedestriansandbicycleswereabout4%ofall
crashes,butnogeneralconclusionscanbedrawnduetothesmallsamplesizeandthelackofgood
exposuredata.Thesafetyandoperationalanalysishadsomewhatconflictingresultsforroundaboutsat
activitycenters.Roundaboutswiththreeorfourlegs,withdirectaccesstoactivitycenters,areequallyas
safeasroundaboutswithoutdirectaccesstoroundabouts.However,inactivitycenterswithhighvolumes
ofpedestriansandbicyclists,erroneousdriverbehavior,suchasstoppinginthemiddleoftheroundabout
topick‐upordrop‐offpedestrians,causesdelaysforotherdrivers.Theoperationalanalysisdidnot
identifyotherconcernsfoundintheliterature,includingspillbackintotheroundaboutfromadownstream
bottleneck,whichwouldresultincompletelylockingtheroundabout.
Amajorconclusionofthisresearchisthat,whilemuchresearchhasbeenconductedaboutroundabouts
andaboutaccessmanagement,littleresearchhasbeenconductedonroundaboutsincombinationwith
accessmanagementandroundaboutsasaformofaccessmanagement.Roundaboutsareaformofaccess
managementbecausetheycanaccommodateleftturnsandallowtheremovalofdirectionalleft‐turnlanes;
yettheyfunctionasintersections.Howqueuesformandtrafficoperatesinthefunctionalareaaround
roundaboutsislesswellunderstoodthanforothertypesofintersections.Thedifferencesinroundabout
safetyandoperationalcharacteristicsfromothertypesofaccessmanagementandotherintersections
meansthatthesitedistances,stoppingdistances,functionalareacharacteristics,andintersectionand
drivewayspacingmaybedifferentfromothertypesofintersections.
Recommendations
AsFloridastartsincorporatingroundaboutsintoitspractices,consistentguidanceontheuseof
roundaboutsthataddressthediversesituationsunderwhichroundaboutsareimplementedshouldbe
provided.Ofthe283roundaboutsinFlorida,onlyfourarelocatedonthestatehighwaysystem;therest
arelocatedinavarietyofregionalcontexts–urban,suburbanandrural–withdiversedesignsandaccess
considerations,andatdifferentdistancesfromthenearestcommunitycenters,highways,interstates,and
statehighways.Essentialtothisguidanceisconsiderationofthedifferencesbetweenroundaboutsand
othertypesofintersections,andtotypesofaccessmanagement,suchasdriveways,andmedians.Itis
essentialtounderstandtheeffectsofroundaboutsontrafficconditions,safetyandtrafficnetwork
operations.Thefindingsofboththesafetyandoperationalanalysisreinforcetheneedtoaccommodate
bicyclistsandpedestriansaroundroundabouts.Whilethisresearchdidnotidentifysignificantproblems
withtrucksandotherlargevehicles,theneedtoaccommodatethemislikelytobecomeanissueas
roundaboutsaremorewidelyusedalongstateroadwaysandotherhigh‐capacityroadwayswhere
roundaboutdesignneedstoaccountforadequatelateralclearanceandlargerradius.Floridahasalready
adoptedNCHRP672,Roundabouts,AnInformationalGuidebutthestateshouldconductandsupport
additionalresearchontheuseofroundabouts.TheFDOTshouldsupportnationalresearchthat
specificallyfocusesonthefunctionalareaofroundaboutsonmajorarterials.Thestateshouldconsiderthe
useoflocally‐developedparametersforvariousaspectsofdesignandoperationalanalysisofroundabouts.
Recently,theCityofSarasota,inconsultationwiththeFDOT,hasproposedaseriesofroundaboutsonUS
41.TheFDOThasauniqueopportunitytocompleteabefore‐and‐afterstudyontheoperationalandsafety
characteristicsofcorridorsofroundaboutsinsteadofconventionalintersectionsinthiscorridor.
Benefits
Roundaboutsofferseveraladvantagesoverothertrafficcontrols:theymaycostlesstoinstall,havegreater
safetypotentialbyreducingthenumberofconflictpoints,canaccommodateaseriesofU‐turnsandleft‐
turnlanesandreducedelayinthecorridor,andcanhaveandmayhaveloweroperationsandmaintenance
costs.Theguidanceresultingfromthisresearchcancertifythatroundaboutsareimplementedinamanner
thatensuresimprovedsafetyandcapacitywhilemaintainingaccesstonearbybusinesses.
ThisresearchprojectwasconductedbyRuthL.Steiner,oftheUniversityofFlorida.Formoreinformation,
contactGinaBonyani,ProjectManager,at850‐414‐4707,[email protected].
Roundabouts and Access Management
Page vii
TableofContents
Disclaimer...................................................................................................................................................................................................ii MetricConversionTable.....................................................................................................................................................................iii TechnicalReportDocumentationPage.........................................................................................................................................iv Acknowledgements.................................................................................................................................................................................v ExecutiveSummary...............................................................................................................................................................................vi Background..........................................................................................................................................................................................vi ListofFigures..........................................................................................................................................................................................xii ListofTables..........................................................................................................................................................................................xiv ListofAbbreviations............................................................................................................................................................................xv ChapterOne:Introduction...................................................................................................................................................................1 1.1Background...................................................................................................................................................................................1 1.2ResearchQuestions....................................................................................................................................................................2 1.3ObjectiveofResearch................................................................................................................................................................3 1.4ScopeofWorkandSupportingTasks.................................................................................................................................3 1.5OrganizationoftheReport.....................................................................................................................................................6 ChapterTwo:LiteratureReview.......................................................................................................................................................7 2.1Overview........................................................................................................................................................................................7 2.2Roundabouts.................................................................................................................................................................................7 2.2.1ModernRoundabouts......................................................................................................................................................7 2.2.2GeometricDesign...............................................................................................................................................................8 2.2.3ContextsofRoundabouts..............................................................................................................................................11 2.2.4ComparingRoundaboutstoOtherTypesofIntersectionTrafficControls..............................................13 2.3AccessManagement.................................................................................................................................................................13 2.3.1AccessManagementElements...................................................................................................................................14 2.3.2SpacingStandardsandRoadwayClassifications................................................................................................14 2.3.3AccessManagementMechanismsandIntersectionControls........................................................................16 2.3.4ImpactofRoundaboutsonAccessManagement................................................................................................18 2.4OperationalEffectsofRoundabouts.................................................................................................................................19 2.4.1EffectofTrafficFlowandDriverBehavior............................................................................................................19 2.4.2EffectofGeometry...........................................................................................................................................................20 2.4.3OperationalAnalysisofRoundabout.......................................................................................................................20 2.4.4RoundaboutCapacityunderDifferentConditions.............................................................................................21 2.4.5SummaryofRoundaboutOperationLiteratureReview..................................................................................22 2.5RoundaboutsandSafety........................................................................................................................................................23 2.5.1OverallSafetyEffectsoftheRoundabouts............................................................................................................24 Roundabouts and Access Management
Page viii
2.5.2AspectsofSafetyPerformanceofRoundabouts.................................................................................................25 2.5.3SafetyforDifferentRoundaboutUsersandModes...........................................................................................29 2.5.4MethodsinRoundaboutSafetyAnalysis................................................................................................................35 2.5.5RoundaboutsandSafety:Conclusion......................................................................................................................39 2.6EvaluationofGapsinRoundaboutLiterature..............................................................................................................40 2.6.1LiteratureGapsinAccessManagement.................................................................................................................40 2.6.2LiteratureGapsinRoundaboutOperationsandCapacity..............................................................................40 2.6.3LiteratureGapsinRoundaboutSafety....................................................................................................................41 ChapterThree:Methodology............................................................................................................................................................43 3.1AccessManagementandRoundaboutGuides’Selection.........................................................................................43 3.2SiteIdentification......................................................................................................................................................................45 3.3SafetyAnalysis...........................................................................................................................................................................46 3.3.1CategorizeRoundaboutLocations............................................................................................................................46 3.3.2ExtractCrashData...........................................................................................................................................................47 3.3.3CorrectCrashLocationsandReviewPoliceReports........................................................................................47 3.4OperationalAnalysis...............................................................................................................................................................50 3.4.1DataCollectionSiteSelection.....................................................................................................................................50 3.4.2DataCollection..................................................................................................................................................................53 3.4.3DataAnalysis.....................................................................................................................................................................54 ChapterFour:ReviewofNationalandStatePractices..........................................................................................................56 4.1NationalandStateGuidebooksforRoundaboutsandAccessManagement....................................................56 4.1.1NationalGuidanceforAccessManagement..........................................................................................................56 4.1.2States’GuidanceforAccessManagement..............................................................................................................61 4.1.3NationalandStateGuidebooksforRoundabouts..............................................................................................62 4.1.4StateGuidanceforRoundabouts...............................................................................................................................65 4.2StateofFloridaGuidance.......................................................................................................................................................68 4.2.1AccessManagementGuidanceinFlorida..............................................................................................................68 4.2.2RoundaboutsGuidanceforFlorida...........................................................................................................................75 4.3NationalGuidanceonAccessManagementintheContextofRoundabouts....................................................76 4.4States’GuidanceonAccessManagementintheContextofRoundabouts........................................................77 4.5RoundaboutLocationGuidelines.......................................................................................................................................80 4.6GeometryDesignGuidelines................................................................................................................................................81 ChapterFive:SafetyAnalysis...........................................................................................................................................................88 5.1OverallCrashStatistics...........................................................................................................................................................88 5.1.1AreaType............................................................................................................................................................................88 5.1.2CrashType..........................................................................................................................................................................88 5.1.3CrashSeverity...................................................................................................................................................................90 Roundabouts and Access Management
Page ix
5.1.4NumberofVehiclesInvolved......................................................................................................................................91 5.2ImpactofDrivewayCornerClearancesonRoundaboutSafety.............................................................................91 5.3SafetyImpactofMedianOpeningsintheVicinityofRoundabouts.....................................................................94 5.4SafetyatRoundaboutsThatProvideDirectAccesstoActivityCenters..........................................................100 5.5SafetyofVulnerableRoadUsers.....................................................................................................................................104 5.5.1Pedestrians......................................................................................................................................................................104 5.5.2Bicyclists...........................................................................................................................................................................105 5.6SummaryofFindings...........................................................................................................................................................106 ChapterSix:OperationalAnalysis...............................................................................................................................................109 6.1OverviewofDataCollectionSites...................................................................................................................................109 6.2AnalysisofAccessManagementIssuesAffectingOperations.............................................................................109 6.2.1ConflictsatAccessPointwithinRoundabout’sFunctionalArea...............................................................109 6.2.2ConflictswithPedestrians.........................................................................................................................................110 6.2.3ViolationofTrafficRules...........................................................................................................................................111 6.2.4SummaryofOperationalAnalysis.........................................................................................................................112 6.3AssessmentofSoftware......................................................................................................................................................112 6.3.1HCS......................................................................................................................................................................................113 6.3.2Synchro.............................................................................................................................................................................114 6.3.3SIDRA.................................................................................................................................................................................115 6.3.4RODELandARCADY....................................................................................................................................................115 6.3.5VISSIM...............................................................................................................................................................................115 6.3.6CORRIDORSIMULATION(CORSIM).....................................................................................................................116 6.3.7Summary..........................................................................................................................................................................118 ChapterSeven:Discussion..............................................................................................................................................................119 7.1Overview...................................................................................................................................................................................119 7.2RoundaboutsandAccessManagementinFlorida...................................................................................................120 7.2.1SummaryofSafetyAnalysis.....................................................................................................................................120 7.2.2SummaryofOperationalAnalysis.........................................................................................................................122 7.3RoundaboutsandAccessManagementGuidance....................................................................................................124 7.3.1SummaryofNationalandStateGuidanceonRoundabouts.......................................................................124 7.3.2SummaryofNationalandStateGuidanceonAccessManagement.........................................................125 7.3.3SummaryofNationalandStateGuidanceonRoundaboutandAccessManagement......................126 7.3.4SummaryofFlorida’sGuidanceonRoundaboutsandAccessManagement........................................127 7.4SynthesisofFindingsoftheResearch...........................................................................................................................128 7.5Recommendations.................................................................................................................................................................131 7.5.1RecommendationsforFlorida’sGuidanceonRoundaboutsandAccessManagement...................131 7.5.2RecommendationsforAdditionalResearch......................................................................................................134 Roundabouts and Access Management
Page x
ChapterEight:Conclusions............................................................................................................................................................136 8.1ConclusionsoftheReviewofNationalandStateGuidance.................................................................................136 8.2ConclusionsAboutSafetyAnalysisofRoundaboutsinFlorida..........................................................................136 8.3ConclusionsAboutOperationalAnalysisofRoundaboutsinFlorida..............................................................137 8.4FinalRemarks.........................................................................................................................................................................137 8.5AdditionalResearchNeeds................................................................................................................................................138 ReferencesCited.................................................................................................................................................................................140 AppendixA:RoundaboutsFeaturesandDimensions.........................................................................................................149 KeyFeaturesofaModernRoundabout..........................................................................................................................149 Dimensions.................................................................................................................................................................................149 AppendixB:StatePolicies..............................................................................................................................................................151 AppendixC:AccessManagementTechniquesinStateGuidelines................................................................................161 AppendixD:SiteSelection..............................................................................................................................................................165 Roundabouts and Access Management
Page xi
ListofFigures
Figure1.GeometricDesignFeaturesofaTypicalModernRoundabout(FDOT,2007)...........................................8 Figure2.GeometricDesignFeaturesofaTypicalModernRoundabout:Single‐lane(b)andMultiple‐Lane
Roundabouts(c)......................................................................................................................................................................................9 Figure3.IntersectionSightDistance(FHWA,2006,p.18)................................................................................................11 Figure4.StoppingSightDistance(FHWA,2006,p.19).......................................................................................................11 Figure5.AccessandRoadClassification....................................................................................................................................15 Figure6.RelationshipbetweenAccessManagement,RoadwayDesign,TrafficOperationsandLandUse
(Roseetal.,2005)..................................................................................................................................................................................16 Figure7.CrashTypesonaTypicalRoundabout(ArndtandTroutbeck,1998,p.28‐3)........................................24 Figure8.VehicleConflictsandVehicle‐PedestrianConflictsatSignalizedIntersectionsandSingle‐Lane
Roundabouts(Rodegerdtsetal.,2010,Exhibit5‐2,p.5‐7).................................................................................................26 Figure9.DifferentMarkingSystems(Bieetal.,2005).........................................................................................................28 Figure10.(1)Mixedtraffic;(2)adjacentbikelanes;(3)separatedbikelaneswithpriorityforbicyclists;
and(4)separatedbikelaneswithoutpriorityforbicyclists(DanielsandWets,2005,p.6‐8)............................31 Figure11.CrashFrequenciesinRoundabouts(Isebrands,2009b)...............................................................................36 Figure12.DataRequiredforChi‐SquareAnalysis(FlanneryandDatta,1996,p.6)..............................................36 Figure13.ExamplesofRoundaboutsLocatedinEachLandUseType.........................................................................47 Figure14.CrashesDisplayedbyCrashTypeataRoundabout........................................................................................48 Figure15.CrashesDisplayedbyCrashSeverityataRoundabout..................................................................................48 Figure16.AnExampleofaCrashThatWasNotDirectlyRelatedtotheRoundabout...........................................49 Figure17.DataCollectionusingWeb‐basedTool..................................................................................................................50 Figure18.RoundaboutsitesinFloridaSelectedforOperationalAnalysis.................................................................52 Figure19.CameraLocationofVideoRecordingforIndependentDriveandSouthLauraStreetin
Jacksonville..............................................................................................................................................................................................54 Figure20.RoadwayFunctionClassificationinFlorida(FDOT,2010,p.24)..............................................................69 Figure21.DrivewayDesignandSpacing(FDOT,2008,p.9)............................................................................................69 Figure22.EffectiveRadiusandCurbRadius(FDOT,2008)..............................................................................................70 Figure23.RampSpacing(FDOT,2008,p.78).........................................................................................................................71 Figure24.RoundaboutatanInterchange(FHWA,2006,p.8).........................................................................................72 Figure25.CornerClearance(FDOT,2008,p.73)..................................................................................................................72 Figure26.CornerClearanceforDownstream(FDOT,2008,p.76).................................................................................73 Figure27.SightDistanceandDriverEyeSetbackDrivewayInformationGuide(FDOT,2008,p.62)...........74 Figure28.JointandCrossAccess(FDOT,2008,p.86)........................................................................................................74 Figure29.TypicalDimensionsforLeft‐turnAccessnearRoundabouts(Rodegerdtsetal.,2010,p.6‐98)...77 Figure30.MeasuredDistancefromSplitterIslandtoFirstAccessPoint(KsDOT,2013,p.4‐26)....................79 Figure31.MinimumSpacingStandardsforCommercialEntrances,Intersections,andCrossovers(VDOT,
2007,p.F‐23)..........................................................................................................................................................................................80 Figure32.TheEffectofDesignElements(WisDOT,2011,p.38).....................................................................................81 Figure33.ExampleSolutionDesignwithCirculating‐ExitingPathConflict(Caltrans,2007,p.62).....................83 Figure34.SolutionOptionsforCirculating‐ExitingPathConflict:(i)ModifyLaneConfiguration,and(ii)
RealignApproaches(Caltrans,2007,p.63‐64).........................................................................................................................83 Figure35.AngleofVisibility:(i)theAngleisTooSevere(ii)RealignedRampTerminalApproachtoHave
BetterAngleofVisibility(Caltrans,2007,p.65)......................................................................................................................84 Figure36.StatisticsbyAreaType.................................................................................................................................................89 Figure37.TotalandNighttimeCrashStatisticsbyCrashType......................................................................................90 Figure38.StatisticsbyCrashSeverity........................................................................................................................................90 Figure39.UpstreamandDownstreamDrivewayCornerClearances...........................................................................92 Roundabouts and Access Management
Page xii
Figure40.RoundaboutonSRA1A,NassauCounty,FloridawithReducedSightDistanceatDownstream
CornerClearance...................................................................................................................................................................................94 Figure41.Case1‐VehiclesTurningontoaDrivewayDownstreamoftheRoundabout.....................................95 Figure42.Case2‐VehiclesTurningLeftfromaDrivewayUpstreamofaRoundabout.......................................96 Figure43.AnExampleofaCrashataMedianOpeningInvolvingaVehicleTurningLeftFromtheMain
StreetOntoaDriveway.......................................................................................................................................................................97 Figure44.AnExampleofaCrashataMedianOpeningInvolvingaVehicleTurningLeftFromaDriveway
OntotheMainStreet............................................................................................................................................................................97 Figure45.ANon‐incapacitatingInjuryInvolvingaVehicleTurningLeftfromDrivewayandaBicyclist....97 Figure46.ExamplesofCrashesInvolvingHeavyVehiclesatRoundabouts..............................................................98 Figure47.ClosingMedianOpeningsPreventVehiclesFromTurningLeftFromtheDriveway.......................99 Figure48.ClosingMedianOpeningsPreventVehiclesFromTurningLeftFromtheMainStreet...................99 Figure49.ACorridorwithTwoRoundaboutsonSegoviaStreet,MiamiDadeCounty,Florida....................100 Figure50.AnActivityCenterwithAccessThroughaMajorDriveway....................................................................101 Figure51.AnActivityCenterwithDirectAccessfromaRoundabout......................................................................101 Figure52.ExamplesofSix‐leggedRoundaboutsthatExperiencedHighCrashes.................................................103 Figure53.FatalCrashInvolvingaPedestrian(CrashID:772427040)....................................................................104 Figure54.CorridoronSW2ndAvenue,Gainesville,AlachuaCounty,Florida........................................................106 Figure55.ConflictofLeft‐turnVehicleatRoundabout(SW2ndAvenueandSW6thinAlachuaCounty)..110 Figure56.RoundaboutObservationonSpillBackofEnteringTrafficintoanAdjacentAWSCIntersection
(NE10thCt.andSW152ndAve.,Miami)...................................................................................................................................110 Figure57.RoundaboutObservationwithPedestrianConflict(IndependentDr.andS.LauraSt.,Duval
County)...................................................................................................................................................................................................111 Figure58.RoundaboutObservationwithDriverViolationofTrafficRules(IndependentDr.andS.Laura
St.,DuvalCounty)...............................................................................................................................................................................111 Figure59.RoundaboutObservationwithSpillBackfromDrivewayintoCirculatingLanes(CausewayBlvd.
andMandalayAve.,PinellasCounty).........................................................................................................................................112 Figure60.InterfaceofHCS2010................................................................................................................................................113 Figure61.UserInterfaceofSynchro(Trueblood,2013).................................................................................................114 Figure62.ExampleofRoundaboutSimulationinVISSIM(FHWA,2011)................................................................116 Figure63.ExampleofModelingRoundaboutinCORSIM(Elias,2009).....................................................................117 Figure64.ConditionalTurnMovementinCORSIM(Elias,2009)................................................................................117 Figure65.ConflictandSpillbackassociatedwithLeft‐turnAccesstoDriveway...................................................122 Figure66.Solution1‐DedicatedLeft‐turnLaneforAccesstoDriveway..................................................................123 Figure67.Solution2–Right‐laneAccess...............................................................................................................................123 FigureD.1.CameraLocationofRoundaboutatCausewayBlvdandMandalayAve.............................................165 FigureD.2.CameraLocationofRoundaboutatSW2ndAveandSW6thSt...............................................................166 FigureD.3.CameraLocationofRoundaboutatMLKBlvd.andN.CentralAve......................................................167 FigureD.4.CameraLocationofRoundaboutatEagle’sReserveBlvdandDyerBlvd..........................................168 FigureD.5.CameraLocationofRoundaboutatIndependentDr.andS.LauraSt..................................................169 FigureD.6.CameraLocationofRoundaboutatCR‐707andAveA.............................................................................170 FigureD.7.CameraLocationofRoundaboutatCR‐210andMicklerRd...................................................................171 FigureD.8.CameraLocationofRoundaboutatNE10thCt.andSW152ndAve.....................................................172 FigureD.9.CameraLocationofRoundaboutatGreenwayDr.andSegoviaSt.&CoralWay............................173 FigureD.10.CameraLocationofRoundaboutatBiltmoreWayandSagoviaSt....................................................174 FigureD.11.CameraLocationofRoundaboutatHolmbergRd.andParksideDr.................................................175 FigureD.12.CameraLocationofRoundaboutatPonceDeLeonBlvd.andRuizAve..........................................176 FigureD.13.CameraLocationofRoundaboutatMargateBlvd.andNW58thSt...................................................177 Roundabouts and Access Management
Page xiii
ListofTables
Table1.SelectionofAnalysisTool(Rodegerdtsetal.,2010)............................................................................................23 Table2.DetailedCountermeasuresforDesignElements(Lordetal.,2007,p.429)..............................................32 Table3.AdvantagesandDisadvantagesofRoundaboutforPedestrians(Furtado,2004)...................................33 Table4.MainDocumentsonAccessManagement–RelatedStateDOTGuidebooks.............................................44 Table5.TheSourcesofRoundaboutStates’DesignGuidebooks....................................................................................45 Table6.SummaryofRoundaboutsinFloridabyDesignandContext...........................................................................46 Table7.CriteriaforSelectingRoundaboutsforOperationalAnalysis..........................................................................51 Table8.SummaryofRoundaboutSelectionProcess............................................................................................................52 Table9.SummaryofFeaturesandSurveyTimeofSelectedRoundaboutsofThirteenRoundaboutsand
DataCollectionTimesforOperationalAnalysis.......................................................................................................................53 Table10.MainDocumentsoftheAccessManagement‐RelatedStateDOTsGuidebooks...................................62 Table11.RoundaboutStates’DesignGuidebooksReviewedinthisDocument.......................................................65 Table12.AccessManagementStandardsfromRule14‐97(FDOT,2006,p.15)......................................................73 Table13.RoundaboutDesignSpeed...........................................................................................................................................82 Table14.RecommendedHeadwayValues(WisDOT,2011,p31)...................................................................................82 Table15.WisconsinDOTMinimumVisibilityDistance.....................................................................................................84 Table16.TypicalInscribedCircleDiameterRanges(Caltrans,2007,p.67)..............................................................84 Table17.CommonRangesofInscribedCircleDiameters(Caltrans,2007,p.68)....................................................85 Table18.TheGuidelinesComparisonforDesignVehiclesonMulti‐laneRoundabouts(Caltrans,2007).....86 Table19.StatisticsbyAreaType..................................................................................................................................................88 Table20.StatisticsbyCrashType................................................................................................................................................89 Table21.StatisticsbyCrashSeverityandAreaType...........................................................................................................91 Table22.StatisticsofSingle‐vehicleandMulti‐vehicleCrashesbyAreaType.........................................................91 Table23.StatisticsofSingle‐vehicleandMulti‐vehicleCrashesbyCrashSeverity.................................................92 Table24.Driveway‐relatedCrashesThatOccurredwithinUpstreamandDownstreamDrivewayCorner
Clearances................................................................................................................................................................................................93 Table25.StatisticsofRoundaboutswithThreeandFourLegs...................................................................................102 Table26.StatisticsofRoundaboutswithFiveandSixLegs..........................................................................................102 Table27.PedestrianCrashStatisticsbyMedianType.....................................................................................................105 Table28.BicycleCrashStatisticsbyLocationandCrashSeverity.............................................................................106 Table29.InputandOutputforRoundaboutComponentsinHCS2010....................................................................114 Table30.RecommendedSelectionofAnalysisToolforDifferentApplicationsRegardingRoundaboutsand
AccessManagement..........................................................................................................................................................................118 TableA.1.KeyFeaturesofaModernRoundabout..............................................................................................................149 TableA.2.DimensionsofRoundabouts...................................................................................................................................149 TableB.3.StateWebsitesandGuidanceonRoundaboutsandAccessManagement...........................................151 TableB.4.RoundaboutGuidelinesinDrivewayorHighwayManuals.......................................................................153 TableB.5.SpecificManualsonRoundaboutGuidance......................................................................................................154 TableB.6.StateGuidanceonAccessManagementManuals...........................................................................................156 TableB.7.OtherDocumentsRelatedtoAccessManagement........................................................................................160 TableC.8.SpacingRequirements...............................................................................................................................................161 TableC.9.AccessManagementElementsontheStates(GluckandLorenz,2010,page48)............................162 TableC.10.AccessManagementTechniquesappliedbytheStateDOTs(GluckandLorenz,2010,pages49‐
50).............................................................................................................................................................................................................163 Roundabouts and Access Management
Page xiv
ListofAbbreviations
AADT AverageAnnualDailyTraffic
AASHTO
AmericanAssociationofStateHighwayandTransportationOfficials
ADA AmericanswithDisabilitiesAct
ANOVA
AnalysisofVariance
ARCADY
AssessmentofRoundaboutCapacityandDelay
AWSC All‐WayStopControlled
AzDOT
ArizonaDepartmentofTransportation
CMF CrashModificationFactors
CORSIM
CorridorSimulation
CS
ConflictingSpeed
DCEE DepartmentofCivilandEnvironmentalEngineering
DOT DepartmentofTransportation(general;appliestoanystateorstatescollectively)
ESSIE EngineeringSchoolofSustainableInfrastructureandtheEnvironment
FDOT FloridaDepartmentofTransportation
FHWA FederalHighwayAdministration
FIU
FloridaInternationalUniversity
ft.
Feet
FTA FederalTransitAdministration
FWSC Four‐wayStopControlled
GIS
GeographicInformationSystems
HCM HighwayCapacityManual
HCS HighwayCapacitySoftware
HSM HighwaySafetyManual
ICD
InscribedCircleDiameter
INDOT IndianaDepartmentofTransportation
IowaDOT
IowaDepartmentofTransportation
ISD
Intersectionsightdistance
ITE
InstituteofTransportationEngineers
km/h Kilometersperhour
KSU KansasStateUniversity
KYTCKentuckyTransportationCabinet
LOS LevelofService
LOSPLAN
LevelofServicePlanning
MEV MillionEnteringVehicles
MDOT MichiganDepartmentofTransportation
mi.
Miles
MNDOT
MinnesotaDepartmentofTransportation
mph Milesperhour
MPO MetropolitanPlanningOrganization
NCHRP
NationalCooperativeHighwayResearchProgram
NHDOT
NewHampshireDepartmentofTransportation
ODOT OregonDepartmentofTransportation
PDO PropertyDamageOnly
Roundabouts and Access Management
Page xv
PennDOT
PHB RCI
RTM SPF SSD TRB TWSC UF
URP VISSIM
WisDOT
WSDOT
PennsylvaniaDepartmentofTransportation
PedestrianHybridBeacon
RoadwayCharacteristicsInventory
Regression‐to–the‐mean
Safetyperformancefunctions
Stoppingsightdistance
TransportationResearchBoard
Two‐waystopcontrolled
UniversityofFlorida
DepartmentofUrbanandRegionalPlanning
VerkehrinStädten–SimulationsModel
WisconsinDepartmentofTransportation
WashingtonStateDepartmentofTransportation
Roundabouts and Access Management
Page xvi
Chapter1Introduction
ChapterOne:Introduction
1.1Background
Transportationengineersandplannersareincreasinglyinterestedinusingroundaboutstoaddressaccess
andsafetyconcernsinthetransportationsystem.Severalstateshavestronglyencouragedtheuseof
roundaboutsbecausetheymaycostlesstoinstallthansignalizedintersections,mayhaveagreatersafety
potentialbyreducingthenumberofconflictpoints,anddependinguponthecontext,loweroperationsand
maintenancecosts(TRB,2010a).Roundaboutshave“seenunprecedentedgrowthacrosstheUnitedStates,
fromjustahandfuladecadeagotomorethan2,000andcounting”(Schroederetal.,2011,p.1).Arecent
FederalHighwayAdministration(FHWA)andFederalTransitAdministration(FTA)(Rueetal.,2010)
publicationdescribesthebenefitsofroundaboutsfromalivabilityperspective:
…theymanagequeuingandcongestionatintersectionsbyallowingsimultaneousoperation
ofsomecrossingmovements;theybreakpotentialvehicle‐pedestrianconflictsintotwo
discretepointsbyuseoftheirsplitterislands;andtheyslowtrafficmovingthroughthe
intersection,whileincreasingcapacity.Theyoffergreatersafety,eliminatingthepotential
forhead‐oncollisionsandfocusingdrivers’attentionontheroadwayahead,andtoward
othercarsandpedestrians.Althoughtheyrequireconstructionadjustmentstoexisting
geometryoftheintersectingroadways,theyoffersafetyandoperationalbenefitsthatmake
themworkmoreeffectivelythantrafficsignalsbymostmeasures(Rueetal.,2010,p.6).
Althoughroundaboutsareinuseinmanycontexts,existingresearchdoesnotprovidedetailedguidanceon
howtoevaluatetheuseofroundaboutsasaformofaccessmanagementoraspartofalargerroadway
network.Roundabouts,AnInformationalGuidesuggeststheadvantageofroundaboutsasamethodto
“facilitateU‐turnsthatcansubstituteformoredifficultmid‐blockleftturns,especiallywherethereisno
leftturnlane”(Rodegerdtsetal.,2010,p.29).Examplesfromothercommunitiessuggestthatacorridor
usingmultipleroundaboutscanaccommodateaseriesofU‐turnsandleft‐turnlanesandreducedelayin
thecorridor.However,Roundabouts,AnInformationalGuidealsosuggeststhatroundabouts“mayreduce
thenumberofavailablegapsformid‐blocksignalizedintersectionsanddriveways”(Rodegerdtsetal.,
2010,p.29)andthusreducethecapacityoftheseaccesspoints.Attheveryleast,thetrafficalonga
corridorchangeswiththeintroductionofroundabouts;thetrafficmaybemoreuniformlydistributedwith
alargernumberofsmallergapsratherthanfewerlargeones.Thechallengesofusingroundaboutsalonga
corridoraredescribedingreaterdetailinthefollowing:
Itiscommonpracticetocoordinatetrafficsignalsonarterialroadstominimizestopsand
traveltimedelayforthroughtrafficonthemajorroad.Aroundaboutwithonlyyieldcontrol
cannotbeactivelymanagedtoprovideprioritytomajorstreetmovementsinthesameway.
Asaresult,thecoordinatedplatoonsoftrafficthatimprovetheefficiencyoftrafficsignals
canbedisruptedbyroundabouts,thusreducingtheefficiencyofdownstreamintersections.
Roundaboutscannotbemanagedusingacentralizedtrafficmanagementsystemto
facilitatespecialevents,diverttrafficflows,andsoonunlesssignalsattheroundaboutorin
thevicinityareusedforsuchapurpose(TRB,2010a,pp.2‐6).
However,thebenefitsofaroundaboutmayvaryfordifferentusers.Priorresearchshowsgenerally
consistentresultsaboutcrashratesbuttheperceptionsofthesafetyofroundaboutsvariesamong
diverseusers.Researchisalsoneededontheoperationalaspectsofroundabouts,especiallyasit
relatestoallroadwayusers;priorityforonetypeofusermaycausedelaysforothertypesofusers.
Accessmanagementmayalsorequireestablishingpriorityforspecificmovementsatornear
roundaboutsthataffecttheiroperations.
Roundabouts and Access Management
Page 1
Chapter1Introduction
AccessmanagementbenefitshavebeendocumentedinvariousNationalCooperativeHighway
ResearchProgram(NCHRP)reports,bothforsignalizedandunsignalizedintersections;
roundaboutsaregenerallyincludedasunsignalizedintersections.Themostrecentdocumenton
accessmanagement—NCHRPReport548(TRB,2003)—statesthataccessmanagementhasa
numberofpositivebenefits:improvedsafety,reductionindelay,increasedenvironmental
friendlinessintermsoffuelconsumptionandemissions,improvedaccesstoproperties,integration
oflanduseandtransportation,andtheprovisionofappropriatefunctionforhighwayswith
reducedcut‐throughtraffic.Tomaximizeroundaboutbenefitsandtoachievethemainpurposesof
roundaboututilization,theintegrationofroundaboutandaccessmanagementisrequired.
Insummary,roundaboutshavethepotentialtoincreasesafetyandreducedelaybycontrollingaccessand
morereadilyaccommodatingU‐turnandleft‐turnmovements.However,lessisknownabouthowto
evaluateroundaboutscomparedtootherformsofaccessmanagementandintersectioncontrolwith
respecttotraveldelay,safety,andothercommunityperformancemeasures.Additionally,manyofthe
micro‐scaledetailsaboutaccessmanagementnearroundaboutsandalongcorridors,likethelocationof
drivewaysandtheplacementanduseofmedians,arenotwelldefinedintheliteratureandarepotentially
moreflexiblewithroundaboutsthanconventionalintersectiondesigns.
NCHRPProject03‐65:ApplyingRoundaboutsintheUnitedStates,hasresultedintwomajornational
researchreportsontheuseofroundabouts:NCHRPReport572:RoundaboutsintheUnitedStates
(Rodegerdtsetal.,2007)andNCHRPWeb‐OnlyDocument94:AppendicestoNCHRPReport572;
RoundaboutsintheUnitedStates(Rodegerdtsetal.,2006).Thesereportsincludeaninventoryof
roundaboutsintheUnitedStatesatthetimeofthepublicationofthedocument,andadatabaseof
geometric,operational,andsafetyinformation.Theresultsofthisresearchhavebeenincorporatedinto
theHighwaySafetyManual(HSM)(TRB,2010b)andtheHighwayCapacityManual(HCM)(TRB,2000).
Roundabouts,AnInformationalGuidewasfirstpublishedin2000andupdatedthroughNCHRPProject03‐
65AtoproduceNCHRPReport672:Roundabouts:AnInformationalGuide,SecondEdition(Rodegerdtsetal.,
2010).Thisguidecontainssectionsonroundaboutconsiderations,planning,operationalanalysis,safety,
geometricdesign,applicationoftrafficcontroldevices,illumination,landscaping,andconstructionand
maintenance.TheFHWAOfficeofSafetyhasaRoundaboutOutreachandEducationToolbox(FHWA,2013)
thatincludesavarietyofcasestudiesfromdifferentstates,focusingonhowtoeducatethepublicto
properlyandsafelyuseroundabouts.
1.2ResearchQuestions
Themainquestionaddressedinthisresearchis,“Whataspectsofaccessmanagementshouldbe
incorporatedintothestateguidancedocumentsinthestateofFloridaonroundaboutswithrespecttotheir
usageneardrivewaysandalongcorridors?”
Thismainquestionisaddressedthroughanexplorationofthefollowingsub‐questions:
(a) Whatcanwelearnfromexistingliteratureabouttheoperation,capacity,safetyandaccess
associatedwithroundabouts?
(b) Howhaveroundaboutsbeenincorporatedintonationalandstateguidancedocumentsonaccess
management?
(c) Whatguidanceonoperation,capacity,safety,accessmanagement,anddesignhasbeen
incorporatedintonationalandstateguidancedocumentsonroundabouts?
(d) Howhaveaccessmanagement,safety,operations,andcapacityconsiderationsassociatedwith
roundaboutsbeenincorporatedintocurrentpractices?
(e) HasaccessmanagementinfluencedthesafetyofexistingroundaboutsinFlorida?
(f) HastheFloridastategovernmentincludedroundaboutsintheiraccessmanagementanddriveway
managementdocuments?Howdoesaccessmanagementfigureintoroundaboutdesigndocuments?
Roundabouts and Access Management
Page 2
Chapter1Introduction
(g) WhatdoStateofFloridadocumentsrecommendinregardtoaccessmanagementinthevicinityof
roundabouts?
1.3ObjectiveofResearch
The main objective of this research is to provide guidance for transportation professionals in Florida on
how access management around roundabouts should be managed. This objective is achieved through
severaltasksstartingfromareviewofpreviousliteratureandotherstateguidelinesonroundaboutstosee
how these guidelines are applied throughout the United States. The goal is to understand how access
management, capacity, and safety are addressed; to evaluate the gaps in knowledge regarding the use of
roundabouts; to analyze crashes near roundabouts; to conduct an operational analysis of a sample of
roundabouts; and to assess the primary software tools for analyses of roundabouts. The research
recommends changes to guidance documents in Florida, including the access management resources,
MedianHandbook,andDrivewayInformationGuide.
ResearchersattheUniversityofFlorida(UF)andFloridaInternationalUniversity(FIU)accomplishedthese
goals through a series of tasks including: review of literature and other research on roundabouts,
evaluationofthegapsinknowledgeregardingtheuseofroundabouts,safetyanalysisofcrasheswithin500
feetofall283roundaboutsinthestateofFlorida,operationalanalysisofasampleofthirteenroundabouts,
review of software used to evaluate roundabouts, and development of recommendations for additional
research and specific guidance on the deployment of roundabouts. The Department of Civil and
Environment Engineering at FIU completed the safety analysis, made recommendations regarding their
analysis and reviewed the entire document. Faculty from the UF’s Transportation Institute in the
Engineering School of Sustainable Infrastructure and Environment (ESSIE) directed the operational
analysisandthereviewofsoftwareforanalysisofroundabouts.ResearchersintheDepartmentofUrban
andRegionalPlanning(DURP)atUFcompletedtheremainingtasks,includingthereviewofliterature,the
evaluationofthegapsinknowledgeabouttheuseofroundabouts,thereviewofnationalandstatepolicy
documentsandthepreparationofthefinalreport.
1.4ScopeofWorkandSupportingTasks
Task1:LiteratureandBackgroundReview
Literaturerelatedtothesafety,accessmanagement,multimodaltransportation(especiallyforbicyclists
andpedestrians),androadwaycapacityassociatedwiththeuseofroundaboutswasreviewed.The
researchteamalsoexaminedroundaboutpoliciesandguidelinesfromotherstates.Documentationonthe
designandplacementofroundaboutsissummarizedinaseparatespreadsheet.Inataskthatwas
completedaftertheliteraturereview,nationalandstatepoliciesandguidelinesonroundaboutsafety,
access,andcapacitywerereviewedanddocumented;theresultsofthispolicyscanareincorporatedintoa
separatechapterthatreportstheresultsofthisresearch.
Task2:EvaluationofGapsinKnowledgeRegardingUseofRoundabouts
Inthistask,theresearchteamcriticallyevaluatedavailableliteratureandstatepoliciesandidentifiedthe
gapsinknowledgeregardingtheuseofroundabouts,especiallyastheyapplytosafety,access,operations,
androadwaycapacity.Theliteratureisusedtodefineatypologyofcontextsinwhichroundaboutsare
implemented.Thistypologyexpandsthedefinitionofcontextfromurban,suburban,andrural,toinclude
otherfactorsthataffectsafety,access,androadwaycapacitysuchasaccesspoints(threevs.four);number
oflanes(onevs.two);isolatedroundaboutsvs.roundaboutsinacorridor;roundaboutsinaresidential
neighborhoodvs.roundaboutsincommercialdistrictsornearinterchanges;andotherfactorsasdefinedin
theliterature.Thistaskassessedanddocumentedthestateoftheartinaccessmanagementinthevicinity
Roundabouts and Access Management
Page 3
Chapter1Introduction
ofroundabouts(forexample,policiesandassessmentregardingthepositioningofdrivewaysclosetoa
roundabout,oronalinkconnectingtworoundabouts).Thisevaluationalsodevelopedatypologyof
contextsinwhichroundaboutsareimplemented,andthiswasusedintheselectionofroundaboutsfor
detailedinvestigationintheoperationalanalysis.
Task3:SafetyAnalysis
Theresearchteamusedthetypologydevelopedintheprevioussteptounderstandsafetyissuesassociated
withroundabouts.Thesafetyanalysisdetermineswhethercrashcausationisrelatedtothepresenceof
specificdrivewayandmediancharacteristicsandprovidesrecommendationsforaccessdesignfeatures
withrespecttosafety.
Subtask3‐1:IdentifyPotentialStudyLocations
Inthistask,FDOT’sRoadwayCharacteristicsInventory(RCI)wasusedtoidentifythelocationofall
roundaboutsinthestate.TheRCIincludesroadwaydataforallstateroadsandafewoff‐systemroads.The
2011RCIhas219locationsclassifiedas“roundabouts.”Anadditional64roundaboutswerefoundusing
GoogleMapforatotalof283roundaboutsthroughoutthestate.Usingsatelliteimagesalreadycaptured
fromGoogleMapsforeachoftheselocationsandGoogle’sStreetView,allpotentialstudylocationswere
identifiedforuseinthesafetyandoperationalanalysis.Forthesafetyanalysis,allroundaboutlocations
wereusedtounderstandthegeneraltrendsincrashesnearroundaboutsandalargersamplewasusedfor
specificanalysis.Asdescribedbelow,theoperationalanalysisconsidersseveralfactorsusedtoselect
roundaboutsfordetailedstudy:thepresenceofsignificantmainlineanddrivewaytraffic,andthe
proximityoftheroundaboutstodrivewaysand/ormediandesignfeatures,aswellascommercialormixed
residentialandcommerciallanduseareas.
Subtask3‐2:CreateConditionDiagrams,CollectFieldData,andEstimateDrivewayTraffic
UsingacombinationofGoogleEarth,BingMaps,andGoogle’sStreetView,scaledconditiondiagramsof
eachpotentiallocationidentifiedintheprevioussubtaskwereconstructedinMicroStation.Eachsitewas
visuallyinspectedtocollectinformationonthelandusesassociatedwithadjacentdriveways,aswellasto
verifyexistinggeometricconditions.Theinformationcollectedincludeslandusetypes(e.g.,restaurants,
gasstations,apartments,etc.),numberofunits,yearestablished,andwhereapplicable,numberof
employees,floorspace,numberofgaspumps,andotherrelatedcontextinformation.Thelanduse
informationwasthenusedtoestimatedrivewaytrafficusingtheInstituteofTrafficEngineers(ITE)Trip
GenerationManual.
Subtask3‐3:ReviewPoliceReportsandCompileCrashInformation
Hardcopiesofpolicereportsdocumentinguptofiveyearsofcrashesthatoccurredwithinthefunctional
area(500feet)ofeachselectedroundaboutlocationweredownloadedfromageographicinformation
system(GIS)currentlybeingdevelopedbyDr.IlirBejlerioftheUFDURP.Crashdatafrompolicereports
wereextracted,includingcrashlocation,crashtype,crashseverity,vehicletype,driver’sage,lighting
conditions,andothercontributingfactors.Additionally,theillustrativesketchanddescriptionofeachcrash
wasrecorded.Sincetheconstructiondateofsomeofthelocationswasnotavailableandthegeometric
conditionshavechangedovertime,policesketchesanddescriptionswereusedtofurtherverify,tothe
extentpossible,thatgeometricconditionsdidnotchangeoverthestudyperiod.Inthosecaseswherepolice
reportsindicategeometricchanges,crashesthatoccurredbeforethechangeswereexcludedaswere
crashesnotdirectlyrelatedtotheroundabout.
Roundabouts and Access Management
Page 4
Chapter1Introduction
Subtask3‐4:ConstructCollisionDiagramsandPerformSafetyAnalysis
Inthissubtask,crashinformationcompiledpreviouslywasusedtoconstructacollisiondiagramforeach
studylocation.Fromthesediagramsandtheassociatedcrashcharacteristics,crashpatternsastheyrelate
todrivewayandmediandesignfeatureswereidentified.Thesepatternswerefurtheranalyzedbasedon
vehicletype,timeofday,lightingcondition,driverage,estimateddrivewaytrafficvolumes,andother
factors,toidentifythecausesofover‐representedcrashes.Thestatisticswerealsostratifiedbycrash
injuryleveltodeterminetheseverityofthecrashes.Asampleofthequestionstheanalysisattemptedto
answerincludes:
 Dospecificdrivewayandmedianconditions(e.g.,proximityofdrivewayandmedianopeningto
roundabouts;directvs.indirectdrivewayconnection)contributetocertaintypesofcrashes
involvingaccesstraffic?
 Doesthepresenceofdrivewaysandmedianopeningsresultinmoreseverecrashes?
 Issafetyaffectedbycertaingeometriccharacteristicsofroundaboutswhencombinedwithspecific
drivewayandmedianopenings?
 Arethereasignificantnumberofcrashesinvolvingpedestriansnearroundabouts?
 Howhavepedestriancrossingsbeenaffectedbydrivewaylocations?
Basedontheresultsoftheanalysisdonehere,specificrecommendationsondrivewayandmediandesign
featuresnearoratroundaboutlocationsaremade.Thistaskdocumentstheresultsofthesafetyanalysis
andprovidesinformationabouthowsafetyconsiderationsaffectthecontextinwhichroundaboutsare
placed.
Task4:AnalysisofSelectedFieldRoundaboutSites
Inthistask,theresearchteamidentifiedseveralroundaboutsitesinFloridafordirectstudyandanalysis.
Trafficoperationspotentiallyaffectedbydrivewaysandmediansapproachingandexitingtheroundabout,
werestudied.TheresultsofthisanalysiswerecomparedwiththefindingsofTask1.Duringpeak
operatingtimes,betweentwoandfourhoursofvideodatawerecollectedateachroundaboutlocation.
Task5:DevelopmentofRecommendationsforIncorporatingAccessManagementintoFlorida
Practice
Inthistask,theresearchteamtooktheresultsoftheliteraturereviewandanalysisofgapsinknowledge
andmaderecommendationsonhowtoincorporateaccessmanagementintoroundaboutdesigninFlorida.
Thisincludesrecommendationsforadditionalresearch,andchangestoFDOT’sAccessManagementTools,
MedianHandbookandDrivewayInformationGuide.
Task6:AssessmentofPrimaryFDOT‐UtilizedSoftwareToolsforRoundaboutEvaluation
Asappropriate,FDOTregularlyimplementsvariousanalysismethodologiesintocustomsoftwareproducts,
andrecommendstheuseofcertainsoftwareproductsthatimplementFDOT‐approvedanalysis
methodologies.Forexample,FDOTsupportsthedevelopmentofcustomsoftwarefortrafficoperations
andlevelofserviceanalysis(i.e.,LOSPLAN).LOSPLANisgenerallyintendedforplanningandpreliminary
engineeringanalyses,andemploysdeterministic,macroscopicanalysistechniquesconsistentwiththe
HCM.Fortrafficanalysisscenariosinvolvingahighlevelofcomplexity,themicroscopic,stochastic
simulationprogramCORSIM(corridorsimulation)isgenerallyrecommended.AsFDOThasdecidedto
adopttheHSMmethodologyforsafetyanalysis,thecurrentcapabilityofHSMinanalyzingandpredicting
thesafetyperformanceofroundaboutswasassessed,andpotentialapplicationgapswereidentifiedand
recommendedforHSMimplementation.
Roundabouts and Access Management
Page 5
Chapter1Introduction
Inthistask,anassessmentoftheprimaryFDOT‐utilizedsoftwaretoolswasmade.Thisassessment
focusedonthecurrentsuitabilityofthesesoftwaretoolstoassistwiththeevaluationoftheissues
previouslyidentified.Wheretheymaybedeficient,recommendationsweremadeonhowtoimprovethese
toolstomakethemmoreeffectivefortheevaluationofroundaboutsandaccessmanagement.
Task7:PreparationofDraftandFinalReports
ThedraftfinalreportwaspreparedandsubmittedforreviewbytheFDOTSystemsPlanningOfficeandthe
ResearchCenterstaff.Thedraftfinalreportwasreviewedforgrammar,clarity,organization,and
readabilitypriortosubmissiontoFDOTfortechnicalapproval.Towardtheendofthistask,ameetingwas
organizedwiththestaffoftheSystemsPlanningOfficetodiscussthefindingsandrecommendations,and
thedraftfinalreport.Thereportwasalsodistributedtootherresearchersandpractitionerswithexpertise
inthedesignanddeploymentofroundabouts.Theresearchteampreparedarevisedfinalreportbasedon
thecommentsreceivedbythepanel,andsubmittedittoFDOTandthetechnicalreviewandproject
implementationpanel.
1.5OrganizationoftheReport
ThisreportisorganizedintoeightchaptersbeginningwiththeIntroduction.ChapterTwocontainsthe
literaturereviewthatintroducestheconceptsofroundaboutandaccessmanagement;examinestheprior
studiesandreportsonthesimilartopics;andidentifiesgapsinknowledge.ChapterThreedescribesthe
methodologiesutilizedinthisresearch.ChapterFourdescribesthereviewofnationalandstateguidance
regardingroundabouts,accessmanagementandthecombinationofroundaboutsandaccessmanagement.
ChapterFivereportsthefindingfromthesafetyanalysis.ChapterSixdiscussesthefindingsfromthe
operationalanalysisandexploresthesoftwarethatisavailableforuseinanalysisofroundaboutsand
accessmanagement.ChapterSevendiscussesaccessmanagementintheroundabouts,incorporatinga
comparisonoftheinformationfoundintheliteraturereviewandinthestateguidance,includingwhathas
beenimplementedintheStateofFlorida,tomakerecommendationsforfurtherresearchandguidanceto
improveFloridaguidancedocumentsonroundaboutsandaccessmanagement.InChapterEight,the
researchissummarized.
Roundabouts and Access Management
Page 6
Chapter2LiteratureReview
ChapterTwo:LiteratureReview
2.1Overview
Thisreviewofpriorresearchhastwoparts.First,theavailableliteratureregardingtheuseof
roundabouts,especiallyastheyapplytosafety,roadwaycapacity,andaccessissummarized.Next,a
summaryofthestate‐of‐the‐artinroundaboutpracticeisdeveloped,includinganevaluationofgapsin
knowledgeregardingresearchaboutroundaboutsandaccessmanagement.
Theliteraturereviewisorganizedaroundscholarlyandpractice‐basedresearchonroundabouts,
roundaboutcapacity,roundaboutsafety,andaccessmanagement.Ofparticularinterestinthissectionare
articlesthataddressaccessmanagementandmultimodaltransportation,especiallyforbicyclistsand
pedestrians.Theliteraturedefinesatypologyofcontextsinwhichroundaboutsareevaluated,including:
thetype—urban,suburban,andrural;thenumberofaccesspoints—threeandfour;thenumberoflanes—
oneandmulti‐lane;thenumberofroundabouts—oneandcorridor;andlocationoftheroundabouts—
residential,commercial,mixed‐use,andinterchanges.
2.2Roundabouts
Priorliteraturedifferentiatesmodernroundaboutsfromtrafficcirclesorcircularintersections.Thetraffic
circle,introducedin1905,canbeseenasaprecursortoroundaboutsintheUnitedStates(Jacquemart,
1998).IntheRoundaboutsGuide,2ndedition,Rodegerdtsetal.(2010)definedthreetypesofcircular
intersections:rotaries,neighborhoodtrafficcircles,androundabouts.
TheUnitedKingdominitiatedthemodernroundaboutin1966withthe“give‐way”ruleforenteringtraffic,
byallowingcirculatingtraffictocontinuedrivinginroundaboutsratherthanyieldingtoenteringvehicles.
ThefirstmodernroundaboutsintheUnitedStateswereconstructedin1990,andwerebasedonthe
professionaldesignexperienceofothercountries,particularlyAustraliaandtheUnitedKingdom.The
differencebetweenroundaboutsandothercircularintersectionsisthe“give‐way”rulethatprioritizes
trafficcirculatingintheroundaboutorthesmallerneighborhoodtrafficcircles(Rodegerdtsetal.,2010).
2.2.1ModernRoundabouts.Thisprojectfocusesonthemodernroundabouts;throughoutthedocument
theterm“modernroundabouts”isusedinterchangeablywith“roundabouts”asdefinedhere.Roundabouts
canbedescribedas:
circularintersectionswithspecificdesignandtrafficcontrolfeatures.These
featuresincludeyieldcontrolofallenteringtraffic,channelizedapproaches,
andappropriategeometriccurvaturetoensurethattravelspeedsonthe
circulatoryroadwayaretypicallylessthan50km/h(30mph).Thus,
roundaboutsareasubsetofawiderangeofcircularintersectionforms
(Rodegerdtsetal.,2010,p.5).
Withthisdefinition,threekeyfeaturesofroundaboutsaredistinguishedfromthoseofotherformsof
trafficcircles,suchasrotaries,mini‐trafficcircles,andothernon‐modernroundabouts.Thesefeaturesare
theyield‐at‐entryrule,channelizedapproaches,andgeometriccurvaturedesignstoslowdownthespeed.
AtyandHosni(2001)addedtwoothercharacteristicsofmodernroundaboutsthatareimportanttothis
research:prohibitingbothparkingonthecirculatingroadway,andpedestrianactivitiesonthecentral
island.Figure1andFigure2showthefeaturesofatypicalroundaboutandthedifferencesandsimilarities
betweensingleandmulti‐laneroundabouts.
Roundabouts and Access Management
Page 7
Chapter2LiteratureReview
2.2.2GeometricDesign
Geometricelementsoftheroundaboutinclude:inscribedcirclediameter,entrywidth,circulatoryroadway
width,centralisland,entrycurves,exitcurves,pedestriancrossinglocationandtreatments,splitterisland,
stoppingsightdistance(SSD),intersectionsightdistance(ISD),verticalconsiderations,andbicycle
provisions.
2.2.2.1KeyFeaturesandDimensions.AccordingtothesecondeditionofRoundabouts,AnInformational
Guide(Rodegerdtsetal.,2010),thekeyfeaturesofroundaboutsincludethecentralisland,splitterisland,
circulatoryroadway,apron,yieldline,accessiblepedestriancrossings,bicycletreatments,andlandscaping
buffer.Furthermore,theroundaboutdimensionsaddresstheinscribedcirclediameter,circulatory
roadwaywidth,approachwidth,departurewidth,entrywidth,exitwidth,entryradius,andexitradius.
AdditionalexplanationsabouteachfeatureareincludedinAppendixA.
Figure1.GeometricDesignFeaturesofaTypicalModernRoundabout(FDOT,2007)
Roundabouts and Access Management
Page 8
Chapter2LiteratureReview
(a)single‐laneroundabout(FDOT,2007,p.2‐21)
(b)Multi‐laneroundabout(FDOT,2007,p.2‐21)
Figure2.GeometricDesignFeaturesofaTypicalModernRoundabout:Single‐lane(a)andMultiple‐Lane
Roundabouts(b)
Roundabouts and Access Management
Page 9
Chapter2LiteratureReview
Designspecificationsandguidelinesforeachindividualgeometrycomponentareprovidedinnationaland
stateguides(e.g.,GluckandLorenz,2010;FDOT,2007;IowaDOT,2010;Maryland,2012;andWisDOT,
2013).Thefirstelementsthatshouldbedefinedandoptimizedinthegeometricdesignofaroundaboutare
thesize,position,alignment,andarrangementofapproachlegs.Then,otherdetailsofgeometrycanbe
determined.Eachtypeofroundabout(single,double,multi‐lane,rural,ormini)hasspecificdesign
guidelines,soitisdifficulttostandardizethem.However,basedonNCHRP672,Roundabouts,An
InformationalGuide(Rodegerdtsetal.,2010,pp.6‐8),everyroundaboutdesignshouldmeetthefollowing
setofobjectives:
1. “Slowentryspeedsandconsistentspeedsthroughtheroundaboutbyusingdeflection;”
2. “Theappropriatenumberoflanesandlaneassignmenttoachieveadequatecapacity,lanevolume
balance,andcontinuityoflanesthroughtheroundabout;”
3. “Smoothchannelizationthatisintuitivetodriversandresultsinvehiclesnaturallyusingthe
intendedlanes;”
4. “Adequateaccommodationforthedesignvehicles;”
5. “Adesignthatmeetstheneedsofpedestriansandbicyclists;”and
6. “Appropriatesightdistanceandvisibility”(Rodegerdtsetal.,2010,pp.6‐8).
2.2.2.2EntryandExitDesign.Sinceaccessmanagementfocusesonlandusesanddrivewaysadjacenttoa
roundabout,thetwomostobviouslocationstoexamineaccessinrelationtogeometricdesignaretheentry
andtheexit.Entrywidthshouldbedesignedtoaccommodatethedesignvehiclewhileensuringadequate
deflection(Layton,2012,44).Typically,theminimumwidthforasingle‐laneentranceonastatefacility
roundaboutis14ft.Whenacurbispresentonbothsides,andthesplitterislandislongerthan33ft.,the
minimumwidthshouldbe17ft.(thecriteriaforpassingastalledvehicle).
Deflectionisdefinedas:“thechangeintrajectoryofavehicleimposedbygeometricfeaturesofthe
roadway”(Rodegerdtsetal.,2010,Glossary,p.3).Itisusuallydesignedfortheentrancetoaroundabout
andshouldsupportthedesignprinciplesofdeflectiontoslowdriversdown,althoughitcanbesignificantly
affectedbythelocationandspacingofdrivewaysbeforetheroundabout.Deflectionisanimportantaspect
ofroundaboutdesign,bothforsafetyandcapacity.Aspectsofdeflectioninroundaboutsforcethedriverto
reachtheintendedcirculatingspeedrange(usuallybetween20‐30mph),andincreasethedriver’s
awarenessoftrafficbeforeenteringtheroundabout,whileinit,andafterexitingtheroundabout.
Deflectionisoftenachievedwiththeuseofreversecurvesontheentrancetoaroundabout.Accordingto
theOregonDOT,areversecurve“shouldhavethesameoraslightlylargerradiusthantheradiusofthe
curvedpaththatavehiclewouldbeexpectedtotravelthrough.Thespeedofthecurveoftheapproach
shouldbenomorethan10mphfasterthanthemaximumnegotiationspeedthroughtheroundabout”
(Taekratok,1998,p.45).
Toslowtrafficandindicatetheupcomingpresenceofaroundabout,splitterislandsorlanemarkingsare
usedinconjunctionwithreversecurves.Ifdrivewaysorotheraccesspointsareplacedtooclosetoa
roundabout,properlevelsofdeflectioncanbeinhibited,potentiallyaffectingtheoperationofthe
roundaboutandmakingitlesssafeforusers.Toavoidthis,roundaboutsplitterislandsshouldextendback
fromtheroundaboutentryatalengthadequatetohinderdrivewayaccessmovementsthatcouldcause
safetyorqueuingconcerns.
2.2.2.3SightDistance.AccordingtoTaekratok(1998,p.52),“visibilityisanimportantconcerninthe
designofroundabouts.”Severalaspectsofsightdistanceshouldbeevaluatedtodetermineadequate
spacingdistanceandaccesstoaroundabout:SSD,decisionsightdistance,ISD,minimumaccessspacing,
andrecommendedspacing.SSDsarecalculatedbasedonapproachspeedsandotherfactors,andcanbe
foundintheHCM2010.Evaluationsaboutsightdistanceandconflictpointsaresignificantfactorsin
relationtothesafetyofaroundaboutandadjacentlanduses.SeeFigure3andFigure4,below.
Roundabouts and Access Management
Page 10
Chapter2LiteratureReview
Figure3.IntersectionSightDistance(FHWA,2006,p.18)
Figure4.StoppingSightDistance(FHWA,2006,p.19)
2.2.3ContextsofRoundabouts
2.2.3.1Single‐LaneRoundabouts.Convertingcontrolledintersectionsintoaroundabout,especially
single‐laneroundabouts,hasreceivedalotofresearchattentionbecauseofthesafetyeffects.Asan
example,Flannery,Elefteriadou,KozaandMcFadden(1998)studiedthesafetyandoperational
Roundabouts and Access Management
Page 11
Chapter2LiteratureReview
performanceoffivesingle‐laneroundaboutsthatwereconvertedfromstop‐controlledintersections.
Overall,thestudylocationsexperiencedareductionincrashfrequencies,crashrates,andcontroldelay.
TheauthorscomparedcontroldelaymeasuredinthefieldwiththedelaypredictedbySIDRA,asoftware
packagethatanalyzesat‐gradecontrolledintersectionsandroundabouts.Roundaboutdesignersshould
carefullyconsiderthenumberoflanesplannedforinclusioninaroundaboutbeforeinitiatingitsdesign,
construction,andimplementation.Studiesshowthatfewercrashesoccurinsingle‐laneroundaboutsthan
double‐laneroundabouts(Wang,OngandRakha,2013;Mahdalová,SeidlerandCihlářová,2010).However,
two‐laneroundaboutswerefoundeffectiveregardlessofthedegreeofdemand.Also,anincreaseinthe
totalnumberofcrashesoccursatthree‐laneroundaboutsthatwereconvertedfromsignalized
intersections(Mcintosh,RedingerandBagdade,2011).
2.2.3.2UrbanandRuralRoundabouts.Designingforroundaboutsinurbanareascanbechallenging
whentheimpactsofdrivewayaccessandnearbyintersectionsaretakenintoaccount.Thesizeand
geometricdesignofaroundaboutishighlydependentuponthenatureofthearea(urbanvs.suburban),
speedlimits,roadwaynature,ornumberoflanes,anditmaybecomplicatedbytheneedtoensureaccess
forotherlandusesinneighboringurbanareas(IsaacsandBarrett,2003).
Itiseasiertomakeanevaluationforaccessmanagementforruralareasforroundaboutsascomparedwith
urbanareasbecausetherearefewerspatialconstraints.Thegreaterdistancesbetweentrafficintersections
resultinlessinteractionwiththeroundaboutfromneighboringdriveways.However,becauseroadwaysin
ruralareastypicallyhavehigherspeedlimitsthanthoseinurbanareas,trafficsafetyissuesmustbe
seriouslyconsideredregardingaccessandsafety.
2.2.3.3UrbanRoundabouts.Increasedsafetyatroundaboutscomparedtocontrolledintersectionsisa
functionofreducedspeedandfewerpotentialconflictpoints(IsaacsandBarrett,2003).However,higher
crashfrequencymaybecausedbyinadequatedesignstandardsandproblematicdriverbehavior(Sacchi,
BassaniandPersaud,2011).Sacchietal.(2011)showedthatinadequategeometricdesign,particularlyan
excessiveradiusofdeflectionandalowangleofdeviationoftheenteringapproach,contributedto60%of
thecrashesintheItaliancitiesofNovaraandTrento.Anotherissueregardingthedesignandconstruction
ofurbanroundaboutsistheaccommodationofdifferenttypesofroadusers,especiallypeoplewith
disabilitiesandvisuallyimpairedpedestrians(IsaacsBarrett,2003).Whenitcomestoroundaboutsand
peoplewithdisabilities,theliteraturefocusesmoreonvisuallyimpairedpedestriansbecausethose
individualshavedifficultyinidentifyingwhenandwheretocrossaroundaboutlegduetothelackof
detectable warnings.
2.2.3.4RuralRoundabouts.Aconversiontoroundaboutusealongruraltwo‐laneroadwaysreduced
crashfrequencies,crashrates,injurycrashes,andanglecrashes(Isebrands,2009b;Isebrandsand
Hallmark,2012).Thetwostudiesdefinedruralareasas“completelyruralorlessthan2,500urban
population,notadjacenttoametroarea.”Inthefirststudy,Isebrands(2009a)studied17roundabouts,the
majorityofwhichwereconvertedfromtwo‐way‐stopcontrolled(TWSC)intersectionswithflashingyellow
orredwarninglights.Thestudyfounda52%reductionintotalcrashes,a67%reductionincrashrate,an
84%reductionininjurycrashfrequency,andan89%reductionininjurycrashrate.Especiallysignificant
isthefactthatfatalcrasheswerereducedfrom11inthebefore‐periodtononeintheafter‐period.In
addition,thefrequencyofanglecrasheswasalsoreducedby86%(Isebrands,2009b).Inanotherstudy,
IsebrandsandHallmark(2012)evaluatedthesafetyeffectivenessofconverting19intersectionsthatwere
locatedonhigh‐speedruralroadwaysintoroundabouts.Specifically,therewasa62to67%reductionin
totalcrashesandan85to87%reductionininjurycrashes.Moreover,anglecrashesweresignificantly
reducedby91%.
2.2.3.5RoundaboutsWithinaCorridor.Roundaboutsinteractwithotherstreetsaspartoflarger
corridors,oftenwithotherroundaboutsorothertrafficcontroldevicessuchassignalizedintersections.
Streetsystemsshouldbedevelopedtocirculateanddistributetraffictomanageaccessto“landusesinthe
Roundabouts and Access Management
Page 12
Chapter2LiteratureReview
areawithaminimalimpactonthemainlineandcrossroad”(Layton,2012,p.3).Forspecialevents,which
mayexceedsuitabledesign‐hourconditionsfortheroundaboutandothertrafficdevicesinthecorridor,
thedesignofaccessfacilitiestospecialeventlandusesshouldtakeintoaccountincreaseddelays,queues,
safetyimpacts,andlargerthannormalspacingstandards(Layton,2012).ProjectNCHRP03‐100
EvaluatingthePerformanceofCorridorswithRoundaboutswasrecentlycompletedonthistopic;thefinal
reportthefinalreporthasbeenacceptedandwillbepublishedintheNCHRPseries(seeTRB,2014)
2.2.4ComparingRoundaboutstoOtherTypesofIntersectionTrafficControls
Thereviewofnationalandstateguidanceonroundaboutsandaccessmanagementsuggestthatoperations
of roundabouts are similar to unsignalized intersections. HCM 2010 mentions that “[t]he operation of
roundaboutsissimilartothatoftwo‐waystop‐controlledintersections.Inroundabouts,however,entering
driversscanonlyonestreamoftraffic—thecirculatingstream—foranacceptablegap.”(TRB,2010a,p.4‐
14).Also,“roundaboutsdischargevehiclesmorerandomly,creatingsmall(butnotnecessarilyusable)gaps
in traffic at downstream locations” (p. 8‐5). These gaps are different than signalized intersections, a
characteristicsharedwithall‐waystopcontrolled(AWSC)intersections.
2.2.4.1Roundaboutsvs.Stop‐ControlledIntersections.Right‐anglecollisionsarethemostcommon
crashtypesatAWSCintersections.Roundaboutsareconsideredtobeunfavorableatlocationswheretraffic
flowonapproachlegsisunbalanced,atlocationswherespaceislimited,andatlocationsnearpersistent
bottlenecks(Vlahosetal.,2008).Whenroundaboutsareproperlylocated,theyprovidebetterperformance
(i.e.,reduceddelayandincreasedcapacity)comparedtoAWSCintersectionswithsimilartrafficvolume
andright‐of‐waylimitations(Vlahosetal.,2008,pp.88).Inaddition,totalcrashfrequencies,totalcrash
ratesandinjurycrashratesmaybereducedafterstop‐controlledintersectionsareconvertedto
roundabouts(Flannery,2001).Thesestudieswereconductedasbefore‐and‐aftersafetyevaluationsusing
video‐recordeddataforfourhoursduringthepeakperiodsateightsingle‐laneroundaboutswitha
minimumoftwoyearsofdataaftertheroundaboutswerebuilt(Flannery,2001).
2.2.4.2Roundaboutsvs.SignalizedIntersections.Manypriorstudiesagreethatconvertingsignalized
intersectionstoroundaboutsresultsinabettersafetyperformance(Saccomanno,Cunto,GuidoandVitale,
2008;Mcintoshetal.,2011;JensenandApes,2013;Gross,Lyon,PersaudandSrinivasan,2013;Uddin,
HeadrickandSullivan,2012;Wangetal.,2013;andDixonandZheng,2013).However,specificconditions
suchasgeometry,trafficvolumes,andapproachspeedarerelatedtosafetyperformance.First,theconflict
inthesignalizedintersectionisaffectedbygeometryandvolume(Saccomannoetal.,2008).Inturn,fewer
rear‐endcrashesoccuronroundaboutsthanonsignalizedintersections(Saccomannoetal.,2008).Jensen
andApes(2013)madeasimilarargumentwhentheyconcludedthatcentralislandsthataremorethantwo
m(6.6ft.)high,hadabettersafetyperformancecomparedtolowercentralislands.However,Dixonand
Zheng(2013)foundthatthewidthofthecirculatinglaneandtheradiusoftheinscribedcirclewere
insignificantinthemodels.Mostlikely,thisconclusionisduetothesimilarityofgeometricfeaturesinthe
studycomparisonofOregonroundabouts.Saccomannoetal.(2008)andGrossetal.(2013)makesimilar
arguments,andagreethatthesafetybenefitsofroundaboutconversiondeclineswithanincreaseintraffic
volumeintermsoftotalcrashes(Grossetal.,2013).Safetyimprovementswerealsodocumentedwhen
intersectionswithhighapproachspeedswereconvertedtoroundabouts(JensenandApes,2013).
Observationsshowasignificantsafetybenefitforinjurycrasheswithroundaboutconversions;evenin
caseswhereoverallcrashfrequencyincreases(i.e.somemultilaneroundabouts),thereareconsistent,
notabledecreasesinseverecrashes(Grossetal.,2013).
2.3AccessManagement
Accessmanagementisdefinedas“thesystematiccontrolofthelocation,spacing,design,andoperationof
driveways,medianopenings,interchanges,andstreetconnectionstoaroadway”(TRB,2003,pp.3).Much
Roundabouts and Access Management
Page 13
Chapter2LiteratureReview
ofaccessmanagementisachievedthroughpolicyandgovernance,unlikedesignstrategiesmandatedby
nationalguidelinesforotheraspectsoftransportationdesignandplanning.Accessmanagementishighly
context‐sensitive;however,theAccessManagementManualdoesofferguidanceandgeneralconsiderations
foruse.Thoughaccessmanagementcanoftenbethoughtofassimpleregulationofdrivewaysandaccess
ontoroadways,thetermencompassesasignificantlymorediverserangeofprinciples,particularlyinthe
contextofroundaboutdesignandplanning.Accessmanagementrepresentsatoolboxofstrategiesthat
municipalities,planners,andengineerscanemploytoprovidemobilitytousersoftheroadwaysystem
whilealsoensuringaccesstopropertiesinuse,surroundingandadjacenttotheroadway.Foraccess
management,“safety,capacity,continuity,andconnectivityoftheroadwaynetworkarekey”(Williamsand
Levinson,2008,p.26).Clearconnectionsexistbetweenaccessdesign,capacity,andsafety,sinceaccess
managementhasseveralimplicationsonsomeaspectsofroadwaysystems(WilliamsandLevinson,2008).
Accessmanagement,asappliedtotransportationplanningingeneral,enablesaccesstolanduseswhile
providingsignificantbenefitsto“motorists,bicyclists,pedestrians,transitriders,businesspeople,
governmentagencies,andcommunities”(Roseetal.,2005,p.4).AccordingtoFrawleyandEisele(2005,p.
3),accessmanagementhasthreegoals:toimprovesafetyandmobility,toprovidereasonableaccessto
developments,andtopromotelocalgovernmentpartnerships.Itcanalsobedefinedas“asetoftoolsused
tobalancetheneedsofmobilityonaroadwaywiththeneedsofaccesstoadjacentlanduses”(Frawleyand
Eisele,2005,p.2).AccordingtotheTRBAccessManagementCommittee,thetenkeyprinciplesofaccess
managementare:
 Provideaspecializedroadwaysystem
 Limitdirectaccesstomajorroadways
 Promoteintersectionhierarchy
 Locatesignalstofavorthrough‐movements
 Preservethefunctionalareaofintersectionsandinterchanges
 Limitthenumberofconflictpoints
 Separateconflictareas
 Removeturningvehiclesfromthroughtrafficlanes
 Usenon‐traversablemedianstomanageleft‐turnmovements
 Provideasupportingstreetandcirculationsystem
Accessmanagement,inthecontextofroundabouts,seekstodefinehowroundaboutsrelatetoadjacent
landuses,particularlythesupportingstreetandcirculationsystem,drivewaysandotheraccesspointsto
theroadway,andenteringandexitingtheroundabout,aswellasmovementwithinit.Sinceboththeuseof
roundaboutsandthestudyofaccessmanagementarerelativelynewintheUnitedStatesatboththe
nationalandstatelevels,littleliteratureexistsregardingtheapplicationofaccessmanagementto
roundaboutdesignandplanning.
2.3.1AccessManagementElements
Eventhoughgeometricdesignelementsdonotregulateaccessmanagementdirectly,theygreatlyinfluence
theoperationofandaccesstotheroundaboutforusersandneighboringlandusesandplayasignificant
roleinthespacingofdrivewaysandnearbyintersections.AsseeninFigure8,thedistancebetween
drivewaysaffectsthenumberofconflictpointsforpotentialvehiclecollisions.
2.3.2SpacingStandardsandRoadwayClassifications
AccordingtotheAccessManagementGuidebook,NCHRPReport548(Roseetal.,2005,p.39),higher
functionroadscommonlyhavefeweraccessopportunities.Similarly,localstreetsmaximizeaccessto
residenceswhilesupportinglessthroughtraffic.However,abasicprincipletodeterminetheaccesslevelis
Roundabouts and Access Management
Page 14
Chapter2LiteratureReview
theproposedfunctionoftheroadways.TheAccessManagementGuidebookalsoshowsthat,asthe
proportionofthroughtrafficincreases,accessdecreases.Forexample,freewayshaveverylimited
controlledaccesswhilelocalstreetsprovidefullaccess.
Figure5.AccessandRoadClassification
TheAccessManagementGuidebook(Roseetal.,2005)proposesroadwayclassificationdefinitionsbased
oncharacteristics(Roseetal.,2005,p.49)suchasfunctionalclassification,traveldistanceofmotorists
(e.g.,shortvs.longtrips),natureofthetravel(e.g.,throughvs.local),travelspeeds,landuse,locationofthe
roadwayfacility(e.g.,urbanvs.rural),andphysicalcharacteristicsoftheroadway(e.g.,dividedvs.
undivided).Inadditiontothesecharacteristics,theplanninganddesignelementsincludedintheaccess
managementforeachroadwayclassificationarethefollowing:
 Permittedandprohibitedaccesslocations;
 Drivewaydesignandspacing;
 Cornerclearance;
 Medianopeningdesignandspacing;
 Signallocation,spacing,andcoordination;
 Turn‐lanelocationanddesign;
 Auxiliary‐lanelocationanddesign;and
 Service/frontageroadlocationanddesign.
Inaddition,accordingtoDemosthenes(2007),roadwaydesignandtrafficoperationsintersectwithaccess
managementandlandusedesign(seeFigure6).
Roundabouts and Access Management
Page 15
Chapter2LiteratureReview
Figure6.RelationshipbetweenAccessManagement,RoadwayDesign,TrafficOperationsandLandUse
(Roseetal.,2005)
2.3.3AccessManagementMechanismsandIntersectionControls
AccordingtotheAccessManagementGuidebook,NCHRPReport548(Roseetal.,2005),themostreliable
methodsofaccessmanagementforgeneralhighwaymanagementintersectioncontrolsinclude:acquisition
ofaccessrights;accessmanagementregulations;policies,directives,andguidelines;landdevelopment
regulations;geometricdesign;anddevelopmentreview/impactassessments(Roseetal.,2005,pp.8‐10).
2.3.3.1AcquisitionofAccessRights.Localmunicipalitiescanacquirerightstopropertiesthatadjoinor
areadjacenttoroundaboutstomaintainaccess.Ifthelocationofaroundaboutwouldblockaccesstoa
neighboringproperty,sometimesthemunicipalitymaypurchasethepropertyandprovidefinancingto
helptheownerrelocatetoanalternatelocationwithadequateaccess(Roseetal.,2005).Inother
circumstances,however,drivewaysmayremainincloseproximitytoaroundabout,oreveninthemiddle
ofaroundabout,asseeninsomeroundaboutsinWisconsin(M.Johnson,Personalcommunication,
February7,2013).
2.3.3.2AccessManagementRegulations.Mostmunicipalitiesincludetransportationdesignpolicy
regulationsaspartofaccessmanagementstandards.Theseareoftenbaseduponnationalandstate
standards,althoughtheycanvoluntarilygointofurtherdetailtoaddressissuesofcontextoroflocal
transportationpatterns.Thesearecommonfortraditionalstop‐controlledandsignalizedintersections,and
arebecomingincreasinglypopulartoaddressroundaboutdesignandplanningissueswithinalocality.
Agencieswhichfrequentlyuseroundaboutsgenerallyhaveinternalconsensusaboutthetypesofcontexts
inwhichroundaboutsareappropriateandwheretomanageaccess(P.Demosthenes,Personal
communications,March14,2013).
2.3.3.3Policies,Directives,andGuidelines.Comprehensiveplanningandzoningdesignationsshould
recognizetheroleofcontextsensitivetransportationfacilities,whichmayincludeincorporatingminimum
spacingstandards,andaddressanyuniquecharacteristicsofthespecificroundaboutinpolicies.The
relevantlocalgovernmentoragencyshoulddesignatetheappropriatelandusecontrolsand
comprehensiveplanningguidelines,becausenationalpolicyalwaysincludesexemptions(P.Demosthenes,
Personalcommunications,March14,2013).
Roundabouts and Access Management
Page 16
Chapter2LiteratureReview
Comprehensiveplansshouldincluderegulationsanddesignguidelinesforaccessmanagementoftraffic
controldevices.Whenconsideringfutureexpansionofcertaincorridors,alternativetrafficcontroldesigns
suchasinterchangesorroundaboutsrequiremoreplanninganddesignconsiderationsthanacorridorthat
ofonlysignalizedintersections(Layton,2012).
Thephysicalexpansionofintersectionsshouldbeexaminedincomprehensiveplans,specificallythe
numberoftravellanes,auxiliarylanes,high‐occupancyvehiclelanes,transitways,modificationstoexisting
interchanges,andplannednewinterchanges.Eachoftheseprojectedchangesrequiresadditionalright‐of‐
wayconsiderationsforthemunicipality.Inthesecases,Layton(2012,p.4)arguesthatthemunicipality
shouldinsurepropertyforexpansion,notingthatprotectivebuyingmaybemorecost‐effectivethan
purchasingthepropertyinthefuture.
2.3.3.4GeometricDesign.Geometricdesignforroundaboutsshouldacknowledgetheneedfor
roundabouttraffictobedistributedtoavoidatrafficqueueintheroundabout,andensureaccessto
neighboringproperties.Inlocaltrafficdesignregulationsandpolicies,designguidelinesshouldbeincluded
thatensurebothmobilityandaccesstoneighboringproperties(Schroeder,2011).
Evaluationofthelanduseandgeographiccontextsoftheroundaboutisakey.Theoptimumspacing
betweenurbanroundaboutswithinadowntownurbancorecoulddifferfromthatofruralroundaboutson
countyroads.Minimumspacingandgeometricdesignoftheroundaboutmustallowforweavingdistance
andaqueuelengthsetatacomfortableoperatingcondition(Layton,2012,p.5).
2.3.3.5SightDistance.Themostpertinentguidelinesforsightdistancerelatingtoaccessmanagementare
thoseoftheexternalapproachexitandthecirculatingroadway.Theexternalapproachsightdistanceisthe
distanceadriverhastotravelfromthemomentofapproachingtheyieldlineoftheroundaboutentranceto
anyentrancepath.AccordingtoTaekratok(1998),“adriverwhoisapproachingtheyieldlineshouldhave
aclearlineofsighttoapproachingtrafficenteringtheroundaboutfromanapproachimmediatelytothe
left,foratleastadistancerepresentingthetraveltimeequaltothecriticalgap.Aminimumdistanceis70m
(230ft.)”(1998,p.38).
Driversenteringtheroadwayfromadrivewayoraccesspointshouldbeabletoseevehiclesupstreamon
theroadwaytoensureasafeturn.Forinstance,thespacingandlocationofthedrivewayclosesttothe
roundaboutshouldenableadriverexitingthatdrivewaytobeabletoturnontotheroadwaywithaclear
viewofvehiclesapproachingandexitingtheroundabout.Thisappliestodrivewayaccesspointsforboth
theenteringandexitingsidesoftheroundabout.
Whilethepreviousexampletakesintoaccountlocationandsightdistancewithnoqueue,theeffectsof
queuesmustalsobeconsideredwithregardtosightdistance.Anexaminationofstoppingdistanceand
queuelengthshouldbeconsideredwhendeterminingminimumspacingbetweenadrivewayaccesspoint
andanintersection(Layton,2012).
2.3.3.6DevelopmentReview/ImpactAssessments.Oneofthemostimportantwaysaccessmanagement
canbecontrolledwithinamunicipalityisinthedevelopmentreviewprocess.Evenifaroundaboutdesign
claimstofollowaccessmanagementprinciples,itistheresponsibilityofthemunicipalorregionaltraffic
engineertoreviewthedesignandpoliciestoensurethedesigndoesachievethestatedgoalsandensures
accesstoneighboringlanduses.
2.3.3.7ImplementingMechanisms.Agenciesneedtoworktogetheracrosstheboardtoimplement
accessmanagementmechanisms.Theseentitiesincludestateagencies,statelegislatures,metropolitan
planningorganizations(MPOs),regionalplanningagencies,localplanningagencies,andlocalelected
officials.Roseetal.(2005)identifyaccessmanagementimplementingmechanisms,classifiedbyauthority,
Roundabouts and Access Management
Page 17
Chapter2LiteratureReview
agencypolicy,accessmanagement,advocacy,managementaccountability,projectprogramming,and
projectdevelopment,andtheimplementingagency.Collectively,theirworkreinforcestheimportanceof
thewiderangeofstateandlocalpoliciesandguidanceonaccesscontrol,landuseandsiteplanreview,
drivewayandotherpermittingstandardsandprocesses,fundingforcorridorpreservation,design
standards,andarea‐wideandcorridoraccessmanagementplans.
2.3.4ImpactofRoundaboutsonAccessManagement
Forthemostpart,thesmallbodyofexistingliteratureonaccessmanagementandroundaboutssuggests
theymayhaveperformancecharacteristicssuperiortosignalizedintersections.Roundaboutsenhancethe
achievementofaccessmanagementgoalsinmultipleways:maintainingthecontinuityoftheroadway’s
flow,improvingsafety,reducingcongestion,balancingmobilityandaccess,andbyextendingthelifeof
infrastructure(personalcommunications,MarkJohnson,February7,2013).Thedifferingoperational
characteristicsofroundaboutscanprovideversatilityandflexibilityintheapplicationofaccess
managementtechniques:lessqueuing,slowerspeedsatentryandexit,consistentspeeds,reducedspeed
differentials,geometricflexibility,anddriveway/intersectionspacingflexibility.Insomecases,
roundaboutsmayalsoprovideincreasedcapacityatintersections,reducingtheneedtoexpandentire
roadways.Physicalgeometriccharacteristicsofroundaboutscanalsoalteraccessmanagementpatterns,
changingthesideofstreetanddrivewayaccessspacingneedsandrequirements.Often,drivewayaccess
andspacingcanbeeasiertoplanbecauseoflessqueuing,slowerspeeds,andeasierdecisionmaking.
Inresponsetothescarcityofliteratureonthetopic,KansasStateUniversity(KSU)studiedtheimpactof
roundaboutinstallationonbusinessaccess.Russell,LandmanandGodavarthy(2012)concludethatthe
operationalcharacteristicsofroundaboutsallowbusinessestobelocatedmuchclosertointersectionsthan
dotraditional,signalizedintersections(Russelletal.,2012,p.16).Intraditional,signalizedintersections,
queuedtrafficatredlightsforthroughtrafficandturnmaneuverscanblockaccesstobusinesses.With
properaccessmanagementofroundaboutandflowingtraffic,“roundaboutscanbedesignedwitha
commercialorbusinessentrancedirectlyofftheroundabout”(Russelletal.,2012,p.16).Johnsonand
Isebrands(2008),reachthesameconclusionsasRusselletal.(2012),thattheoperationalcharacteristics
ofroundaboutsprovide“lowdelayandimprovedsafety,providesexcellentmobility,ingress,andegress
throughequalopportunityforlefts,throughmovements,andU‐turns”(JohnsonandIsebrands,[2008]as
citedinRusselletal.,2012,p.16).
2.3.4.1BusinessAccess.Inseveralcases,roundaboutshaveincreasedaccesstobusinesses.Inthe
previouslymentionedstudy,Russelletal.,(2012)foundthat76.9%ofbusinessesinTopeka,Kansas
classifiedtheimpactoftheroundaboutsasfair,good,orverygood(Russelletal.,2012,p.vi).Inadditionto
interviewswithTopekabusinessowners,simulationstudiesoftheroundaboutinstallationdepicted
significantreductionsindelayandqueuingforalltrafficmovements.Intheirstudy,Russelletal.,(2012)
referredtoseveralbusinessownerswhosaidtheyowedtheirsuccesstotheconstructionofthe
roundabout.Priortotheroundabout,heavytrafficandqueueshadbeendiscouragingpeoplefrommaking
leftturnsinandoutofbusinesses.However,aftertheroundaboutwasinstalled,trafficdelaywasreduced
anddriverswereabletomakeleftturnsmoreeasilyandaccesstheadjacentbusinessesmorefrequently
(Russelletal.,2012,p.7).
InGolden,Colorado,theintroductionofaseriesofroundaboutsprovedmoreefficientinmanagingtraffic
flowandcreatedacorridorthatslowedtrafficandallowedpedestrianstoaccessmanybusinessesalong
thecorridor(Ariniello,2004).MarkLenters,presidentofOurstonRoundaboutEngineering,found
roundaboutshadapositiveinfluenceonbusinessaccessinanumberoflocations,including(Lenters,n.d.):
LinvilleRoadinBrownCounty,Wisconsin;SouthGoldenRoadinGolden,Colorado;LeeRoadinBrighton,
Michigan;numerousintersectioninCarmel,Indiana;VailInterchangesinVail,Colorado;RockyMountain
AvenueinLoveland,Colorado;andAvonRoad;Avon,Colorado.
Roundabouts and Access Management
Page 18
Chapter2LiteratureReview
However,roundaboutconstruction,likeallintersectionconstruction,isnotoriousforinhibitingaccessto
adjacentpropertiesandbusinessesduringthatperiod.Decreasedaccessduringthistimecancontributeto
negativeconnotationsofroundabouts,eventhoughaccesswillreturntonormalorevenimproveonce
constructioniscompleted.
2.3.4.2AccessPoints.Severalstudiesfindthatroundaboutsaresuccessfulwhenthe“reorganization”of
accesspointsispartoftheroundaboutdesignandengineeringprocess.Theaforementionedcasestudy
fromGolden,Coloradoinvolvedacorridorthatwasdescribedasbeingan“unpleasanttravelcorridor”with
wideroads,poorsafetyconditions,acenterturnlane,and“numerousunorganizedaccesspoints”(Russell
etal.,2012,p.9).Inevaluatingdifferentoptions,thecityfavoredtheroundaboutselectionbecauseit
“wouldprovidebetteraccessoptionsandbetterpedestrianaccess”thantraditionaltrafficsignals(Russell
etal.,2012,p.10).Aftertheconstructionoffourroundaboutsinplaceofsignalizedintersectionsandafter
makingsignificantstreetscapeimprovements,thecorridorwascitedasa“vibrantcommunitycorridor,”
with“improvedbusinessaccess,”includingbetterpedestrianaccesstobusinesses,improvedsafety,anda
6%increaseinretailsalestaxrevenue(Russelletal.,2012,p.10).Adescriptionofthecorridorandits
characteristicsispresentedbelow:
SouthGoldenRoadisatypicalsuburbanstripcommercialcorridor.Theinstallationoffour
roundaboutswithinthishalf‐milelongarterialhasresultedinslowerspeeds,butlowertraveltimes
andlessdelayatbusinessaccesspoints.…[S]alestaxrevenueshaveincreased60%since
installationoftheroundabouts,and75,000squarefeetofretail/officespacehasbeenbuilt.In
Golden,Colorado,businesseshavesaid,“Yes,roundaboutsaregoodforbusiness.”(Ariniello,2004
inRusselletal.,2012,p.12).
2.4OperationalEffectsofRoundabouts
Ingeneral,operationalaspectsofroundaboutscanbeassessedintermsofcapacityandthelevelofservice
(LOS),whichcombinesseveralmeasuresofeffectivenesssuchasdelayandqueuelength.Thefollowing
designaspectshaveanimpactontheoperationsofroundabouts:geometricdesignofroundabouts;traffic
flowanddriverbehavior;placementofdrivewaysnearroundabouts;andseriesofroundabouts.
2.4.1EffectofTrafficFlowandDriverBehavior
Thecapacityofaroundaboutentrydecreasesastheconflictingflowincreases(Rodegerdtsetal.,2010).In
capacitymodelspecifications,thecapacityofaroundaboutdecreasesfromthemaximumentryflowrate
perhourwiththeincreaseofthevehicleconflictrate.Additionally,avarietyofconditionsexistinreal‐
worldsituationsthatmightaffecttheaccuracyofagivenmodelingtechnique.Rodegerdtsetal.(2010)
summarizetheseconditionsasfollows:
 Effectofexitingvehicles.Exitingflowattheimmediatelyupstreamlegcanaffectadriver’sdecision
onwhetherornottoentertheroundabout.
 Changesineffectivepriority.Whentheenteringflowandcirculatingflowvolumesarebothhigh,a
circulatingvehiclemightadjustsitsheadwaytoallowentering,andagap‐acceptancemodelmay
notgivereliableresults.
 Capacityconstraint.Thismayoccurwhenanapproachoperatesovercapacity.Duringthis
condition,theactualcirculatingflowislessthanthedemandresultingfromtheover‐saturated
approach.Thereductioninactualcirculatingflowmaythereforedecreasethecapacityoftheother
affectedentries.
 Origin‐destinationpatterns.Thiscouldcauseanunbalancedflowataroundaboutwithcertain
approachesoperatingovercapacity.
Roundabouts and Access Management
Page 19
Chapter2LiteratureReview
2.4.2EffectofGeometry
Geometriccharacteristicsgreatlyaffecttheoperationofroundabouts.Roundaboutsarenormallysaferif
theyaredesignedtoforcevehiclestoreducetheirspeedwhenenteringthecirculatoryroadway.Onthe
otherhand,lowspeedsdecreaseroundaboutcapacity.Therefore,geometricdesignshouldbebalanced
betweensafetyandoperationalrequirements(Rodegerdtsetal.,2010).Generally,theoperational
performanceofaroundaboutisdeterminedbyitsgeometricdesign,alongwiththetrafficvolumeusingthe
roundaboutatagiventime.
Geometricelementsthatinfluenceoperationsincludeentrycurvesandwidth,circlediameter,circular
roadwaywidth,exitcurves,centralandsplitterislands,stoppingandISD,bicycleprovisions,sidewalk
treatments,parkingconsiderations,busstoplocations,andright‐turnbypasslanes(Rodegerdtsetal.,
2010).Manyoftheaforementionedgeometricparametersdependonthedesignvehicleandthe
accommodationofheavyvehicles,bicyclesandpedestrians.However,allareessential,andsmallchanges
toevenonecouldresultinsignificantchangestotheoverallroundaboutoperationperformance.Geometry
alsodictatesthenumberoflanesthatarerequiredtofacilitatethetrafficdemandandaffectsdrivers’
perceptionoftraveltime,theirenteringandcirculatingspeed,andthegapbetweenvehicles.
2.4.3OperationalAnalysisofRoundabout
AccordingtotheHCM,thecapacityofafacilitycanbedefinedas“themaximumsustainablehourlyflow
rateatwhichpersonsorvehiclesreasonablycanbeexpectedtotraverseapointorauniformsectionofa
laneorroadwayduringagiventimeperiodunderprevailingroadway,trafficandcontrolconditions.”
(TRB,2010a,p.4‐1).
TheHCMdefinesspecificperformancemeasure(s)foreachhighwayfacilitytype.Controldelayisusedto
definethelevelofservice(LOS)atalltypesofintersectionsincludingroundaboutsandsignalizedand
unsignalized.Anotherperformancemeasureisgeometricdelay,i.e.,theadditionaldelaycausedbythe
intersectiongeometry.Forroundabouts,thisadditionaldelayisexperiencedwhendriversslowdownto
negotiatetheroundabouts’curvature(TRB,2010a).Otherrelevantperformancemeasurementsinclude
degreeofsaturationandqueuelength.
Besidesroundaboutperformancemeasures,afewfeaturesarecommontothemodelingtechniquesto
calculatecapacitythatisincorporatedintoallanalysistools(Rodegerdtsetal.,2010).Modern
roundaboutsuseyieldcontrolatapproachlanesanddriversmustyieldtheright‐of‐waytocirculating
vehiclesandacceptgapsinthecirculatingtrafficstream.Therefore,theoperationalperformanceofa
roundaboutisdirectlyinfluencedbytrafficpatternsandgapacceptancecharacteristics.Also,the
operationalperformanceofroundaboutsisinfluencedbytheirgeometricfeatures(Rodegerdtsetal.,2010,
pp.4‐3to4‐4).
Onewaytoconstructaroundaboutcapacitymodelisthroughempiricalmodeling,whichusesstatistical
methodologytomodelcapacitybasedonobserveddata(Al‐MasaeidandFaddah,1997;PolusandShmueli,
1997;Wei,GrenardandShah,2011).Typicallyaresearchprocessforcreatinganempiricalroundabout
capacitymodelistouseregressiontofindtherelationshipbetweenvolumeperhourandthegeometric
characteristicsofaroundabout.
Mostoftheliteraturerelatedtoroundaboutcapacitymodelsconsistsofdescriptionsofanalyticalmethods
andtypesofmeasurement.Theanalyticalmodelisprimarilybasedondriverbehavior,measuredingap
acceptance(Fisk,1991;Akçelik,ChungandBesley,1997;Al‐Masaeid,1999;FlanneryandDatta,1997;
Polus,LazarandLivneh,2003;Hagring,Rouphail,andSorenson,2003).
Roundabouts and Access Management
Page 20
Chapter2LiteratureReview
2.4.3.1GapAcceptanceintheRoundaboutandCapacityModel.Akçeliketal.(1997)presenteda
methodforestimatingthecapacityandperformanceofroundaboutentrylanes.Thismethodisbasedon
modelingthegapacceptanceprocessundertheadjustmentofthecharacteristicsoftheapproachflows.
Theauthoralsopresentedacasestudythatisanapplicationofthemethod.Themodelinthispaper
combinedtheconceptofoverflowqueueandsignalanalogytoanalyzethecapacityandperformanceso
thatitisagoodfitforheavyandunbalanceddemandcasesinreallife(Akçeliketal.,1997).
2.4.3.2ComparisonbetweenDifferentModelsandApproachesforCapacityMeasurement.
Roundaboutcapacitycanbemodeledbasedontwotypesofapproaches.Lane‐basedmodelsmeasureand
predictroundaboutcapacitylanebylane,andcanbeextremelyusefulinthecaseofmulti‐lane
roundaboutswithdifferentlanecapacities.Incontrast,approach‐basedmodelscombinetheentrylanesas
ananalytical“lanegroup.”AstudybyHagringetal.(2003)showedthatalane‐basedmodelisbetterthan
theapproach‐basedmodelincomparingobservedheadways.Theyfoundthecriticalgapsfortheleftand
rightentrylanesweredifferentandtypicallylargerfortheleftlanes.However,forthecirculatinglanes,the
criticalgapswerefoundtobesimilar.Akçelik(2011)concludedthattheHCM2010modelisauniquelane‐
basedmodelandifcalibratedwithdriverbehavior,couldbeaveryaccuratemodelforcapacityanalysis.
Akçelik’sstudyalsoshowsthattheuseofVISSIMandSIDRAyieldedsimilarresultsforcontroldelayand
queuelength.However,otherstudiesshowthatVISSIMpredictedlargerdelayvaluesthanSIDRA(Yinand
Qui,2011).
2.4.4RoundaboutCapacityunderDifferentConditions
Variousresearchershavestudiedthecapacitymodelforroundaboutsunderdifferentcircumstances.In
thisresearch,thecontextusuallyaddressestheimportanceofthenumberoflanescirculatingandentering
theroundabout,thepresenceofsliplanes,thespecificshapeofroundabouts(e.g.,turbo),andthe
approachingflowintotheroundabout.
2.4.4.1UnconventionalRoundaboutCapacity.Roundaboutswithtwoormoreentrylanescanalsohave
differentcapacity.Lindenmann(2006)concludedthatasmallroundaboutwithtwo‐laneentriesanda
single‐lanecirculatingroadwayhasacapacitymorethan20%greaterthanthosewithone‐laneentries.
SisiopikuandOh(2001)determinedthatatwo‐laneroundaboutisthebestdesignforintersectionswith
highthroughandleft‐turningtraffic.Theirstudyalsoconcludedthatroundaboutscouldhaveahigher
capacitythansignalizedintersections(SisiopikuandOh,2001).Anothertypeofconventionalroundabout
isaturboroundaboutwhichisatypeofmodernroundaboutwithspiralroadmarkings,designatedlanes,
andraisedlanedividers.Thereforecapacityforturboroundaboutscanalsobedifferent.
2.4.4.2RoundaboutswithUnbalancedFlow.Unbalancedtrafficoccurswhereoneapproachvolume
dominatestheotherapproachvolume,orthereisasignificantdifferencebetweenapproachvolumes.The
capacitymodelofroundaboutswithunbalancedflowconditionswasstudiedandresultsshowedthatthose
withunbalancedflowconditionsweresignificantlydifferentfromotherroundabouts(Akçelik,2004;
SisiopikuandOh,2001;Valdez,CheuandDuran,2011).SisiopikuandOh(2001)foundthatfroman
operationalperspective,unbalancedtrafficpatternsinroundaboutscouldsometimescarryhighervolumes
thantraditionalintersections.
2.4.4.3RoundaboutCapacitywithSlipLanes.Asliplaneinaroundaboutfacilitatesright‐turningtraffic
toreducedelayandincreasecapacityandsafety.Threetypesofsliplanesareincorporatedinto
roundaboutdesigns:free‐flowsliplanes,yield‐controlsliplanesandstop‐controlsliplanes.Al‐Ghandour,
etal.(2012)believedthatallsliplanetypescouldreduceaveragedelayinasingle‐laneroundaboutand
thatafree‐flowstylesliplaneperformsthebest.Theresultsofthesestudiesshowedthattheaveragedelay
isexponentiallyrelatedtosliplanevolumes.Allthreetypesofsliplaneshaveasignificantpositiveeffect
Roundabouts and Access Management
Page 21
Chapter2LiteratureReview
oncapacity,withthefree‐flowsliplanehavingthemostpositiveeffect,followedbyyieldandstop‐control
sliplanes.Howeverwhenpedestrianshavepriority,afree‐flowsliplanecanincreaseroundaboutdelayby
fivetimesifthepedestrianvolumeandright‐turnvolumearebothhigh(Al‐Ghandouretal.,2012).
2.4.4.4Roundaboutsinseriesofsignalizedintersections.Thecapacityofroundaboutscanbe
dramaticallyaffectedbylocation,aswellasthetrafficprogressionbeforeandaftertheroundabout.Several
studiesexaminetheimpactoncapacitythatroundaboutshaveonaseriesofsignalizedintersections.Bared
andEdara(2005)foundthatifaroundaboutiswithinone‐quartermileofasignalizedintersection,it
resultsindelayscomparabletoafullysignalizedarterial.Hallmark,Fitzsimmons,Isebrands,andGiese
(2010)foundthattheuseofroundaboutsinasignalizedcorridordidnotappeartoadverselyaffecttraffic
floworoperations.
2.4.5SummaryofRoundaboutOperationLiteratureReview
Rodegerdtsetal.(2010)summarizedhowtoconductroundaboutoperationalanalysesasfollows:
 Datacollectionandprocessing.Trafficdatacanbecollectedwithliverecordingsofturning
movementsinroundabouts,trafficflowinintersections,andorigin‐destinationpatterns.Field
observationisnecessaryformeasuringsomeoftheoperationalperformancemeasuressuchas
controldelay(Rodegerdtsetal.,2010).Dataprocessingincludesdeterminingroundaboutflow
ratesbyconvertingturn‐movementvolumestoroundaboutvolumesandadjustingforheavy
vehicles.
 Determinestudymethodsandtools.Avarietyofmethodologiesareavailableforstudying
roundaboutsdependinguponthestageinthedevelopmentoftheroundabout.Intheearlierstages
ofanalysis,suchasplanning‐levelsizing,andpreliminarydesign,thepractitionerwilluse
deterministicsoftwareortheHCM.Inlaterstages,suchastheanalysisoftheimpactofthe
roundaboutonspecialusers,suchaspedestrians,oronthetransportationsystemandfor
communicatingtothepublic,simulationtoolsbecomemoreimportant.Thedecisiononwhich
methodtouseisbasedontherequiredoutputandtheavailabledata.Rodegerdtsetal.(2010)
presentedatable(seeTable1)specifyingthemethodselectionstandard.
Roundabouts and Access Management
Page 22
Chapter2LiteratureReview
Table1.SelectionofAnalysisTool(Rodegerdtsetal.,2010)
2.5RoundaboutsandSafety
SafetyisoneoftheprimaryreasonsfortheincreaseduseofroundaboutsintheUnitedStatesandaround
theworld.Thevolumeofliteratureonroundaboutsafetyisquiteextensivecomparedwiththeavailable
literatureonroundaboutcapacityandaccessmanagement.NCHRPReport674CrossingSolutionsat
RoundaboutsandChannelizedTurnLanesforPedestrianswithVisionDisabilitieshighlightstheclose
relationshipbetweensafetyandaccessibility,particularlyinthecaseofroundabouts(Schroederetal.,
2011).AccordingtoSchroederetal.(2011),“afacilitycouldbeconsideredsafeifthecrashrateatthe
facilityislow.”Consequently,crashrateisthemostfrequentlyusedmeasuretoestimatesafetyintraffic
engineeringingeneral,andforroundaboutsaswell;however,theuseofthecrashcanbeachallenge
becausethecrashrateisseldomalinearrelationship.
Theliteraturethatexploressafetyasitpertainstomodernroundaboutsplacesemphasisondifferent
areas:safetyeffectiveness,safetyofvehiclesandvulnerableusers(i.e.,bicyclistsandpedestrians),
comparisonofthesafetyperformanceofroundaboutswithothercontrolledintersections,andother
factorsrelatedtodriversafety.Crashratesbasedonbefore‐and‐afterorcross‐sectionalstudiesareoften
usedtoevaluatesafetyatroundabouts.Duetothelackofexposuredata,thesafetyofvulnerableroadusers
isoftenestimatedusingdirectobservation.Despitedifferentviewsaboutsafetyandaccessibilityat
roundabouts,mostoftheliteratureconfirmsthatmodernroundaboutshavesignificantsafetybenefitsfor
alltypesofroadusers.
TheFHWASafetywebsiteonroundaboutshasconsiderableinformationregardingroundaboutsafety,
includingseveralreportsandmanualsontheapplicationofbestsafetypracticesinroundaboutdesignand
planning.Themostcommonlyusedsafetyguidebooksinclude:
Roundabouts and Access Management
Page 23
Chapter2LiteratureReview





Roundabouts,AnInformationalGuide(Robinsonetal.,2000)
PedestrianAccesstoModernRoundabouts:DesignandOperationalIssuesforPedestrianswhoare
Blind(USAB,2006)
NCHRPReport672:Roundabouts:AnInformationalGuide,2ndEdition(Rodegerdtsetal.,2010).
NCHRPReport572:RoundaboutsintheUnitedStates(Rodegerdtsetal.,2007).
NCHRPReport674:CrossingSolutionsatRoundaboutsandChannelizedTurnLanesforPedestrians
withVisionDisabilities(Schroederetal.,2011)
2.5.1OverallSafetyEffectsoftheRoundabouts
Inthepastresearchershavestudiedthesafetyperformanceofroundaboutsandcomparedthefindings
withothertraffic‐controlledintersections,suchasstop‐controlledintersections,andsignalized
intersections.Mostresearchersusecross‐sectionalstudiesthatcomparetheroundaboutseitherwith
previousmeansofintersectioncontrolorwiththosemeansoftrafficcontrolwithinanareaclosetothe
roundabouts.Safetyperformancemeasuresorindicatorscommonlyusedarecrashfrequency,crashrate,
crashseverity,andcrashtype(Isebrands,2009b).Specifically,differentlocationswithintheroundabout
mayaffectthesafetyperformanceofroundabout.AccordingtoArndtandTroutbeck(1998),crashescanbe
categorizedassingle‐vehicleandmultiple‐vehiclecrashes.Formultiple‐vehiclecrashes,thefollowing
characteristicsareincluded:wherethecrashoccurred;whetherthevehiclewasentering/circulatingthe
roundabout;exiting/circulatingtheroundabout;whetheritwasitasideswipecrash;andotherlow
frequencytypesofcrashes.Thelocationsincludedepartureleg,exitpoint,approachingrearend,
entering/circulatingcrash,entrypoint,andsideswipecrashes.Figure7illustratesthelocationsofthe
typesofcrashesinroundabouts.
Figure7.CrashTypesonaTypicalRoundabout(ArndtandTroutbeck,1998,p.28‐3)
Previousstudiesfoundthemagnitudeofsafetyeffectsrangedfroma17to70%reductioninthenumberof
crashes.FlanneryandDatta(1996)foundanaverageofa60‐70%reductionincrashfrequencyforthe
Roundabouts and Access Management
Page 24
Chapter2LiteratureReview
safetyeffectof13roundaboutsinthreestates:Maryland,Florida,andNevada.Retting,Persaud,Garder,
andLord(2001)foundthatachangeto24roundaboutinstallationsfrom20stop‐controlledintersections
andfoursignalizedintersectionsledtoa38%reductionintotalcrashfrequencyanda76%reductionin
injuryseverity.Similarly,Persaudetal.(2001)foundasafetyeffectforroundaboutsthatledtoa40%
reductionintotalcrashfrequencyandan80%reductionininjuryseverity.Isebrands(2009b)foundthat
roundaboutsreduceinjurycrashfrequencyandinjurycrashrateby84%and89%,respectively.She
(Isebrands,2009b)alsofoundthatroundaboutsreducedtotalcrashfrequencyandtotalcrashrateby52%
and67%,respectively.DeBrabander,Nuyts,andVereeck(2005)evaluatedthecrashfrequencyfor95
roundaboutsand119comparableintersectionsinFlanders,Belgiumandfounda34%reductioninthe
numberofinjurycrashes.Similarly,inanotherstudy,DeBrabanderandVereeck(2007)foundthat
roundaboutsresultedina39%reductionininjurycrashes,a17%reductioninseriousinjurycrashes,anda
38%reductioninminorinjurycrashes.Churchill,Stipdonk,andBijleveld(2010)concludedthat
roundaboutsreducedthenumberoffatalandseriousinjurycrashesby76%and46%respectively.Elvik
(2003)foundconversionfromanintersectiontoaroundaboutresultedina30‐50%reductioninthetotal
crashrate.Thefatalcrashratewasreducedby50‐70%.
Despitethesegenerallypositiveresults,notallconversionofroundaboutssignificantlyreducesthenumber
ofcrashoccurrences.Forexample,Rodegerdts(2007)concludesthattheconversionfromfour‐waystop
controlled(FWSC)intersectionstothemodernroundaboutsdonotappreciablyreducethetotalandinjury
crashrates.Thisstudyalsohighlightsdesignfeatures,suchasthenumberoflanes,whichwerefoundto
performbetterthanmulti‐laneroundabouts,whicharemoresensitivetosuchcharacteristics.Theresult
mayalsobedependentontheprevioustrafficcontroltype,priortoroundaboutconstruction,andthe
numberofapproachlegs(Elvik,2003).Furthermore,placementrequirementsshouldbeconsidered
beforeroundaboutconversion.Forexample,roundaboutsareconsideredunfavorableforlocationswhen
trafficflowonapproachlegsisunbalanced,atlocationswheregeometryislimited,andatlocationsneara
persistentbottleneck(Vlahosetal.,2008).
Incontrasttotheeffectsofroundaboutsonsingleormultipleautomobilecrashes,priorstudiesmake
variousargumentsregardingcrashesinvolvingvulnerableusers,i.e.pedestriansandbicyclists.First,the
argumentisthatroundaboutinstallationsreducesafetyforvulnerableusers(DeBrabanderandVereeck,
2007;Danielsetal.,2008).Intheirmeta‐analysisstudy,DeBrabanderandVereeck(2007)foundthat
crashesinvolvingvulnerableroadusersincreasedbyabout28%.Moreover,Danielsetal.(2008)concluded
thatinbuilt‐upareas,crashesinvolvingbicyclistsincreasedby48%.Inbuilt‐upareas,bicycle‐vehicle
crashesatroundaboutsthatwereconvertedfromstop‐controlledandsignalizedintersectionsincreasedby
55%and23%,respectively.Outsidebuilt‐upareas,thechangeinbicycle‐vehiclecrashesbeforeandafter
roundaboutconstructionwasstatisticallyinsignificant.AstudyinSwedenreachedseveralconclusions
relatedtocrashesinvolvingbicyclistsandpedestrians:(1)single‐laneroundaboutsaremuchsaferfor
bicyclistsandpedestriansthanformultilaneroundabouts;(2)forpedestrians,roundaboutsarenolesssafe
thanconventionalintersections;(3)issaferforbicyclisttobypassaroundaboutonabicyclecrossingthan
totravelonacarriageway;and(4)fewercyclistcrashesoccurwhenthecentralislandisgreaterthan10m
(33ft.)andwhenbicyclecrossingsareprovided(Rodegerdtset.al,2006).Otherresearcharguesthatno
significantproblemswerefoundforpedestriansatroundabouts(HarkeyandCarter,2006).Thesedifferent
resultsmaybecausedbydifferentareasofstudy,thenumberofvulnerableusers,andtypeofanalysis;at
theveryleast,theyreinforcetheimportanceofconsideringthecontextoftheroundaboutintheanalysis.
2.5.2AspectsofSafetyPerformanceofRoundabouts
Severaldesignaspects,suchasconflictpoints,roundaboutdesign,speed,geometry,sightdistance,and
pavementmarkings,determinethesafetyperformanceofroundabouts.Theimportanceofeachofthese
aspectsisexploredbelow.
Roundabouts and Access Management
Page 25
Chapter2LiteratureReview
2.5.2.1ConflictPoints.Aconflictpointisdefinedasalocationwherethepathsoftwomotorvehicles,ora
vehicleandabicycleorapedestrianpath,diverge,merge,orcrosseachother(Rodegerdtsetal.,2010,p.5‐
5).Thenumberofpotentialconflictpointscouldbeasurrogatemeasureofsafety;fewerconflictpoints
couldresultinenhancedsafety.Roundaboutshavefewerconflictpointscomparedtoconventional
intersections,withtheresultingpotentialforimprovedsafety.Figure8showstheconflictpointsata
traditionalstop‐controlledorsignalizedintersectionandatasingle‐laneroundabout.Atraditionalstop‐
controlledorsignalizedintersectionwithfourlegshas32conflictpoints,whilearoundaboutwithfourlegs
hasonlyeightconflictpoints(Bie,Lo,Wong,HungandLoo,2005;Rodegerdtsetal.,2010;Stoneetal.,
2002).Byreducingthenumberofconflictpoints,roundaboutscanincreasesafetyatanintersection(Elvik,
2003;HydenandVarhelyi,2000).
( ) V hi l
Figure8.VehicleConflictsandVehicle‐PedestrianConflictsatSignalizedIntersectionsandSingle‐Lane
Roundabouts(Rodegerdtsetal.,2010,Exhibit5‐2,p.5‐7)
Theone‐waytrafficflowthroughroundaboutsgivesasenseofeasetodriverswhenobservingoncoming
traffic,andhasbeenshowntoimprovesafetybymakingdriversmorecautious(DanielsandWets,2005).
Roundabouts and Access Management
Page 26
Chapter2LiteratureReview
Certaincrashtypes,includingright‐turn,angle,andleft‐turncrashesareeliminatedasvehiclesmoveinone
directionthroughtheroundabout.Further,crashesatroundaboutsareoftenlesssevere;mostcrashes
resultinminorinjuriesorpropertydamageonly(Rodegerdtsetal.,2010).Adesirableroundaboutdesign
establishesahighpriorityonspeedreductionandspeedconsistency(Robinsonetal.,2000).Vehiclesmust
beabletonavigatetheroundaboutthroughaseriesofturningmovementsatlowerspeeds,usuallyless
than20mph.Geometricfeaturescanalsocontrolvehiclespeeds.Someofthesafetybenefitsforagood
roundaboutdesigninclude:
 Areductionincrashseverityforpedestriansandbicyclists;
 Moretimefordriversenteringtheroundabouttomakeproperdecisions,adjusttheirspeedandentera
gapincirculatingtraffic;
 Safermergesintocirculatingtraffic;
 Moretimefordriverstodetectandcorrecttheirmistakesorcompensateforthemistakesofothers;
 Makingintersectionssaferfornoviceusers;and
 Eliminatingleft‐turncrashes.
Whenproperlydesigned,roundaboutsreducethespeedofvehiclesapproaching,circulating,andexiting
theroundabout.Lowertravelspeedsreducethespeeddifferentialsamongvehicles.Vehicleshavelowand
homogenousrelativespeedsinroundabouts,forcingtraffictoslowdownbecauseoflateraldisplacement
(DanielsandWets,2005).Consequently,drivershavemoretimetoanticipateandreacttopotential
conflicts.Ingeneral,higherspeeddifferentialsyieldedhighercrashratesfortotalcrashesandentryrear‐
endcrashtypes(Zirkel,Park,McFadden,AngelastroandMcCarthy,2013).Asaconsequence,speed
standardsontheroundaboutsarenecessary(Montella,Turner,Chiaradonna,andAldridge,2013).Studies
alsoshowuneventrafficflowisacontributingfactortospeedvariations(St‐Aubin,Saunier,Miranda‐
Moreno,andIsmail,2013).ResearchatfiveroundaboutsinQuébec,Canadaalsoreportedthatlargeand
inconsistentspeedvariationwasmainlyduetoregionaldifferencesindesignandroaduse(St‐Aubinetal.,
2013).
Insafetyperformancemodels,speedmayperformasasurrogatevariableindesigningroundabouts(Chen,
PersaudandLyon,2011).Afteranalyzingcrashdataandapproachleveldatafor33approachesat14
roundaboutsfromeightstates,theauthorsconcludedthatspeed‐basedmodelsperformedbetterthannon‐
speedbasedmodels.Afterrelatingspeedtogeometricfeaturesusingcorrelationanalysisandcalibrating
themodel,theauthorsidentifiedtheinscribedcirclediameter(ICD),andentrywidthassignificant
geometricfeatures.Higherapproachspeedsresultinincreasedcrashratesatroundabouts(Mahdalová,et
al.,2010)
Furthermore,“relativespeedsamongadjacentgeometricelementsshouldbeminimizedforoptimum
safety”(ArndtandTroutbeck,1998,p.16).Vehiclespeedscouldbereducedby“reducingtheradiusofthe
approachcurve,minimizingtheentry,exit,andcirculatinglanewidth;betterpositioningoftheentryand
departurelegs;andincreasingthecentralislanddiameter”(ArndtandTroutbeck,1998,p.13).Inthis
study,otherrelevantconclusionsinclude:theidealdifferentialspeedbetweentheupstreamintersection
andtheroundaboutisabout20km/h;andlargerradiidecreasethefrequencyofsingle‐vehiclecrashes,but
potentiallyincreasemultiple‐vehiclecrashrate.Tokeepdriversfromcuttingintoanadjacentlane,this
studysuggeststhattheapproachroadwayshiftlaterallyby7m.Theauthoralsosuggeststhatthe85th
percentilespeedsonalltheapproachlegsbelimitedtoabout60km/h.Thiscanhelpminimizerear‐end
crashes.Finally,theentering/circulatingvehiclecrashescouldbeminimizedbylimitingtherelativespeed
betweenvehiclesenteringandcirculatingintheroundabouttoabout35km/h.
Thesizeoftheinscribedcirclediameter,theentry/exitradii,trafficflow,andgeometricallayoutinfluence
safetyatroundabouts(Mahdalová,etal.,2010).Speedlimitalsohasaneffectonsafety.Forexample,
higherapproachspeedsresultedinrelativelyhighercrashrates,especiallyiftheapproachspeedwas
Roundabouts and Access Management
Page 27
Chapter2LiteratureReview
above70km/h.Furthermore,thecrashratewasfoundtoincreasewithanincreaseinthenumberof
approachlegs.Daniels,Brijs,Nuyts,andWets(2011)foundthatthree‐legroundaboutsperformedless
effectivelythanfour‐legroundabouts.TheauthordevelopedPoissonandgamma‐modelstopredictcrashes
using148roundaboutsinFlanders,Belgium.Thestudyalsoconcludedthatroundaboutswithacyclepath
hadfewercrashesthanthosewithotherbicyclefacilities,whilethosewithlargecentralislandshadmore
single‐vehiclecrashes.
2.5.2.3SightDistance.Indeterminingpropersightdistancesattheroundabouts,designersshould
considertheISD,upstreamapproachsightdistance,andcirculatingsightdistance.Whileaninadequate
sightdistanceisconsideredunsafe,agreaterdistancemayincreasethepercentagesfortotalandrear‐end
crashfrequenciespossiblybecauselargersightdistancesencouragehigherspeeds(Angelastro,McFadden
andMehta,2012).Theauthorsdevelopedcrashpredictionmodelsasafunctionofaverageannualdaily
traffic(AADT)andsightdistanceattributestopredicttotalandrear‐endentrycrashesperyearper
roundaboutapproach.Themodelsshowthatsightdistanceparameterscouldbetterexplainthevariations
ofcrashfrequencieswhencomparedtobasemodelsthatuseAADTastheonlypredictor.Moreover,
exceedingsightdistancethresholdsincreasedtheriskofcrashoccurrenceandyieldedgreaterspeed
differentialsbetweentheapproachandtheentrytotheseroundabouts(Zirkeletal.,2013).
2.5.2.4PavementMarkings.Severalstudiesexaminedtheimpactofdifferentpavementmarkingsonthe
safetyoftheroundabouts(Bieetal.,2005;Fortuijn,2009).Thefirststudycomparedconventionaland
Alberta‐typelanemarkingsinroundabouts(asshowninFigure9).Alberta‐typemarking,alsoknownas
spiralmarkingsystem,isusedfortwoormorelaneroundaboutsandincludespavementmarkingsto
indicatetodriversatwhichlanetheyneedtobetoexistfromtheroundabout.Asafetyanalysiswas
performedusingacell‐basedmodeltodeterminepotentialconflictswhentwoormorevehiclesare
projectedtocollideinthesamecellatthesametimeinterval.AlthoughAlberta‐typemarkingtendsto
centralizetheconflictspotsandpotentiallyinfluencesafety,thisstudyfindsnostatisticallysignificant
differenceinthesafetyofroundaboutswithconventionalandAlberta‐typemarkings.
Figure9.DifferentMarkingSystems(Bieetal.,2005)
Inthelaterstudy,Fortuijn(2009)reviewedraisedlanedividers,alsoknownasturbodividers,and
evaluatedtheireffectivenessinminimizingsideswipecrashesattwo‐laneroundabouts.Fortuijin(2009)
evaluatedthenewtypeofdesignatsevenroundaboutlocationsintheNetherlandsandfoundthatit
reducedcrashesby72%.Theroundaboutswithturbodividersarecalledturboroundabouts.Turbo
roundaboutscanbedefinedasaspecifickindofspiralmarkingroundabout.
2.5.2.5CrashTypes.Differenttypesofcrashoccurrencesdeterminetheemphasisofroundabout
geometricdesign.Forexample,singlecrashesatroundaboutsmayoccurwhendriverslosecontroloftheir
vehiclesandcollidewithapartoftheroundabout,orasaresultofweather‐relatedfactorsandroad
conditions.Forinstance,wetroadconditionsresultinalowercoefficientoffrictionandcollisionswiththe
Roundabouts and Access Management
Page 28
Chapter2LiteratureReview
apronorcurbsofroundabouts.Also,visibilityisreducedatnightandduringfoggyconditions.Single‐
vehiclecrashratesarefoundtobehigheratroundaboutswiththefollowinggeometry:highabsolute
speedsonaparticulargeometricelement,highdifferentialspeedsbetweenadjacentroadsandthe
roundabouts,longcurves,andcurvesthatrequiredhighvaluesofsidefriction(ArndtandTroutbeck,
1998).Thepredominanttypesofmultiple‐vehiclecrashesincluderear‐endcrashes,crashesinvolving
vehiclesentering/exiting/circulatingtheroundabout,andsideswipecrashes.Thesecrashesaremainlydue
tohighdifferentialspeedsbetweenvehicles,orobstructiontodrivers’viewofothervehiclesorthe
roundabout(ArndtandTroutbeck,1998).
Insingle‐laneroundabouts,safetycouldbeimprovedbyprovidingadequatevisibilityandsufficientright‐
of‐wayforgooddeflectiononthecenterisland(Flannery,2001).Byobservingcrashstatisticsafterthe
roundaboutconstructionofninesingle‐laneroundaboutsinMaryland,Nevada,andFlorida,theauthor
foundthat27.3%oftotalcrashesweresideswipes,24.2%wererear‐endcrasheswitharelativehighof
45.5%oftotalcrashesduetoalossofcontrol.Thiscouldbeattributedtohighspeedsonentryapproaches
andpossibledriverviolations.Specifically,safetycouldbeimprovedattheselocationsbyimprovingthe
geometricdesignoftheapproaches.
2.5.2.6Signing.Signageandclearinformationhavearoleinimprovingsafetyeffects.Lowsafetyeffectsin
two‐laneroundaboutsraisedstudyconcernsabouttheimpactofsignage(Inmanetal.,2006b).Thestudy
showsthatroundaboutuserseitherdonotuseordonotunderstandassociatedsignage.Richfieldand
Hourdos(2013)hadasimilarconcernaboutsafetyontwo‐laneroundaboutsandevaluatedtheimpactof
changesmadetostripingandsigningatatwo‐laneroundaboutinRichfield,Minnesotaondrivingbehavior.
Thestudyfoundthatimproperturnsandfailuretoproperlyyieldwerethemaincausesofamajorityof
crashes.Changesinsignageandstripingresultedina55%reductioninimproperturnsanda59%
reductionineventswheredriverschoseincorrectlanes.
2.5.3SafetyforDifferentRoundaboutUsersandModes
Safetyisalsorelatedtodifferenttypesofusers.Inthissection,literaturereviewforsafetyofvulnerable
roadusers,pedestrians,bicyclists,andheavyvehiclesarediscussed.
2.5.3.1VulnerableRoadUsers.Thesafetyperformanceofmodernroundaboutsforvulnerableroadusers
haslongbeendebated.Althoughseveralstudieshavefoundnosignificantissues(HarkeyandCarter,2006;
Schroederetal.,2006);vulnerableroadusers,particularlybicyclistsandvisually‐impairedpedestrians,
couldencounterpotentiallyunsafesituationsatroundabouts.Researchresultsareextremelydependent
onthelocationofthestudies.Forexample,studiesfromcountriesoutsidetheUnitedStates,particularly
Belgium(DeBrabanderandVereeck,2007)andDenmark(HelsandOrozova‐Bekkevold,2006;Møllerand
Hels,2008),concludethatthesafetyofbicyclistsandpedestriansworsenedafterroundabout
implementation.Thiscouldbebecause,comparedwiththeUnitedStates,pedestrianandbicyclisttrafficis
significantlyhigherinthesecountries.
Crashdataofvulnerableroadusersislimitedbecausefewercrashesarereported.Additionally,
pedestriansandbicyclistsmaytendtoavoidroundabouts,resultinginlimitedexposure.Consequently,
studiesconductedintheUnitedStatesonpedestrianandbicyclesafetyrelyprimarilyonobservational,
ratherthanstatisticaltechniques.SafetystudiesintheUnitedStatestypicallyfindeithernosignificant
issueswithroundaboutconversionsoranimprovementinsafetyforpedestriansandbicyclists
(Stone,ChaeandPillalamarri,2002;HarkeyandCarter,2006;Schroederetal.,2006).
Eventhoughdifferentargumentsexistonthesafetyeffectsofmodernroundabouts,amajorityofthe
literatureconcludesthattwo‐laneroundaboutsaremoredangerousforpedestriansandvisually‐impaired
pedestriansthansingle‐laneroundabouts.Inman,DavisandSauerburger(2005)proposedadditional
crossingtreatmentforvisually‐impairedpedestriansintwo‐laneroundabouts.Schroeder(2013)also
Roundabouts and Access Management
Page 29
Chapter2LiteratureReview
concludedthatadditionaltreatmentwasnecessary.However,Inmanetal.(2006a)foundthatsoundcue
treatmentsdonothelpandmayresultinnumerousfalsealarms.
Unlikevehiclecrasheswhereroundaboutsresultedinfewerseriousinjuries,forvulnerableusers(i.e.,
pedestrians,bicyclists,mopeddrivers,andmotorcyclists)thepercentagesgoup.Conversionfroma
signalizedintersectiontoaroundaboutincreasedthenumberoffatalpedestrianandbicyclistscasualties
perseriousinjuryratefrom0.03to0.17(DeBrabanderandVereeck,2007).Theirstudyfocusedon
roundaboutintersectionswithapproachspeedsof50km/h(31mi/h).Conversionfromastop‐controlled
intersectiontoaroundaboutresultedina14%reductionininjurycrashfrequency.Ontheotherhand,
conversionfromasignalizedintersectiontoaroundaboutresultedina28%increaseininjurycrash
frequency.Similarly,conversionfromastop‐controlledintersectiontoaroundaboutincreasedthenumber
offatalcausalitiesperseriousinjuryratefrom0.12to0.19(DeBrabanderandVereeck,2007,p.588).
Conversely,HarkeyandCarter(2006)havenotfoundsubstantialsafetyproblemsforpedestriansand
bicyclists.Theauthorsuseddigitalvideoforobservationalanalysisatsevenroundabouts.Theyobserved
thedigitalvideosandcodeddifferentreactionsfrompedestriansandbicyclistsas“normal,”“hesitant,”
“retreat,”and“run.”Further,motorist‐yieldingbehaviorwascodedas“activeyield,”“passiveyield,”and
“didnotyield.”Thestudyshowednosubstantialproblemsforpedestriansandbicyclists.Nonetheless,the
researchhighlightedtheneedforamorepedestrian‐friendlydesignofroundaboutsinexitlegsandthe
needtoprovideadditionaltreatmentsformulti‐laneroundabouts.
2.5.3.1.1Bicyclists.Bicyclistsinroundaboutscanbetreatedaspedestriansorasdrivers;thisdistinction
influencesthenumberofconflictsexperiencedbycyclists.DanielsandWets(2005)addedthatthedetails
ofroundaboutdesigninfluencethenumberofconflictpointsforbicyclists.Thenumberofconflictpoints
increasesifbicyclistsaretreatedasdriversduetothespeeddifferentialandthedifferenceinvisibility
betweenbicyclistsandothermotorizedvehicles(Brown,1995;DanielsandWets,2005;Robinsonetal.,
2000).
Figure10showsfourtypesofalternativetreatmentsforbicyclistsatroundabouts:(1)mixedtrafficwith
motorizedtraffic,(2)adjacentbikelanes,(3)separatedbikelaneswithpriorityforbicyclistsatcrossings,
and(4)separatedbikelaneswithoutpriorityforbicyclistsatcrossings.Alternative(3)wasfoundtobe
saferthanAlternative(4)becausemotorizedvehiclesyieldtobicyclistswhenpriorityisgiventobicyclists
(DanielsandWets,2005).Alternative(3)hadaslightlyhighernumberofseriousinjuriescomparedto
Alternative(4)(DanielsandWets,2005).Bothalternatives(i.e.,3and4)performedbetterthan
Alternative(1)andAlternative(2)forinjurycrashes(DanielsandWets,2005).However,specific
recommendationswerenotmadeduetolackofsufficientevidence.
Roundabouts and Access Management
Page 30
Chapter2LiteratureReview
Figure10.(1)Mixedtraffic;(2)adjacentbikelanes;(3)separatedbikelaneswithpriorityforbicyclists;
and(4)separatedbikelaneswithoutpriorityforbicyclists(DanielsandWets,2005,p.6‐8)
Stillonthesafetyperspectiveofbicyclists,roundaboutsinbuilt‐upareasperformedworsecomparedwith
thoseoutsidebuilt‐upareasresultingina48%increaseinbicyclecrashfrequencyatroundabouts
constructedinsideabuilt‐uparea.Noincreaseinbicyclecrasheswasfoundatroundaboutsconstructed
outsidebuilt‐upareas(Danielsetal.,2008).Furthermore,theauthorsestimateda15‐24%increasein
severe‐injurybicyclecrashes.Despitethosefouralternatives,twootheralternativesthatwerenot
discussedbytheauthorincludetreatingbicyclistsaspedestriansandprovidinggrade‐separatedcrossings
attunnelsandbridges.
Incontrast,bicyclistsappearedtogainmorerespectfromdriversafterroundaboutconstructionasthe
percentageofyieldingincreasedfrom13to77(HydenandVarhelyi,2000).Thisstudyconductedon‐site
observationswiththeobjectiveofviewingtheinteractionsbetweenroadusersatjunctionsafterthe
roundaboutconstruction.HydenandVarhelyi(2000)alsoperformedaconflictanalysisandfoundthatthe
frequencyofbicycle‐vehicleconflictsdroppedfrom77to45,withtheexpectednumberofinjurycrashes
peryeardownfrom4.2to1.7.
Thebehaviorofviolenceinfluencedsafetyperformance.Forexample,usingobservationforallbicycle
movementsandanyobservedbicycle‐vehicleinteractionsonsingle‐laneroundaboutslocatedin
Massachusetts,BerthaumeandKnodler(2013)foundthatwhenthenumberofbicyclesthatperformed
unsafemaneuverswascomparedtothetotalnumberofbicyclesobservedtraversingtheroundabout,
about3%oftotalbicyclemaneuverswerefoundtobeunsafe.Inaddition,bicycle‐vehiclecollisionsat
roundaboutswerefoundtobemorefrequentwhenbicyclistsunderestimatedtheriskand/orhadlittle
knowledgeoftherelevanttrafficrules(MøllerandHels,2008).Theperceivedlevelofriskataroundabout
withoutabikefacilitywashigherthanthatforbicyclistsataroundaboutwithabikefacility.Additionally,
theperceivedlevelofriskwasalsoinfluencedbyage,gender,involvementinanearcrash,trafficvolume,
andwhetherthereisabikefacility.Apossiblecountermeasuretoincreasetheperceivedriskandtocorrect
unsafepracticesistoimplementefficientsignageforbicyclists.Aftergeneratingamodelusingdata
Roundabouts and Access Management
Page 31
Chapter2LiteratureReview
collectedbetween1987and1993with1,385observationsandcomparingbicyclelanesinroundabouts
withandwithoutpedestriansignals,DabbourandEasa(2008)recommendusingpedestriansignalsat
roundabouts.
2.5.3.1.2Olderpopulation.Clearsignageinfluencessafetyforolderroadusers(i.e.,≥65years)usinga
roundabout(Lord,Schalkwyk,ChryslerandStaplin,2007).Thestudywasconductedusingstructured
interviewsandfocusgroupsinCollegeStation,TX,andTucson,AZ.Theparticipantsincluded14menand
17women.Inthisstudy,designelementswerereviewed,includingadvancewarningsigns,lanecontrol
signs,directionalsigns,yieldtreatments,andexitsigntreatments.ALikert‐typescalewithsevenpoints
wasused.Researchersthenusedtheanalysisofvariance(ANOVA)tounderstandifthereweresignificant
differencesbetweenthebasecondition,countermeasure#1,andcountermeasure#2.Table2providesa
detaileddescriptionofthebaseconditionandtestedcountermeasures.
Table2.DetailedCountermeasuresforDesignElements(Lordetal.,2007,p.429)
DesignElement
A.Advance
WarningSigns
BaseCondition
Theadvancewarningsign
template[W2‐6]wasused
accordingtotheguidelines
proposedintheMUTCD
(FHWA,2003).
B.Roundabout
LaneControl
Signs
TheBaseConditionwas
modeledaftertheR3‐8
seriesofadvanceinter‐
sectionlanecontrolsigns
(FHWA,2003).
TheBaseConditionshowsa
centralislandwithoutany
guidesignsorspecial
pavementmarkingtoguide
trafficcirculatinginsidethe
roundabout,asperthe
guidelinesproposedbythe
MUTCD(FHWA,2003).
C.Directional
Signs(one‐way
sign)
D.Yield
Treatment
ThestandardR1‐2yieldsign
wasprovidedonbothsides
oftheroadattheentranceof
theroundabout.This
conditionrepresentsthe
standardsetbySection
2B.10oftheMUTCD(FHWA,
2003).
E.ExitTreatment TheBaseCondition
consistedofplacingastreet
exitsign(basedontheD1
series)priortoreachingthe
exit;thesignwasplaced
betweentwointersecting
streetsfacinginwardtoward
thetrafficinthecircle.
Roundabouts and Access Management
Page 32
Countermeasure#1
Twochangesweremade
comparedtotheBase
Condition:(1)asolidblack
circlewasaddedinthemiddle
ofthesign,and(2)aplaque
withthetext"ROUNDABOUT"
wasattachedbelowtheadvance
warningsign.
Asolidblackcirclerepresenting
thecentralislandwasaddedto
theleftlane'sroute,butnotfor
therightlane'sroute.
Aone‐waysign(templateR6‐1)
wasplacedonthecentral
island,positionedtofacethe
centerlineoftheapproaching
roadwayata90ºangle.Inthis
position,driverswillseethe
signastheyapproachthe
roundabout.
Ayieldlineconsistingofsolid
whiteIsoscelestriangleswas
addedtotheBaseCondition.
Thesamestreetexitsignfrom
theBaseConditionwasused,
butwasmovedontothesplitter
islandoftheintendedstreet
exit;thissignstillfacedinward
towardthetrafficinthecircle.
Countermeasure#2
Aplaquewithanadvisory
speedof30mphwasplaced
belowthewarningsignused
forcountermeasure#1(i.e.,
thesignwiththesolidblack
circle).
Thetext"LEFTLANE"and
"RIGHTLANE"underthe
correspondingrouteswere
addedtothesignusedforthe
BaseCondition.
Thesameone‐waysignwas
placedonthecentralisland,
butdirectlyinfrontofthe
driver'sentrypointatthe
gorearearatherthanfacing
thecenterlineofthe
approachingroadway.This
placementputsthesignmore
directlyinthedriver'slineof
sightfromtheyieldline.
Thistreatmentincludedallof
thecomponentsnotedfor
Countermeasure#1,but
addedaplaquereading"TO
TRAFFICINCIRCLE"below
theyieldsigns.
Anarrowpointingtotheexit
legwasaddedonthestreet
namesignusedfor
countermeasure#1.
Chapter2LiteratureReview
Theresultsofthisstudyforeachdesignelementareasfollows.A“ROUNDABOUT”legendispreferredas
anadvancewarningsignupstreamofaroundabout.Addingdirectionalsignsarefavored;however,the
resultsforthisdesignfeaturewerenotstatisticallysignificant.Fortheyieldtreatmentelement,adding“TO
TRAFFICINCIRCLE”undertheYIELDsignwasfoundtobestatisticallysignificant.Thearrowforexitsign
treatmentyieldedamorepositiveresponsefromparticipants.
2.5.3.1.3Pedestrians.RoundaboutseliminateseveralpotentialconflictsforpedestriansasTable3shows.
However,pedestrian‐vehicleconflicts,whentheyexist,involvehigh‐speed,right‐turning,andleft‐turning
vehicles(DanielsandWets,2005).
Theincreaseinpedestrian‐vehicleconflictshasbeenshownbyseveralstudies(Hyden,2000;Stone,Chae
andPillalamarri,2002).Thefirststudyexaminestheeffectofroundaboutinstallationatoneintersectionin
Raleigh,NCbyconductingthreeanalyses:thepedestrian‐vehiclecrashhistorieswithandwithoutthe
proposedroundabout;astatisticalanalysisforpedestrian‐vehiclecrashesversusstreetandintersection
characteristics;andatrafficsimulation.TheresearchersusedParamicssoftwarebecauseitmodeled
roundaboutsexplicitlyratherthanasone‐waystop‐controlledintersections.Thestudyconcludedthatthe
proposedroundaboutseemedpromisinginthatthereisa7%reductioninpedestrian‐vehiclecrashesin
theroundaboutcomparedwiththoseonthestreetoratintersections.Inaddition,thesimulationshowed
thattheproposedroundaboutwouldimprovepedestriansafetycomparedwithaFWSCintersection.This
isduetofewerconflictpointsandlowerspeedsofvehicles.Thesecondstudyshowedthatthatafterthe
installationofroundabouts,theproportionofvehiclesyieldingtopedestriansincreasedfrom24%to51%,
andthenumberofconflictswasreducedfrom19tofour.HydenandVarhelyi(2000)observedthenumber
ofpedestrian‐vehicleconflictsbeforeandafterinstallationofroundaboutsusingthe30‐hourobservation
period.Additionally,theresultsalsoshowedthatroundaboutconstructionresultedinareductioninthe
expectedfrequencyofinjurycrashesfrom0.6to0.1.
Fordesign‐specificconcerns,Furtado(2004)foundthatroundaboutswithcentralislandsthathavea
diametergreaterthan10m.performbetterthanthosewithsmallerdiameters.Furthermore,theauthor
madethefollowingrecommendations:(a)theminimumoffsetfromtheyieldlinetothecrosswalkshould
tobe7.5m.,(b)adetectablewarningsurfacedelineatingthetravellanefromtherefugeareashouldbe
installed,and(c)signingandpavementmarkingtreatmentsforcrosswalkfacilitiesshouldbeprovided.
Theythenpointouttheadvantagesanddisadvantagesofroundaboutsforpedestrians,asshowninTable3.
Table3.AdvantagesandDisadvantagesofRoundaboutforPedestrians(Furtado,2004)
Advantages




Vehiclespeedisreducedascomparedtoother
intersections
Pedestrianshavefewerconflictpointsthanat
otherintersections
Splitterislandsandresultingpedestrianrefuge
areasallowuserstofocusononedirectionof
trafficatatime
Crossingmovementcanbeaccomplishedwith
lesswaittimethanatconventional
intersectionsthathavemultipleprotected
phases
Disadvantages




Vehicletrafficisyieldcontrolled;therefore,
trafficdoesnotnecessarilystopanditcould
causepedestrianstohesitate
Maycauseanxietyinpedestrianswhoarenot
confidentaboutjudginggapsintraffic
Crossinglocationsandsetbacksfromtheyield
lineoftenresultinlongertraveldistancesfor
pedestrians
NotwidelyusedinNorthAmerica,providing
significantchallengesforthevisuallyimpaired
Inevaluatingthesafetyofroundabouts,pedestrianswithvisualdisabilitiesrequirespecialconsideration
Eventhoughissuesofvisually‐impairedpedestriansatroundaboutshavebeendiscussed,untilrecently
therehadbeennoextensiveresearch.Tofillthisgap,Ashmead,etal.,(2005)conductedastudyto
Roundabouts and Access Management
Page 33
Chapter2LiteratureReview
comparesixnormal‐sightedpedestriansandsixcompletelyblindpedestriansastheycrossedatwo‐lane
roundabout.Theyfoundthatvisually‐impairedpedestriansaremoresusceptibletodangerswhencrossing
aroundabout.Also,visually‐impairedpedestrians’waittimewaslongerthanthatofsightedpedestrians.
ThestudywassimulatedinNashville,TN.Participantswithnormalvisionwalkedaroundoncewithan
experimenterwhopointedoutthesamefeaturesthatweredescribedtothevisually‐impairedpedestrians.
Theexperimenteronlyintervenedasasafetymeasure.Thestudyshowedthatthesightedparticipantsdid
notneedanyinterventionfromtheexperimenter.However,therewere10instanceswherethevisually‐
impairedpedestriansneededinterventionbecausetheydidn’trealizetheywerewalkingintoapotentially
dangeroussituation.Also,outofthe144totalcrossings,therewere15instanceswherethevisually‐
impairedpedestrianbegantocrossandthenabortedthecrossing.
Visually‐impairedpedestriansmayhaveproblemsincrossingmodernroundaboutsbecausetheymayhave
thefollowingdifficulties:locatingthecrosswalkwithintheroundabout;identifyingthedirectionof
crosswalkalignmentthatmightbeperpendiculartothesidewalk;decidingwhenthetrafficiscontinuous,
andidentifyingwhetheravehicleisyielding;andfollowingthepathofcrossingalignmentsandcross
multiplelanesthroughtheendofthecrosswalk(Schroederetal.,2006).Thecurvedgeometryofmodern
roundaboutsoftenforcesvisually‐impairedpedestrianstobefamiliarwithhowtocrossinthese
circumstances,asopposedtotraditionalintersections.Sincemostroundaboutsdonothavetrafficsignals,
thetaskofidentifyinggapsintrafficatroundaboutsisquitedifficultforvisually‐impairedpedestrians.
Modernroundaboutshavecontinuoustrafficandhighnoiselevelsthataddtothedifficultyofvisually‐
impairedpedestriansindeterminingwhetherthevehicleshaveyielded,stopped,orcontinued.
Thetotalnumberofcrashesinvolvingpeoplewithdisabilitiesincreasedaftertheconstructionof
roundabouts;however,crashseveritydramaticallydecreased(SingerandHicks,2000).SingerandHicks
(2000)alsoreviewedthechallengesindesigningamodern,pedestrian‐friendlyroundaboutinTowson,
MD.Thechallengesincludedtheunusuallayoutoftheroundabout;difficultyinaccommodatingpeople
withdisabilitiesandcomplyingwiththeAmericanswithDisabilitiesAct(ADA);theavailabilityofalternate
routes,andliabilityissues.TheauthorsprovidedinsightsonhowtheMarylandStateHighway
Administrationcouldaddressthesechallenges.Theyinvolvedvariousstakeholdersinthedevelopmentof
theroundabout,conducteddriverandpedestrianeducationprograms,andprovidedadditional
informationtothepublic,suchasBraillemaps.
Inresponsetothoseissues,Schroederetal.(2006)testedadditionaltreatmentsforsingle‐lanemodern
roundaboutswhichincludedsoundstrips,apedestrian‐actuatedflashingbeacon,andacombinationofthe
twotreatments.Fortwo‐laneroundabouts,theauthorstestedaraisedcrosswalkandpedestriansignal
withPedestrianHybridBeacon(PHB).Inthisstudy,Schroederetal.(2006)usedthedegreeofriskin
crossingtheroundaboutasaperformancemeasure.Theyusedapre‐andapost‐within‐subject
experimentaldesignwherethesamevisually‐impairedpedestrianscrossedtheroundaboutinbothpre‐
testandpost‐testscenariosaftertheroundaboutconstruction.Inthebefore‐and‐afterstudy,theauthors
usedasimulationofcrossingtheroundaboutsinwhich16peopleparticipated.Thestudyfinallyconcludes
thatasingle‐laneroundaboutdoesnotposesignificantdifficultiesforvisually‐impairedpedestrians.This
isduetolowvehiclespeeds,yieldingfromamajorityofdrivers,properlyinstalleddetectablewarning
surfacesandtheavailabilityofO&Mspecialists.However,tosignificantlyreducepedestriandelayattwo‐
laneroundabouts,additionalcrossingtreatmentsarerequired.
Tofurtherunderstandspecifictreatmentsfortwo‐laneroundabouts,Inman,DavisandSauerburger(2005)
testedwhetherrumblestrip‐likedevicesandpedestrianyieldingsignswouldencouragedriverstoyield
moreforpedestrians.Inmanetal.(2006a)conductedtwoexperimentsonacontrolledandtreatedcourse
withsevenseverelyvisuallyimpairedindividuals.Dataforeachexperimentwascollectedfor1.5hours
everyafternoonforaperiodoftwoweeks.Performancemeasuressuchascorrectlydetectingastopped
vehicle,failuretodetectthestoppedvehicle,falsealarms,andthenumberofcorrectlydetecteddepartures
Roundabouts and Access Management
Page 34
Chapter2LiteratureReview
ofstoppedvehicleswererecorded.Theresultsofthestudysuggestedthatsoundcuesonthepavement
increasedtheproportionofdouble‐yieldingdriversanddecreasedthetimeforvisuallyimpaired
pedestrianstodetectyields;however,falsealarmswerenotaffected.TheYieldtoPedestriansigns,once
installed,increaseddrivers’yieldingactsfrom11.5%to16.7%.However,sincefalsealarmsarestilla
problem,theauthorsconcludedthatthetwotreatmentsdidnothaveasufficientlevelofsafety
improvementtobeimplementedintwo‐laneroundabouts:yet,theyremaineffectiveinthecaseofsingle‐
laneroundabouts.
2.5.3.2HeavyVehicles.Ifroundaboutshavenotbeendesignedproperlytheymayinhibitthesafeand
efficientmovementoflargetrucksduetoroundaboutdesignconstraints(ParkandPierce,2013).Usingan
onlinesurvey,theauthorssynthesizedtruckingindustryobservationsregardingthechallenges
experiencedbycommercialtruckdriverswhileapproachingroundabouts.Themainissuesidentified
includedtheneedforlargerroundaboutcircumferences,moreeducationfordriversofpassengervehicles,
andareevaluationofroundaboutdesign.About73%ofrespondentsbelievedthatroundaboutsweremore
problematicforlargetruckscomparedtoothertypesofcontrolledintersections.Motorcarriers
commentedonroundaboutnavigationproblemsthatareuniquetolargetrucks,specifically,small
roundaboutcircumferences,designfeaturesthatcausedamagetotrucks,andsafeinteractionwith passengercars.Whenaskedtoproposepotentialsolutions,motorcarrierswishedthatroundaboutscould
betteraccommodatelargetruckswithoutsacrificingsafetyandoperationalefficiency.
Daniels,Brijs,Nuyts,andWets(2010)conductedastudytoexplorethecrashseverityatroundaboutsusing
datafrom1,491crashesthatoccurredat148roundaboutsinFlanders,Belgium.Theanalysisperiodvaried
fromlocationtolocationbasedondataavailability.Theminimumperiodwas3years,themaximum10
yearsandtheaverageacrossalllocationswas8.03years.Theydevelopedamodelforheavyvehiclesthat
includedtrucks,trailers,buses,andtractors.Eachroundaboutexperiencedanaverageof1.22annual
injurycrashes;meanwhile,theheavyvehiclecrashratewasfoundtobe0.09annualcrashesper
roundaboutwithavarianceof0.02.Furthermore,atotalof18single‐vehiclecrasheswerefoundbythis7
yearsstudytoinvolveheavyvehicleswithonefatalityandtwosevereinjuriesperyear.Likewise,97multi‐
vehiclecrashesinvolvedheavyvehicleswithnofatalorsevereinjuries.
2.5.4MethodsinRoundaboutSafetyAnalysis
Commonmethodsusedtoanalyzingthesafetyeffectsofroundaboutincludedescriptiveanalysisusing
descriptivestatisticsandchi‐squarestatistics,empiricalobservation,generalizedlinearmodel,odds‐ratio
andmeta‐analysis,ESEprocess,andempiricalbefore‐afterstudy.
2.5.4.1AverageMean(Descriptive).Safetyevaluationofroundaboutscanbeobtainedusingasimple
beforeandafterapproach.Isebrands(2009b)conductedabefore‐andafteranalysisfor17high‐speed
ruralintersectionsusingadescriptivemethodwhichcalculatingtotalcrashfrequency,crashrateandcrash
severityinfivestates:Kansas,Maryland,Minnesota,Oregon,andWashingtonState.Datawereobtained
fromcrashrecordsandaveragedailytraffic(ADT)atthestudylocations.Specificallyforcrashrate,crashes
permillionenteringvehicles(MEV),wasusedasameasureofexposure.Figure11displaysthebefore‐
and‐aftercrashfrequencystatisticsateachofthe17locations.
Roundabouts and Access Management
Page 35
Chapter2LiteratureReview
Figure11.CrashFrequenciesinRoundabouts(Isebrands,2009b)
2.5.4.2Chi‐SquareStatistic.Furthermore,thechi‐squarestatisticandanormalapproximationtestmay
beusedtoseetherelationshipbetweenretrofittedmodernroundaboutandtrafficcrashes(Flanneryand
Datta,1996).Theauthorsconsideredcrashfrequencyandthemeanofcrashesasperformancemeasures.
Theyusedcrashdatabeforeandaftertheretrofittedperiodsforeachlocation.Tounderstandwhetherthe
beforeretrofittedconditionsaredifferentfromthoseoftheafterconditions,theauthorsusedaChi‐square
testwith=0.05,sixlocations,andfivedegreesoffreedom.Theresultindicatedthat,ata95%levelof
confidence,thereisasignificantdifferencebeforeandaftertheconstructionofroundabouts.Figure12
givesthedatausedintheChi‐squareanalysis.
Figure12.DataRequiredforChi‐SquareAnalysis(FlanneryandDatta,1996,p.6)
Theauthorsusedanormalapproximationtesttoprovethatthebefore‐and‐aftergroupdataareneither
correlatednorstatisticallyindependent.Sincethistestrequiressimilartimeperiodsforbothbefore‐and‐
afterconditions,theyuseddatafromtwoyearspriortotheconstructionoftheroundaboutanddatafrom
oneyearaftertheroundaboutinstallation(FlanneryandDatta,1996,p.107).TheauthorsfoundthatX*=
(8.93)andis>X.Thus,the“[r]eductioninthemeanofcrashesforbeforeandafterperiodofroundabout
constructionissignificantata99%levelofconfidence”(FlanneryandDatta,1996,p.108).
However,resultsfromIsebrands(2009b)andFlanneryandDatta(1996)shouldbeusedwithcaution.
First,thenumberofcrashesalwaysfluctuatesinastochasticprocess(DanielsandWets,2005).Second,
othergeneraltrendsmayinfluencethenumberofcrashes,includingpolicies,law,andchangesintraffic
volume.Third,theinstallationofroundaboutsissometimestheresultofhighcrashratesthatcanhavea
regression‐to‐the‐mean(RTM)affectthatisnotaccountedforinasimplebefore‐and‐afterstudy.
Roundabouts and Access Management
Page 36
Chapter2LiteratureReview
2.5.4.3EmpiricalObservation(ConflictStudies).InSweden,HydenandVarhelyi(2000)usedabefore‐
and‐afterstudytotestthelong‐termeffectsofsmallroundabouts.Theyattemptedtoanswerseven
questionspertainingtoroundabouts;dothey:(1)reducespeed,(2)resultinloweredriskofinjury,(3)
promoteuserinteractions,(4)havenoeffectonredistributionoftraffic,(5)increasetimeconsumption
whennogiveawayregulationoccursordecreasetimeconsumptionwithnosignalization,(6)increase
emissionswhennogiveawayregulationoccursanddecreaseemissionwithsignalization,(7)haveno
changeinnoiselevel?Twooftheabovementionedareas,rateofspeedandriskofinjury,arerelatedto
safety.Crashdatawascollectedatthestudylocationssixmonthsafterconstructionandwascomparedto
crashesinthebeforeperiod(1983‐1990).Theauthorsusedconflicttechnique,i.e.,relatingconflictsto
crashes.Theseverityoftheconflictwasbasedontimetoaccident(TA)andconflictingspeed(CS).
Trainedobserversvideorecordedeachofthe12intersectionsfor30hours.Additionally,theauthors
calculatedthenumberofexpectedinjurycrashesperyearbymultiplyingtheratioofseriousconflictsand
injurycrashesdependingonthetypeofroadusersinvolved.Abehavioralstudywasalsoconductedtosee
theinteractionsamongtheroundaboutusers.Conflictsbetweenmultiplevehicles,bicyclesandvehicles,
andpedestriansandvehicleswereexamined.Theresultsshowedthatseriousconflictsbetweenvehicles
andvehiclesincreasedwhilepedestrian‐vehicleandbicycle‐vehicleconflictsdecreased.Thisbefore‐and‐
afterstudyisslightlybiasedbecausetheintersectionsselectedforthisstudywerechosenbecausetheyhad
ahighfrequencyofcrashespriortotheconstructionofroundabouts.
2.5.4.4GeneralizedLinearModels.Churchilletal.,(2010)conductedbothacross‐sectionalstudyanda
before‐and‐afterstudytounderstandtheoverallsafetyeffectofroundabouts.Crashdatafromall
roundaboutsbuiltintheNetherlandsfrom1999to2005wasanalyzed.Theauthorswerelimitedinterms
ofthetotalnumberofconventionalintersectionsandthetrafficvolumesrelatedtobothconventional
intersectionsandroundabouts.Asaresult,theyexaminedtheaggregatefatalcrashdataandfoundthat
whilethenumberoffatalitiesatconventionalintersectionsdecreased,thenumberoffatalitiesat
roundaboutsincreased.However,thismaybeduetothefactthatthefatalcrashfrequencywasnot
normalized(i.e.,totalnumberofroundaboutswasnotincludedintheanalysis).Theresultsmaynot
representactualconditionsforeitherroundaboutsorconventionalintersectionsbecausethecross‐
sectionalanalysisinthisstudywasfoundtobebiased.
Forthebefore‐and‐afterstudy,datawasobtainedfromtheDutchNationalroadsdatabaseandtheDutch
databaseofregisteredcrashes.ArcGISwasusedtogeocodethedataintoamap.Theresearchersassumeda
bufferof40metersaroundtheroundaboutforcrashes.Thisproceduremightinducesomebiasbecause
thepreciselocationoftheintersectionsisunknown.Ageneralizedlinearmodelwasbuiltwiththe
assumptionthat“thecountspercrashyearandperreconstructionyeararelinearlydependentonthe
numberoflocationsretrofittedinthatyear”(Churchilletal.,2010,p.38).
2.5.4.5Odds‐ratioandMeta‐Analysis.Branbander,Nuyts,andVereeck(2005)conductedanother
before‐and‐afterstudythatincludedacomprehensiveanalysisofthesafetyofexistingroundaboutsto
othercontrolledintersections.Usingodds‐ratiomatching,theauthorsfirstmadesurethecomparison
groups(intersections)hadthesamecharacteristics(i.e.,speedlimit)astheroundabouts.Anodds‐ratio
matchingisdefinedas“theratioofthechangeinthenumberofcrashesattheroundaboutlocationsbefore
implementationandthechangeinthenumberofcrashesinthecomparisongroup”(Branbander,Nuyts
andVereeck,2005,p.290).Theodds‐ratioforoneyeariscomparedtothepreviousyear.
Sincethenumberofcrashesataspecificlocationfluctuatesaroundanunknownaverage,theexpected
numberofcrashesataroundabout,takingintoaccountthereversiontomean(RTM)affectcanbe
calculatedusingtheexpectednumberofcrashesatthelocationwheretheroundaboutwastobebuilt,after
correctionforRTMeffect,theaveragenumberofcrashesperyearforthecomparisongroup,includingthe
crashesatthelocationwheretheroundaboutisimplemented;(beforetheconstructionoftheroundabout,
Roundabouts and Access Management
Page 37
Chapter2LiteratureReview
thelocationisconsideredcomparabletothecomparisongroupandcouldbeincluded).Nextthenumberof
yearsisconsidered,thenumberofcrashesinyeart,atthelocationwhereroundaboutswereconstructed,
andtheweightgiventotheaveragenumberofcrashesofthegroup(forthecomparisongroup)are
calculated.Then,theeffectivenessratioiscalculatedandfinally,theoverallsafetyeffectivenessisdefined
as"theweightedaverageoftheresultsoverthedifferentyears,wheretheweightassignedtothegroupof
roundaboutsistheinverseofthevariance"(Branbander,NuytsandVereeck,2005,p.292).
SimilartoBranbanderetal.,(2005),Elvik(2003)performedthelog‐oddsmethodofmeta‐analysis.The
authorestimatedthesafetyeffectforroundaboutinstallationbycomparingthenumberofcrashesafterthe
conversiontothenumberofcrashesbefore,andthencomparingthisratiototheratioofthenumberof
crashesafterandbeforeinacomparisongroupofintersections.
Inthisstudy,Elvik(2003)reviewed28studiesthatevaluatedsafetyonroundabouts.Thestudyalso
conductedtraditionalmeta‐analysis,wherethedataweregroupedbasedonnumberofapproachlegsand
crashseveritytoexplorethesourceofvariation.Additionally,meta‐regressionanalysiswasusedto
supplementthetraditionalmeta‐analysis.
Danielsetal.,(2008)alsousedodds‐ratiomatchingandmeta‐analysistoevaluatebicyclists’safetyat
roundabouts.Takingasampleof91roundaboutsinFlanders,Belgium,andcrashdatafrom1991to2001,
theygroupedtheroundaboutsarounddifferentspeedlimits,andtheirlocations(i.e.,insideoroutside
built‐upareas).Theyalsotookthecomparisongroupofothercontrolledintersections,76forinsidebuilt‐
upareas,and96intersectionsforoutsidebuilt‐upareas,andthenprioritizedthenearbyintersections
basedonapproachspeeds.
Meta‐analysishastwobasicweaknesses.First,meta‐analysiscannotimprovethequalityoftheevaluation
ofthestudy(Elvik,2003).Forexample,afterevaluatingdifferentstudydesigns,Elvik(2003),statedthat
thequalityofsimplerstudydesignsmightweakenthequalityofmoreadvancedstudies.Anotherpotential
weaknessofmeta‐analysisisthatitcanbebiased.Thebiasmayoccurwhenpreviousstudies’findingsgo
againstconventionalwisdomsotheyareregardedashavinglittlevalue.Therefore,thisstudyadoptsthe
trim‐and‐fillmethodtohelpconvertthebias,whichisdefinedas“anon‐parametricmethodfordiagnosing
andcorrectingforpublicationbias,basedontheassumptionthatafunnelplotofresultsshouldbe
symmetricaroundthemeanintheabsenceofpublicationbias.”(Elvik,2003,p.5)
2.5.4.6ESEProcess.TurnerandBrown(2013)usedtheESEprocesstoassessthesafetyimprovementsof
roundabouts.“ThethreekeyelementsoftheESE(orEASY)processare:1.estimationofexpectedcrashes
usingthebestavailablebase(crash)model;2.safetyobservationbasedonexperience;and3.evidence
fromnationalandinternationalroadsafetyresearch.Togiveconfidenceintheresults,theESEprocess
includescheckingthroughouttheprocessbyreviewingandcomparingwithotheravailableinformation
sources.”(TurnerandBrown,2013,p.2).
2.5.4.7EmpiricalBasedBefore‐and‐AfterStudies.AccordingtoPersaudetal.,(2001),asimplebefore‐
and‐afterstudymaybebiasedduetotheRTMeffectbecauseroundaboutsareusuallyconstructedwhenan
intersectionhassafetyproblems.Consequently,ifthestudyfailstocontrolthiseffect,thestudyislikelyto
overestimatethesafetyeffectoftheroundaboutconversion.TorespondtotheneedtoaddresstheRTM
effect,Persaudetal.,(2001)employedtheempiricalBayesbefore‐and‐afterprocedure.Rettingetal.,
(2001)andRodegerdtsetal.,(2007)alsousethisprocedure.
Rodegerdts(2007)evaluated310roundaboutsintheUnitedStateswithdifferentcharacteristics,suchas
urban‐suburban‐ruralsetting,numberoflegs,numberofcirculatinglanes,previousintersectiontype,age
ofroundabout,andgeographiclocations.Theauthorsanalyzed90roundaboutsbasedondataavailability,
geometricinformationandenteringdailytrafficvolumes.Roundabout‐levelcrashpredictionmodelsasa
Roundabouts and Access Management
Page 38
Chapter2LiteratureReview
functionofnumberoflanes,numberofapproachlegs,andAADTweredeveloped.Similarly,approach‐level
crashpredictionmodelsrelatedcommontypesofcrashestoAADT,includingkeygeometricfactors.
2.5.5RoundaboutsandSafety:Conclusion
Thissectionreviewedtheexistingliteratureonroundaboutsandsafety.Manystudiesshowedthat
roundaboutshaveincreasedsafetyperformance,withsafetyeffectsrangingbetween17to70%forcrash
reductions.However,theseresultscouldnotbefullytakenastheeffectofroundaboutconversionbecause
thereareothercontextsandissues,suchastheargumentthatconversionsfromFWSCintersectionstothe
modernroundaboutsdonotsignificantlyreducethetotalandinjurycrashrates(Rodegerdts,2007).Asa
consequence,howtheretrofittedprocessesandlocationselectionsweremademayinfluencethesafety
effectcalculation.Furthermore,theliteraturereviewfoundnumerousconcernsfromresearchersaboutthe
effectofretrofittedroundaboutsforvarioususersandmodes.Safetyperformancesofroundaboutsmaybe
reducedforvulnerableuserssuchasbicyclists,pedestrians,peoplewhoarevisually‐impairedorwith
disabilities,andelderlyroadusers.Theconcernisalsohighlightedforbigtrucksthatrequirespecial
treatmentsanddesignontheroundabout.Manymethodsareavailableforperformingsafetyanalysis:
descriptiveanalysis,chi‐squarestatistics,empiricalobservation,generalizedlinearmodel,odds‐ratioand
meta‐analysis,ESEprocess,andempiricalbefore‐afterstudy.
Roundabouts and Access Management
Page 39
Chapter2LiteratureReview
2.6EvaluationofGapsinRoundaboutLiterature
Anevaluationofexistingliteratureonroundabouts,accessmanagement,safety,andcapacityshowed
severalgapsinknowledge.Gapsareidentifiedbaseduponavailableliteratureregardingtheuseof
roundabouts,particularlyastheyapplytoaccess,operationsandroadwaycapacity,andsafety.
2.6.1LiteratureGapsinAccessManagement
Basedontheliteraturereviewonaccessmanagement,majorgapsintheliteraturewereidentified.Little
literatureexistsaboutaccessmanagementasitspecificallyappliestoroundabouts.Aswasdescribed
earlierinthischapter,manystudieshavebeencompletedabouttheuseofaccessmanagementstrategiesat
intersectiontypes(stop‐controlled,signalizedintersections,un‐signalizedintersections)astheyrelateto
variousdesignandplanningelementconsiderations.However,fewsuchstudieshavebeencompleted
relatedtoroundaboutsandaccessmanagement.
2.6.2LiteratureGapsinRoundaboutOperationsandCapacity
Basedonthereviewofliteratureonroundaboutoperationsandcapacity,severalgapsintheliterature
wereidentified:
 Theanalyticalapproachseemstobethemostcommonmethodologyinroundaboutcapacity
analysis;thereisalackofstudiesthatusestatisticalapproaches.Itismoredifficulttouse
statisticalapproachesbecausetherearefewerroundaboutsthatreachcapacity.Theanalytical
approachdoesnothavethatrequirement;itisbasedongapacceptance.
o Theanalyticalapproachneedstoincorporatethecalibrationofdriverbehaviortomatch
specificlocalconditions.
o Amorestreamlinedprocessofcollectingthedatafromlocalroundaboutscouldalsobe
consideredtostandardizethedatacollectionprocess.
 Onlyafewstudiesfocusontheimpactofbicyclesandpedestriansonroundaboutcapacity.
o Forstudiesspecificallyrelatedtoaccessmanagement,moreinformationisneeded
examininghowslowtrafficinfluencesroundaboutcapacitymodels,particularlyasrelated
todriverbehavior.
o However,thisinformationwouldbedifficulttoacquire,sinceeachroundabouthasunique
geometricandpedestriancrossingdesigns.
o Thereiscurrentlynotareliablesimulationtoolforpedestrianmovementatroundabouts.
 Studiesonunbalancedtrafficatroundaboutentrieshaveincompletedata.
o Sinceaccessmanagementistheprimarygoalofthisresearchproject,unbalancedtraffic
issuesshouldbeaddressedwithcare,sinceexistingstudiesshowunbalancedtrafficcould
haveagreatimpactonroundaboutperformanceandcanindirectlyaffectaccessto
businessesnearroundabouts.However,thedegreeoftheimpactisnotyetclear.
 Althoughsomestudiesconsidertheimpactofheavyvehiclesonroundaboutcapacity,thisimpactis
heavilydependentonlocalconditions,especiallythegeometricdesignoftheroundabouts.
o Theuseofastandardizeddesignguiderelatingvehiclecharacteristicstoroundabout
geometricdesignwouldpresentreliablestandardsforengineerstodesignroundabouts.
 Overall,therearefewstudiesexploringtheimpactsofroundaboutsoncorridors.Existingliterature
suggeststhatroundaboutsdonotperformsignificantlybetterthansignalizedintersectionsina
corridor.Roundaboutsseemedtohavehigherperformancewhenthecorridorhasirregular
intersectionspacing(KittelsonandAssociates,Inc.2013).Butwhetheracorridorofroundaboutsis
superiortoothertypesofintersectionsreallydependsonsite‐specificoperationalconditions
Roundabouts and Access Management
Page 40
Chapter2LiteratureReview
(KittelsonandAssociates,Inc.2013).Ofevenmoreinteresttoourresearch,wouldbestudiesalong
corridorswithunbalancedtrafficconditions,orhighlevelsofpedestrianorbicycletraffic,anda
before‐and‐afterstudyoftheconversionfromsignalizedintersectionsintoacorridorof
roundabouts.
2.6.3LiteratureGapsinRoundaboutSafety
Thereissubstantialagreementintheliteraturereviewedthatmodernroundaboutshavesignificantsafety
impactswhencomparedtotraditionaltrafficintersectiontreatments.Whilethesesafetyimprovements
havebeenobservedandstudiedinternationallyusingseveraldifferentmethods,gapsinthisresearchstill
exist.Basedonthereviewofliteratureonroundaboutsafety,severalgeneralgapsintheliteraturewere
identified:
 Longitudinalsafetystudiesgenerallyincludelessthantwoyearsofdata.
o Studiesshouldbemadeoverperiodslongerthantwoyears,becausethenthesafetyeffects
canbemoreclearlyidentified.
o Inthefirsttwoyearsofimplementationoradaptationperiod,usersarestilllearningthe
rulesandguidelines.
 Collectivelythelongitudinalsafetystudieslacklocationvariation.Roundaboutsinagreater
diversityofcontextsneedtobeanalyzedinlongitudinalstudies.
 Insomestudies,thelocationofmodernroundaboutsseemstohavebeenchosenbecausethose
intersectionshavehighcrashfrequencies.Thisselectionbiasweakenstheconclusionsbecauseit
canbedifficulttoknowiftheimprovementsareduetotheunsafeconditionsbeforetheconversion
toaroundabout,changesindriverbehaviorduetotheconversiontoaroundabout(i.e.,the
treatmenteffect)orwhetherthelackofimprovementisduetothedifficultyofdesigningasolution
inahigh‐crashlocation.
o Studiesshouldincorporatedifferentlocationswithdifferentcharacteristics.
 Moststudiesusedsmallsamplesizes.
o Studiesshoulduselargersamplesizes,togiveadditionalstatisticalsignificanceand
accuracy.
 Simplemethodsofbefore‐and‐afterstudiesdonotcomparetheeffectivenessofmodern
roundaboutstootherintersectionswithoutroundabouts.Inotherwords,morecarefullydesigned
controlstudiesneedtobedeveloped.
 Twomethodsthatacknowledgebothbefore‐afterandcrosssectionalconditionsareodd‐ratioand
empiricalBayes.Thesemethodshavebeendeployedindifferentcontexts,whichmaylimittheir
generalizabilitytoothercontexts.
o Theodd‐ratiomethodwasusedbyBranbanderetal.,(2005),Danielsetal.,(2008),and
Elvik(2003)instudiesthattookplaceinEurope.
o TheempiricalBayesmethodwasusedbyPersaudetal.,(2001),Retting,etal.,(2001),and
Rodegerdtsetal.,(2007)intheanalysisofroundaboutsintheUnitedStates.
o Bothmethodsusedthemeta‐analysistoenablethegroupsofcontexts:forexample,
suburbanandurban,thenumberoflegs,andtrafficflow.However,thelattermethod
incorporatesthecharacteristicsofmodernroundaboutsorothercontrolled‐intersectionsin
thepredictionmodel.Inotherwords,empiricalBayesgivesamorecompletepictureofthe
variablesthatinfluencethecrashrate.
 Rodegerdtsetal.,(2007)isthemostcomprehensivestudyusingthelargestnumberofroundabout
inthesample(310roundabouts).However,theevaluationofsafetyforagroupoflocationsthat
sharesimilarusers’characteristics,roundaboutdesign,anddriverbehavior,forexampleinone
state,maybeimportanttoenhancetheknowledgeofthesafetyofroundabouts.
 Someoftheliteratureproposesadditionaldifferentgeometriesontheroundabouts;additional
studytoaccommodatetheneedsofotherusersisanothergapinknowledge.Althoughtheresultof
Roundabouts and Access Management
Page 41
Chapter2LiteratureReview
thegeometryispromising,itmayaffectothergroupsofusersthatmightfindmorechallenging
conditionsincrossingtheroundabouts.
 SpecificallyinFlorida,theClearwaterBeachroundabouthasbeenevaluatedintensivelyto
understandpedestriansafety(Rodegerdtsetal.,2007).Althoughthislocationmaybeagood
locationtounderstandpedestrianbehaviorandsafety,itisnotnecessarilyrepresentativeof
roundaboutlocations.Additionalresearchisnecessarytodeterminehowrepresentativethis
locationisofthepedestrianconditionsatroundabouts.
Basedonthereviewofliteratureonroundaboutpedestriansafety,severalgapsintheliteratureare
identified:
 StudiesontheeffectivenessofmodernroundaboutsintheUnitedStatesexamineveryfew
locations,andthosesamelocationsareexaminedrepeatedly.Assuch,agreaternumberofsample
locationsshouldbeincorporatedintoroundaboutresearch,andagreaterdiversityofboth
pedestrian,bicyclistandlargevehicleconditionsshouldbeincorporatedintothisanalysis.
 Crashreportsandthepotentialforlocationbiasbydisabledpedestriansforcestudiestorelyupon
observationalresearchintheUnitedStates.Observationalresearchshouldbefurtherincorporated
withstatisticalresearchatlocationswithhighnumbersofpedestriansorbicyclists.
 Althoughperceivedriskandactualriskmayleadtodifferentconsequencesinthemodern
roundaboutdevelopment,knowledgeaboutperceivedriskforeachgroupofvulnerableusersis
importantforenhancingthebalanceofusers’needs.
 Understandingtheperceptionsofvulnerableusersmayhelpdesignersofthemodernroundabout
addresstheneedsofthoseusers.
 Treatmentofvulnerableusers,includingbicyclistsandpedestrians,isinconsistentthroughoutthe
differentstates.Nationaltransportationorganizationsshouldprovidegeneralguidelinesregarding
howtoincorporateallusers’needs,especiallyvulnerableusers.
Basedonthereviewofliteratureonroundaboutdesignandsafetymeasures,severalgapsintheliterature
wereidentified:
 ArndtandTroutbeck(1998)showtheimportanceofunderstandingdriverbehavior,traffic
conditions,androundaboutgeometryinonespecificlocation,andtheycompareAustraliaandthe
UnitedKingdom.Consequently,thisimpliesthatthoseconditionsaredifferentintheUnitedStates.
TheenhancementofpreviousmodelsavailabletobeappliedintheUnitedStatesorotherspecific
locationsmaybethegapofknowledge.
 Eventhoughitisacknowledgedthatmulti‐laneroundaboutsarelesssafethansingle‐lane
roundabouts,multi‐laneroundaboutsneedadditionalattentionbecausetheyareoftenusedfor
capacityreasons.Additionalresearchshouldexploretheeffectsofmulti‐laneandcomplex
roundaboutsonbothsafetyandcapacity.
 Althoughthesestudiesshowseveraldesign‐relatedinfluencesonsafetylevels,theroundabout
designshouldbalanceotherfactors,suchas,capacityandconstructioncost.Optimumbalances
betweensafety,capacity,access,andcostshouldbefurtherexplored.
Roundabouts and Access Management
Page 42
Chapter4ReviewofFederalandStatePractices
ChapterThree:Methodology
Thisresearchusedmultiplemethodstounderstandthestateofpracticeinroundaboutsandaccess
managementinthestateofFlorida.Theyincludeareviewofstateaccessmanagementandroundabout
guides,thecollectionandanalysisofcrashinformationatallroundaboutsinthestate,andtheselectionofa
samplingofroundaboutsinthestateandthecollectionofandanalysisofthefieldoperationsofthesesites.
Inaddition,areviewandanalysisofFlorida‐specificsoftwaretoanalyzethecapacityandoperationsof
roundaboutswithinthestatewillbeconducted.AsdescribedintheLiteratureReview,theanalysisofthis
informationforFloridaiscomplicatedbythelackofpreviousresearchthatspecificallyaddressesaccess
managementnearroundaboutsandtheabsenceofstandardmethodsofprovidingguidanceonaccess
managementandroundaboutsbystatedepartmentsoftransportation.
3.1AccessManagementandRoundaboutGuides’Selection.
Thereviewofnationalandstateguidancewascompletedbyreviewingtwotypesofguidance:access
managementguidesandroundaboutguides.Severalsourcesofnationalguidanceonaccessmanagement
wereidentified.Documentsthatcontainaccessmanagementelementswerefoundinthefollowingtypesof
documents:roadwayorhighwaydesign/manuals;accessmanagementmanuals;anddrivewaymanuals.
NCHRPSynthesis404,StateofPracticeinHighwayAccessManagement(GluckandLorenz,2010)is
particularlyusefulforthisresearchbecauseitincludesdataonwheretofindinformationonaccess
managementforeachstate;theinformationinthatreportisupdatedwithareviewofstatedepartmentof
transportationwebsites.Twenty‐oneDOTsincludeaccessmanagementinformationontheirwebsite.
Table4summarizesthevarioustypesofdocumentsthatstateDOTsuseasapartoftheiraccess
managementprogram.Mostwebpagescontaininformationabouttheintroductionofaccessmanagement,
theaspectsthatshouldbeconsideredinanalyzingaccessneedsofnewdevelopment,andlinkstodesign
manualsandotherrelateddocumentsusedbyDOTstaff.Forty‐threestateshaveincorporatedaccess
and/oraccessmanagementontheirplanninganddesignpolicies.Morespecifically,nineteenstateshave
accessmanagementmanuals,separatefromgeneraldesignmanuals.ElevenstateDOTsmentionaccess
managementondesignmanuals;whilesixteenotherDOTshaveadditionaldocumentswithvariousnames.
ThecompletelistandlinkstoDOTwebsitescanbefoundinAppendixB.
Roundabouts and Access Management
Page 43
Chapter4ReviewofFederalandStatePractices
Table4.MainDocumentsonAccessManagement–RelatedStateDOTGuidebooks
AccessManagement
Manual/Guidebook
Alabama(2013)
Florida(2009)
Idaho(2001)
Indiana(2009)
Iowa(2012)
Kansas(2013)
Michigan(2001)
Minnesota(2008)
Mississippi(2012)
Missouri(2003)
Nevada(1999)
NewJersey(2013)
NewMexico(2001)
Ohio(2001)
Oregon(2012)
SouthCarolina(2008)
Texas(2011)
Vermont(1999)
Virginia(2007)
Roadway/HighwayDesign
Manual
Arizona(2012)
California(2012)
Connecticut(2012)
Illinois(2010)
Massachusetts(2006)
Montana(2007)
NewYork(2002)
Utah(2007);
NorthDakota(2009)
SouthDakota(web,2013)
Washington(2012)
OtherRelatedDocuments
StateHighwayAccessCode/Manual:
Colorado(1998)
Delaware(2011)
DistrictofColumbia(2010)
Maryland(2004)
Wyoming(2005)
DrivewayManualor/andEncroachment
Control:
Georgia(2009)
WestVirginia(2004)
AccessConnectionPolicy/Rules:
Louisiana(2012)
Maine(2005)
AccessControlPolicy:
Nebraska(2006)
Washington(2009)
Wisconsin(FDM,2011)
RightofWayManual:
Utah(2006)
Montana(2007)
DrivewayPermit/Access:
NewHampshire(2000)
NorthCarolina(2003)
Source:DOTwebsites
ThereviewofmanualsandguidebooksforthisresearchissimilartothatcompletedinNCHRPSynthesis
404StateofPracticeinHighwayAccessManagement(GluckandLorenz,2010),butthisresearchreviewed
agreatervarietyofaccessmanagementdocuments;assuch,itupdatesthatreport.Oftheforty‐threestates
andtheDistrictofColumbiawithaccessmanagement‐relateddocuments,sixteenstatesandtheDistrictof
Columbiaupdatedtheirguidelinesafter2009.Asahighlight,theNCHRPSynthesis404‐StateofPractice
conductedsurveysforallfiftystatesandobtainedcomprehensiveinformationaboutaccessmanagement
programelementsbeingdevelopedbystateDOTs,suchasguidelines,generaldepartmentpolicies,and
drivewaypermitmanuals,andstandards.
Furthermore,thisreviewspecifiestheaccessmanagementtechniquesandgeometricdesignelementsthat
havebeenadoptedbymanystates.Oncethestateguidancedocumentswereidentified,theanalysisuses
thesixteencategoriesoftypicalaccessmanagementtechniquesthatareusedintheNCHRPSynthesis404:
StateofPracticeanalysis(GluckandLorenz,2010,p.49‐50):
1. Installationofthemedians
2. Spacingformedianopenings/breaks
3. Spacingforun‐signalizedpubicstreetintersections
4. Spacingforun‐signalizedprivatedriveways
5. Spacingfortrafficsignals
6. Prohibitionofcertainturningmovements,
7. Cornerclearance,and
8. Spacingforcross‐streetinthevicinityofinterchanges
9. SetbackandISD
10. Geometricdesignstandardsfordriveways
Roundabouts and Access Management
Page 44
Chapter4ReviewofFederalandStatePractices
11.
12.
13.
14.
15.
16.
Provisionsforright‐turnandleft‐turnlanes
Purchaseofaccessrights
Internalconnectionofparkinglotsbetweenadjacentparcels
Subdivisionrestrictionsforlargeparcels
Requirementsfortrafficimpactstudies
Requirementfortrafficimpactfees
Amongthesetechniques,thesynthesisreportedthat80%ofthestatesappliedthefirsttenaccess
managementtechniquesandrequirementsfortrafficimpactstudiesoftechniques(number15).The
purchaseofaccessrights(number12),wasusedby66%ofstateDOTs.Internalconnectionofparkinglots
betweenadjacentparcels(number13)andsubdivisionrestrictionsforlargeparcels(number14)areused
by48%and30%respectivelyofstateDOTs,andonly16%ofstateDOTshaveincorporatedtrafficimpact
fees(number16).Asummaryoftheuseoftheaccessmanagementelementsandtechniquesbythestates
canbefoundonAppendixC.
Nationalguidanceonroundabouts,accessmanagement,safety,andcapacity,supplementedbyahandfulof
states,whoareleadingthewayinprovidingstatewideroundaboutguidance.ThoseDOTsincluded
roundaboutguidanceinvarioustypesofdocuments.Forexample,somestatesincluderoundaboutdesign
standardsintheroadwaymanual.Somestatesprovidespecificlinkstoinformationaboutroundabout
design.TheVirginiaDOT(VDOT)placestheroundaboutdesigninformationintheaccessmanagement
designstandards;thisistheonlystatethatdirectlyprovidesthisinformationinasingleplace.Overall,26
stateshavevariouslevelsofinformationaboutroundaboutsontheirwebsites.MoststateDOTwebsites
containinformationfordriversabouthowtousearoundabout.Somestatesalsolinktotheroundabout
websiteofotherstatesandthenationalguidance.Oncetheroundaboutinformationforthe26statesand
theDistrictofColumbiawerereviewed,16statesthatrefertoaccessmanagementinthecontextof
roundaboutsintheirguidebookswereselectedforfurtherexaminationonroundabouts:Arizona,Florida,
Kansas,Indiana,Iowa,Kentucky,Maryland,Michigan,Minnesota,NewHampshire,Pennsylvania,California,
Washington,andWisconsin.SeeTable5forinformationonthelocationofstateinformationon
roundabouts.
Table5.TheSourcesofRoundaboutStates’DesignGuidebooks
RoundaboutGuide
Document
Facility
Development
Manual
Wisconsin(2011)
Florida(1996,2000,2012)
Arizona*(2003)
Kansas(2003)
Pennsylvania(2007)
California(2007)
Iowa(2008)
Michigan(2011)
Maryland(2012)
*–cannotbeaccessedonline
Access
Management
DesignStandard
Virginia(2007)
RoadwayorHighway
DesignManual
NewHampshire(2007)
Iowa(2009)
Minnesota(2009)
Kentucky(2010)
Maryland(2011)
Washington(2011)
Arizona(2012)
3.2SiteIdentification
Thefirststepinboththeoperationalanalysisandsafetyanalysiswastheidentificationofthelocationofall
roundaboutsinthestateofFlorida.TheFDOT’sRCIdatabaseincludesanelementcalled“ROTARY,”which
includesthefollowingthreecodes:roundabout,trafficcircleandmini‐roundabout.Atotalof219roadway
segmentscodedas"roundabout"wereidentifiedfromthe2011RCIdatabase.Onlyfourofthose
roundaboutswerelocatedontheon‐system(i.e.,state)roads,whiletheremaining215werelocatedonthe
Roundabouts and Access Management
Page 45
Chapter4ReviewofFederalandStatePractices
off‐systemroads.SincetheRCIdatabasedoesnotincludealltheoff‐systemroads,anextraeffortwasmade
usingGoogleEarthtovisuallyidentifyadditionalroundaboutsontheoff‐systemroadsthatarenotcovered
intheRCIdatabase.Thisnettedanadditional64locations,foratotalof283roundaboutsforthisstudy.
Foroperationalanalysis,226roundaboutsintheStateofFloridawereanalyzedbyviewingthemapusing
GoogleMap,andfinally13siteswereselectedforadetailedanalysis.Thesummaryofthe226sitesare
outlinedinthefollowingtable.
Table6.SummaryofRoundaboutsinFloridabyDesignandContext
Category
Numberoflegs
Numberofcirculating
lanes
LocationofDriveway
Surroundinglanduse
Aspects
Two
Three
Four
Five+
Singlelane
Multi‐lane
Turbo/Spiral
Atapproachlane
Ategresslane
Drivewaydirectlylinktoroundabout
Morethanonedriveway
Nodriveway
Residential
Commercial
Mixed‐use
Other
NumberofRoundabouts
3
85
122
16
164
53
9
24
33
10
128
31
100
63
54
9
3.3SafetyAnalysis
Thissectiondescribesthemethodologyusedtoconductsafetyanalysis.Itincludeshowtheroundabout
locationsinFloridaarecategorized,howcrashdataincludingbothcrashrecordsandpolicereportsforthe
locationsidentifiedwereextracted,howcrashlocationstoimprovedataqualitywerecorrected,andhow
policereportsforin‐depthsafetyanalysiswerereviewed.
3.3.1CategorizeRoundaboutLocations
Afterthe283roundaboutsinthestatewereidentified,additionalinformationsuchaslanduse(i.e.,
commercialorresidential),roundabouttype(i.e.,singleormulti‐lane),presenceofotherroundaboutsin
thevicinity,numberofapproachlegs,numberofcommercialandresidentialdriveways,presenceandtype
ofmedian,presenceofon‐streetparking,presenceofbikelanesandpedestriancrosswalksonroundabout
approachlegswascollected.Forsafetyanalysis,roundaboutswereclassifiedaseithercommercialor
residential.Commercialroundaboutsarethosethatarelocatedincommercialareasthatservemostly
commercialtraffic.Locationswithamixoflanduses,includingbothcommercialandresidential,arere‐
classifiedascommercial.Residentialroundaboutsarethosethatarelocatedinmostlyresidentialareas.
Figure13givesanexampleofeachoftwolandusetypes,respectively.
Roundabouts and Access Management
Page 46
Chapter4ReviewofFederalandStatePractices
(a)CommercialLandUse
(b)ResidentialLandUse
(Location:PierParkDrive.,PanamaCityBeach)
(Location:SW77Avenue.,Alachua)
Figure13.ExamplesofRoundaboutsLocatedinEachLandUse Type
3.3.2ExtractCrashData
Fiveyearsofcrashdatafrom2007‐2011wereusedinthisanalysis.Crashesthatoccurredinthevicinityof
theroundaboutswerespatiallyidentifiedinArcGIS10.0.Thelocationsofthe219roundaboutsidentified
usingtheRCIdatabasewereimportedintoArcGISusingtheirroadwayIDsandbeginandendmileposts.
Theremaining64roundaboutsthatwerevisuallyidentifiedwereimportedintoArcGISusingtheirlatitude
andlongitudecoordinatesobtainedfromGoogleEarth.
Shapefilesofthecrashdatafortheyears2007‐2011weredownloadedfromtheFDOTUnifiedBasemap
Repository(UBR)forbothon‐systemandoff‐systemroads.Thesefileswereseparatelyimportedinto
ArcGIS.A500ft.bufferwasthencreatedaroundeachofthe283roundabouts.Allthecrashesthatoccurred
withinthe500ft.bufferwerespatiallyidentified.Aninfluenceareaof500ft.waschosentoincludeallthe
crashesthatcouldhavebeenpotentiallyaffectedbythepresenceofroundabouts.Atotalof2,941crashes
werefoundtohaveoccurredwithin500ft.oftheroundabouts.Policereportsofallthesecrasheswere
downloadedfromtheHummingbirdwebsystemhostedonFDOT’sIntranet.
3.3.3CorrectCrashLocationsandReviewPoliceReports
Anexistingin‐houseweb‐basedtoolwasadaptedforthisstudytofacilitatetheprocessofreviewingthe
policereports.Thetoolhasthecapabilitytovisuallydisplaycrashesbycrashtypeandcrashseverity,as
showninFigure14andFigure15,respectively.Thetoolhelpstoquicklynavigatefromonepolicereportto
thenextbyeitherclickingthe“Next”and“Previous”buttons,orbyclickingonthecrashiconintheaerial
map.Thetoolalsohasthecapabilitytomovefromoneroundaboutlocationtothenext,andtonavigatetoa
specificroundaboutbasedonroadwayname.
Roundabouts and Access Management
Page 47
Chapter4ReviewofFederalandStatePractices
Figure14.CrashesDisplayedbyCrashTypeataRoundabout
Figure15.CrashesDisplayedbyCrashSeverityataRoundabout
Roundabouts and Access Management
Page 48
Chapter4ReviewofFederalandStatePractices
Afewroundaboutsdidnotexistfortheentirestudyperiodastheywereconstructedafter2006;however,
theexactconstructionperiodwasunknown.Basedontheillustrativesketchesinthepolicereports,
crashesthatoccurredatthestudylocationspriortotheconstructionoftheroundaboutswereexcluded
fromtheanalysis.
Sincetheanalysisfocusesonevaluatingtheinfluenceofaccessfeaturessuchasdriveways,median
openings,etc.,onthesafetyperformanceofroundabouts,accuratecrashlocationsarecrucial.Aquick
reviewofthepolicereportsrevealedthatthecrashlocationsareapproximate,andinsomecases,the
locationsareoffbyseveralhundredft.Toaddressthisissue,crashlocationsofall2,941crasheswere
manuallyverified.Locationsof1,191crashes(40.5%)werefoundtobeincorrectandwereupdated.For
eachcrash,thecrashlocationwasverifiedandupdatedusingthefollowingsteps:
1. IdentifytheroundaboutlocationonGoogleEarth.
2. Reviewpolicereport(s)ofthecrashtopinpointtheactuallocationwherethecrashoccurred.This
stepmightrequirereviewingboththecrashdiagramandthedescriptionfromthepolicereports.
3. ObtainlatitudeandlongitudecoordinatesofthecorrectcrashlocationfromGoogleEarth.
4. Recordthecorrectcoordinatesintheweb‐basedtool.
Oncethelocationsofallcrasheswereverifiedandrecorded,thecrashfileintheweb‐basedtoolwas
updatedbasedonthenewcoordinates.Next,allthecrashesthatdidnotoccurontheroundaboutoronan
approachlegleadingtoaroundaboutwereexcludedfromfurtheranalysis.Forexample,Figure16showsa
crashthatoccurredwithin500ft.fromtheroundabout,butdidnotoccurontheroundaboutandits
approachlegs.Atotalof1,059crasheswerenotfoundtobedirectlyrelatedtotheroundaboutsandwere
removed.Thisresultedinatotalof1,882crashesthatwereincludedinthedetailedanalysis.
Figure16.AnExampleofaCrashThatWasNotDirectlyRelatedtotheRoundabout
Forthepreliminarysafetyanalysis,potentialsafetyissuespertainingtoroundaboutsandaccessfeatures
werefirstidentifiedfromtheliteraturereview.Accordingly,thesafetyanalysisfocusedonthefollowing
fourpotentialsafetyareasassociatedwithroundabouts:
Roundabouts and Access Management
Page 49
Chapter4ReviewofFederalandStatePractices
1.
2.
3.
4.
Impactofdrivewaycornerclearancesonroundaboutsafety.
Safetyimpactofmedianopeningsinthevicinityofroundabouts.
Safetyatroundaboutsthatprovidedirectaccesstoactivitycenters.
Safetyofvulnerableroadusersincludingpedestriansandbicyclists.





K – Fatal Injury
A – Incapacitating Injury
B – Non-Incapacitating Injury
C – Possible Injury
O – Property Damage Only
Oncethecrashlocationswerecorrected,theillustrativesketchesanddescriptionsinthepolicereports
werereviewedindetailtocategorizecrashesintotheaforementionedcategoriesfordetailedanalysis.The
web‐basedtoolwascustomizedtofacilitatethisprocess.Figure17givesthescreenshotofthetool’s
interfaceusedfordatacollection.Inaddition,datafromthepolicereportswereusedtoobtaincrash
severityusingthefollowingcodes:
Figure17.DataCollectionusingWeb‐basedTool
3.4OperationalAnalysis
Thepurposeoftheoperationalanalysisistoevaluatetheperformanceofroundaboutsandidentifythe
potentialissuesrelatedtoaccessmanagement.Thissectionexplainsthemethodforselectionofstudy
sites,thecollectionofdataonroundaboutoperations(videoandsiteobservations),andtheanalysisofthe
datacollectedateachofthesites.AnevaluationofFDOT‐utilizedsoftwareisalsoincludedtoassessthe
suitabilityofthesesoftwarepackagesonanalyzingroundaboutandaccessmanagementissues.
3.4.1DataCollectionSiteSelection
UsingGoogleEarth,wevisuallyinspectedeachofthe283roundaboutstounderstandthedesign,regional
context,andaccesscharacteristicsofeachroundaboutusingthecategoriesshowninTable7.
Roundabouts and Access Management
Page 50
Chapter4ReviewofFederalandStatePractices
Table7.CriteriaforSelectingRoundaboutsforOperationalAnalysis
Category
Aspect
Type—numberoflegs
Designof
roundabout
Type—numberoflanes
Geometric
consideration
Regionallocation
context
Whetherinurbanarea
Regionalcontext
Transportationcontext
Drivewayplacement
Access
Landusetypearound
roundabout
Definition
Numberofapproachinglegs:
Arangefrom3to6legs
Numberofcirculatinglanes:
Singlelane;
Multilane;
ComplexRoundabout(Spiral,turbo)
Thegeometriccharacteristicsoftheroundabout
includes:
Mediansonapproachinglane;
SlipLanes;
Stub‐out.
Relativelocationtonearesttown
Urban,suburban,rural
Whetherornotonastatehighway;
Within1mileofinterstate;
Nearstatehighway;
Nohighwaynearby.
Inthemiddleofroundabout;
Ontheaccessapproachofroundabout;
Ontheegressapproachofroundabout;
Onbothaccessandegressapproachof
roundabout;
Nodrivewaynearby.
Residentialsingle‐familyhousing;
Residentialmulti‐familyhousing;
Commercial;
Mixed‐use.
Onceallsiteswereevaluated,asmallersetofsiteswereselectedfortheoperationalanalysisbasedonthe
followingcriteria:(1)modernroundaboutwithsplitterisland;(2)locatedinanurbanareawithsignificant
amountoftraffic;(3)havepotentialforaccessmanagementissues,e.g.,adjacentdrivewayandintersection
nearby;(4)eitheronelaneormulti‐lane;and(4)couldhaveon‐streetparkingorbeapartofaseriesof
roundabouts.
Fortheoperationalanalysis,theroundaboutlistwasnarroweddowninthreestages.First,100siteswere
selectedfromtheentirelistbymerelylookingatroundaboutgeometricdesignfeaturesandthelanduse
contextaroundtheroundabout.Then,severalteammatesfurthernarrowedthenumberdowntothirty‐
fourbasedonmorestringentcriteria,suchasselectingsiteswithlargertrafficvolume.Afterthat,each
researcherintheteamvotedfortensites,andthehighestrankedeighteensiteswerechosenforactual
visitsthroughareviewprocessthatinvolvedinternalteammeetings,discussions,andasiteselection
meetinginthestateofFloridawiththeFDOTProjectTeam.Finally,theeighteensiteswerevisited,from
whichthirteensiteswereconsideredsuitablefordatacollectionbasedonthetrafficvolumeandgeometric
designofthesites.Thefivesitesthatwereinitiallyselected,butforwhichwedidnotcollectdata,were
eliminatedbecausethereisnodrivewayincloseproximitytotheroundabout,ortheyarelocatedinalow‐
densityareawherethereisnotenoughtraffictocreatesignificantdelayandqueuingneartheroundabout.
Roundabouts and Access Management
Page 51
Chapter4ReviewofFederalandStatePractices
Amongthethirteenselectedsites,onlyoneislocatedonastatehighway.Table8showsthesummaryof
roundaboutselectionprocess.Videowascollectedfromthosethirteensites.Figure18showsthelocations
ofbothselectedroundaboutsandthepoolofroundabouts.Detailsaboutthethirteenselectedsitesare
includedinAppendixD.
Table8.SummaryofRoundaboutSelectionProcess
StepsinSelection
Number
AllRoundabouts
283
ConsideringContextofRoundabouts(e.g.,geometricdesign,landusecontext)
100
DetailedAnalysisbyprojectteam(e.g.,locationofdriveways,leveloftraffic)
34
Rankingbyeachteammemberandreviewbyprojectmanagers
18
Siteobservation‐datacollection
13
Figure18.RoundaboutsitesinFloridaSelectedforOperationalAnalysis Roundabouts and Access Management
Page 52
Chapter4ReviewofFederalandStatePractices
3.4.2DataCollection
Inadditiontothecontextdatacollectedasapartoftheselectionprocess,theoperationalanalysisof
roundaboutsrequiredthecollectionoffielddataonvehicleturningmovements,conflicts,andviolations.
Duringthedatacollection,twotechniqueswereusedtogatherinformationrequiredforoperational
analysis:siteobservationoftheflowoftrafficneartheroundabout,andvideorecordingoftheentire
intersectionfollowedbymanualextractionofvideoclipswithaccessmanagementissues.Table9
summarizesthefeaturesandtimeofdatacollectionfortheselectedsites.
Table9.SummaryofFeaturesandSurveyTimeofSelectedRoundaboutsofThirteenRoundaboutsand
DataCollectionTimesforOperationalAnalysis
County
SiteName
Data
Collection
Dateand
Time
Alachua
SW 2nd Ave. and SW
6th St.
Margate Blvd. and NW
58th Ave
Holmberg Rd. &
Parkside Dr.
Independent Dr. and S.
Laura St.
Biltmore Way and
Sagonia St.
Greenway Dr. and
Sagovia St.
NE 10th Ct. & SW
152nd Ave.
Ponce De Leon Blvd.
and Ruiz Ave.
Eagle’s Reserve Blvd.
and Dyer Blvd.
MLK Blvd. and N.
Central Ave.
Causeway Blvd. and
Mandalay Ave.
CR-210 and Mickler
Rd.
CR-707 and Ave A
4/5/13: 3:00 pm
– 5:30 pm
5/23/13: 7:40
am – 9:40 am
5/16/13: 3:25
pm – 5:30 pm
4/23/13: 11:00
am – 2:00 pm
5/15/13: 4:50
pm – 7:15 pm
5/14/13: 4:50
pm – 7:10 pm
5/13/13: 5 pm –
7:20 pm
5/21/13: 4:50
pm – 7:05 pm
4/14/13: 12:00
pm – 1:00 pm
4/5/13: 11:00
am – 12:00 pm
3/22/13: 3:00
pm – 5:30 pm
5/9/13: 1:00 pm
– 3:00 pm
5/9/13: 1:00 pm
– 3:00 pm
Broward
Duval
MiamiDade
Orange
Osceola
Pinellas
St. Johns
St. Lucie
Numberof
Circulating
Lanes
Number
ofLegs
1
4
Spiral
4
1
3
1
3
Spiral
4
1
5
1
4
Spiral
5
2
4
On both access and
egress approaches
On both access and
egress approaches
On both access and
egress approaches
On both access and
egress approaches
On both access and
egress approaches
On both access and
egress approaches
On both access and
egress approaches
On both access and
egress approaches
On the access approach
Spiral
4
On the egress approach
2
6
1
4
1
4
In the middle of
roundabout
In the middle of
roundabout
On both access and
egress approaches
Presenceof
Driveway
Duringthesitevisits,fiveactivitiestookplace.First,weverifiedthegeometricconditionsinthe
roundaboutdiagrams.Next,wereviewedthetrafficoperationsapproachingandexitingtheroundaboutby
collectingtwotofourhoursofvideodataatthepeakoperatingtimeofeachsite.Informationwascollected
onlandusesassociatedwithadjacentdrivewaysandontrafficvolumeatthelocationofaccesspoints
duringthesitevisit.
Trafficmovementwasvideotapedatall13selectedsites,andusefulvideoclipswithaccessmanagement
issueswereextractedfortheoperationalanalysis.Thecamerasforthedatacollectionateachroundabout
wereplacedbasedonthegeometricdesignanddrivewaylocationsofeachroundabout.Figure19showsan
exampleofthecameralocationforfielddatacollection.Undersomecircumstances,asshownbyCamera1
Roundabouts and Access Management
Page 53
Chapter4ReviewofFederalandStatePractices
inFigure19,inordertorecorddrivewaymovementoneachsitecameraswereputfurtherawayfromthe
roundabouttocapturetheinteractionbetweenadrivewayandtheapproachinglane.Cameras2and3are
placedinordertorecordthepedestrianflowandvehicleconflictsontheothertwoapproachlegsofthe
roundabout.
Figure19.CameraLocationofVideoRecordingforIndependentDriveandSouthLauraStreetin
Jacksonville
In order to collect enough information, data collection took place during the busiest hours of operation
(peakhours)ateachroundabout.Forexample,ifaroundaboutislocatedonamajorarterialsection,data
were collected during the usual peak hour. For roundabouts located near shopping centers, data were
collectedslightlylaterthanthepeakhouroronweekends.
3.4.3DataAnalysis
Theoperationalanalysisaimedatfindingaccessissuesrelatedtoroundabouts.Morespecifically,inthe
dataanalysis,weconsideredtheconflictpointsattheintersectionofdrivewaysandtheapproachinglane
oftheroundabouts,theimpactofthequeueontheoperationofnearbystop‐controlleddriveways,the
conflictsbetweenvehiclesandotherroadwaysusers,e.g.,bicyclistsandpedestrians,andtheimpactof
drivingviolationsontheoperationswiththeroundabouts,e.g.,pickupanddropoffinactivedrivinglanes.
Thisanalysisincludestheimpactofmedianopeningsattheapproachinglaneontheoperationoftheentire
roundabout,andthequeuingassociatedwithadrivewaythatislocatedneararoundaboutwhichmay
disrupttheoperationofeitherthedrivewayortheroundabout.Thevideoscollectedduringthesitevisits
werecarefullyreviewedtoidentifythetypesofaccessissues.
Roundabouts and Access Management
Page 54
Chapter4ReviewofFederalandStatePractices
3.4.4AssessmentofFDOTSoftwareforRoundaboutEvaluation
SoftwarepackagesusedbytheFDOTwereevaluatedtounderstandtheircapabilitytoanalyzeroundabout
operationsandcapacityand,inparticular,toaddress,issuesrelatedtoaccessmanagement.Both
deterministicsoftwareandsimulationpackageswereevaluated.Softwarepackagescurrentlyusedby
FDOT,includingHCS2010,SYNCRO,andCORSIM,arecomparedwithothersoftwarepackagesto
understandthesuitabilityofthesetoolstoevaluateaccessissues.
Examplesofanalysisofroundaboutscapacity,delayandqueue,aregivenintheanalysisinorderto
evaluateitseffectivenessinassessingroundaboutoperations.Wherethesetoolsmaybedeficient,
recommendationsaremadeonhowtoimprovethemtomakethemmoreeffectivefortheevaluationof
roundaboutsandaccessmanagement.
Roundabouts and Access Management
Page 55
Chapter4ReviewofFederalandStatePractices
ChapterFour:ReviewofNationalandStatePractices
Thischapterisorganizedintosixsections.First,thenationalandstateguidebooksforaccessmanagement
androundaboutsarereviewed.Second,Florida’sguidebooksaresummarized.Third,nationalandstate
guidebooksthathavetakenaccessmanagementintoconsiderationinthecontextofroundaboutsare
presented.Then,roundaboutlocationconsiderationguidelinesandgeometricdesignfromthenationaland
stateguidebooksarebrieflymentioned.Next,thefindingsofsafetyandoperationalanalysisof
roundaboutsarepresented.Accessmanagementissuesarediscussedwithconsiderationofsafetyand
operationalaspectsofroundabouts.Thischapteralsoincludesadetaileddiscussionofthelimitationsof
Florida’sroundaboutguidebooks.
4.1NationalandStateGuidebooksforRoundaboutsandAccessManagement
Tounderstandthestate'sroleinroundaboutdesignandaccessmanagement,weidentifiedexisting
roundaboutpoliciesandguidanceatthenationallevelaswellasinall50statesandtheDistrictof
Columbia.Inthissection,thenationalandstatereportsandguidesforroundaboutsandaccess
managementidentifiedinthemethodologysectionareanalyzed.
4.1.1NationalGuidanceforAccessManagement
TheprimaryauthorityonaccessmanagementintheUnitedStatesistheTRBAccessManagement
Committee(AHB70).TheTRBAccessManagementCommitteealongwithFHWAandFDOTpublishedthe
AccessManagementManualin2003asacomprehensiveresourceonstate‐of‐the‐artpracticesfortheuse
ofpractitionersandstakeholdersaffectedbyaccessmanagementactions.BesidestheAccessManagement
Manual,alimitednumberofguidesorinformationalreportsexistatboththenationalandstatelevelsthat
includeaccessmanagementprinciples;evenfeweraddressaccessmanagementprinciplesinthecontextof
roundabouts.BasedupontheirlistingontheFHWAwebsite,thedocumentsbelowarereviewed.The
documentsarepresentedinreversechronologicalorder.
 APolicyonGeometricDesignofHighwaysandStreets(GreenBook),6thEdition,AASHTO,2011.
 NCHRPSynthesis404:StateofPracticeinHighwayAccessManagement(GluckandLorenz,2010).
 NCHRPReport548:AGuidebookforIncludingAccessManagementinTransportationPlanning
(Roseetal.,2005).
 NCHRPSynthesis351:Accessrights:asynthesisofhighwaypractice.(HuntingtonandWen,2005).
 NCHRPReport524:SafetyofU‐turnsatUnsignalizedMedianOpenings(Potts,2004).
 NCHRPSynthesis337:CooperativeAgreementsforCorridorManagement(Williams,2004).
 TRBAccessManagementManual(TRB,2003).
 NCHRPSynthesisofHighwayPractice332:AccessManagementonCrossroadsintheVicinityof
Interchanges(ButoracandWen,2002).
 NCHRPSynthesis304:DrivewayRegulationPractices(Williams,2002).
 NCHRPReport420:ImpactsofAccessManagementTechniques(Gluck,Levinson,andStover,
1999).  NCHRPReport395:CapacityandOperationalEffectsofMidblockLeft‐TurnLanes(Bonnesonand
McCoy,1997).
 NCHRPReport348:AccessManagementGuidelinesforActivityCenters(KoepkeandLevinson,
1992).
4.1.1.1APolicyonGeometricDesignofHighwaysandStreets(GreenBook),6thEdition,AASHTO,
2011.Thisbookcontainstenchapters:highwayfunctions,designcontrolsandcriteria,elementsofdesign,
cross‐sectionelements,localroadsandstreets,collectorroadsandstreets,ruralandurbanarterials,
Roundabouts and Access Management
Page 56
Chapter4ReviewofFederalandStatePractices
freeways,intersections,andgradeseparationsandinterchanges.Sectionsthatdiscussaccessmanagement
arethehighwayfunctions(chapter1),accesscontrolandaccessmanagement(section2.5),elementsof
design(chapter3),ruralandurbanarterials(chapter7),typesandexamplesofintersections(section9.3),
androundaboutdesign(section9.10).Roundaboutsandthetypesofroundaboutsaredefinedinsection
9.3.Section9.10includesaspectsofroundaboutgeometry,sizeandspaceneeds,andfundamental
principles(speeds,lanebalanceandcontinuity,appropriatenaturalpathalignment,designvehicle,non‐
motorizedusers,andsightdistanceandvisibility).Sightdistance,asoneoftheaccessmanagementaspects,
coverstwotypes,SSDandISD.
Thisdocumentprovidesgeneralinformationontheuseofaccessmanagementmeasuresforalltypesof
roadwaysforallcontextsincludingroundabouts,butitdoesnotspecifyanymeasurethatisappliedonlyto
roundabouts.Detaileddesignstandardsareprovidedforlocalruralroads,localurbanstreets,special‐
purposestreetssuchasrecreationalroadsandresourcerecoveryroads,collectors,arterials,andfreeways
(Chapters5through8).Geometricdesignelementsincludesightdistance,vertical,andhorizontal
alignment.Sightdistancefeaturesaredescribedfordifferenttypesofintersections,includingthree‐legand
four‐legwithandwithoutchannelization,androundabouts.Frontageroadsarealsoexploredbecausethey
impactadjacentpropertiesaturbanarterialsorfreewaysthatdonothavedirectaccessduetoaccess
controls.
4.1.1.2NCHRPSynthesis404:StateofPracticeinHighwayAccessManagement(GluckandLorenz,
2010).Thisdocumentprovidesacompletereviewofaccessmanagement,withtheaimofreviewing
currentadministrationandpracticesinall50states.Surveyswereconductedatall50stateagencieswitha
100percentresponserate.Thesurveyscoverthecontentofpoliciesandprograms,program
implementation,anditsreportedeffectiveness.Thereviewincludedaccessmanagementprogramsinthe
statesofVirginia,NorthCarolina,Indiana,Minnesota,Oregon,Louisiana,California,andNewJersey,as
specificexamplesofcurrentpractices.
Basedonthesurveyresults,moststateshaveutilizedaccessmanagementpractices,withtwo‐thirdsof
thosekeepingtheformalprograms.Accessmanagementprogramsarecommonlyusedonthedriveway
permitlevel(92%),theprojectlevel(78%),thecorridorlevel(64%)andthestatewidelevel(60%).The
mostimportantaspectofimplementingaccessmanagementprogramsincludeastrongorganizational
commitment.Meanwhile,thebarrierstoimplementationarepoliticalresistance,humanandfunding
resources,andorganizationalandinstitutionallimitations.“Othercommonbarrierscitedincludedalackof
educationandtrainingopportunities,resistancebythedevelopmentcommunity,limitedcoordinationwith
localgovernments,legalissues,andalackofvision”(pp.106,GluckandLorenz,2010).Inaddition,this
synthesisgivescompletelinkstoallaccessmanagementdocumentsmaintainedbythestateDOTsand
individualresearchers.Inconclusion,thisresearchpresentsaspectsofaccessmanagementthatmay
contributetoprogramsuccess.Theseelementsincludeastrongaccessmanagementauthority,a
frameworkforanaccessclassificationsystem,anaccesscommittee,anaccountableanddedicatedstafffor
accessmanagement,accesschampions,alegalcasehistory,casestudies,educationandtraining,outreach
totheaffectedparties,stakeholderscooperation,astatewidemasterplan,andhavingmonitoringand
evaluationprogramsinplace.
4.1.1.3NCHRPReport548:AGuidebookforIncludingAccessManagementinTransportation
Planning(Roseetal.,2005).Thisreportdescribesbestaccessmanagementpracticesforhighway
systemsacrossthecountry,andoffersguidanceonincludingaccessmanagementintransportation
planning.Thereportidentifiesseveralbenefitsofaccessmanagement,suchasincreasedsafetyforvehicles
andpedestrians,environmentalefficiency,accesstoproperties,protectionofphysicalintegrity,
coordinationbetweenlanduseandtransportation,andprotectionoftheintendedaccessfunctionstateand
regionalroadways.Itisaguidancedocumentfortheimplementationofaccessmanagementelementsona
generalscalefortransportationplanninganditrecognizesdifferentformsandstylesofaccess
Roundabouts and Access Management
Page 57
Chapter4ReviewofFederalandStatePractices
managementacrossthecountry.Thereportisorganizedroundthetypeoftransportationplan;for
example:overallplanning,long‐rangeplans,andcorridorandsub‐areaplanning.Therefore,itisevident
thatthebroadrangeofvariablesandthecontext‐dependentnatureofaccessmanagementhaveresultedin
fewinvestigationsatalocallevelorcasestudieswithspecificexamples.
4.1.1.4NCHRPSynthesis351:Accessrights:asynthesisofhighwaypractice.(HuntingtonandWen,
2005).Thepurposeofthisstudyistounderstandon‐goingpracticesofhighwayaccessmanagement.A
nationalsurveywasconductedwithfollow‐upinterviewstoexplorethreespecificconcernsaboutaccess
rights:acquisition,management,anddisposal.ThreecasestudieswereselectedinMontana,Ohio,and
Oregontoexploretheon‐goingpracticeofaccessmanagement.Whiletheacquisitionofcompleteaccess
controlhasbeenasuccessfulmethodinreducingcurrentandfutureaccesstoaroadway,effortsto
implementpartialaccesscontrolhavenothadsimilarsuccessinsomeagencies.Inthatregard,engineering
andplanninganalysisisrequiredtoplaceboththedrivewaysandtheattachedaccesscontrolforthose
driveways.
4.1.1.5NCHRPReport524:SafetyofU‐turnsatUnsignalizedMedianOpenings(Potts,2004).This
reportcontainstheguidelinesforevaluatingvariousdesignsofunsignalizedmedianopeningsbasedon
safetyandoperationalperformance.Withthefocusonurban/suburbanarterials,thisresearchcategorizes
medianopeningsinto17typesofmedianopeningdesignsandperformsfieldstudiesat26urbansitesand
12medianopeningsonruralarterials.Inaddition,thisreportpresentsthecurrentdesignpoliciesand
practicesofhighwayagenciesobtainedfrommailsurveysof35stateand30localhighwayagencies.Crash
ratesatU‐turnandleft‐turnmaneuversatunsignalizedmedianopeningsarelow.Morespecifically,the
averageofU‐turnplusleft‐turnaccidentspermedianopeningperyearaturbanarterialcorridorsis0.41,
andthesameaverageatruralarterialcorridorsis0.20.Thisstudyrecommendsthatthemidblockmedian
openingsbetakenintoaccountasanoptionforeitherthreeorfour‐legintersections.Also,thecombination
ofdirectionalmedianopeningsanddirectionalmidblockmedianopening(s)maybeconsideredasan
optiontoconventionalmedianopeningsatthreeorfour‐legintersections.
4.1.1.6NCHRPSynthesis337:CooperativeAgreementsforCorridorManagement(Williams,2004).
Thisresearchfocusesoncooperativeagreementsbetweentwoormoreagenciesforcorridormanagement.
Theresearchexaminesongoingpracticesincooperativeagreementsbylookingatsurveysfrom22
agenciesatbothstateandprovinciallevels.Fivecaseswereselected:Arkansas,Wyoming,Colorado,
FloridaandCalifornia.Reviewsofthesecooperativeagreementsinclude:resolutions,memorandumsof
understanding,intergovernmentalagreements,public‐privateagreements,andelementsofcorridor‐
managementagreements.Issuesfoundoncooperativeagreementsforcorridormanagementincludethe
agencies’lackofunderstandingaboutcorridormanagement,alackofagencyleadershipincorridor
management,andoppositionfromthelocalcommunityornopublicacceptance.Intermsof
implementation,theproblemsarelocalcommitment,legalandpoliticalconcerns,andcallsfortechnical
assistance.
Toreacheffectiveagreements,everyaffectedstakeholdershouldcompromiseandinteractwithothersas
equalpartnersandconsiderinputfromallagenciesontheprocessesneededtoimplementthesuggested
agreement.Commonvision,anintegratedpointofviewforcorridormanagement,andthewillingnessof
thosestakeholderstoworktogethertowardsthesamevision,maybuildthefoundationforeffective
corridormanagement.
4.1.1.7TRBAccessManagementManual(TRB,2003).Thismanualexploresthegeneralbenefitsof
managingaccesstoroadways,explaininghowaccessmanagementcanbeachieved,itsaspectsand
principles,aswellastherolesofvariousinstitutionsinaccessmanagement.
Roundabouts and Access Management
Page 58
Chapter4ReviewofFederalandStatePractices
Accessmanagementaffectssafety,operations,economicfactorsrelatedtotheretailorcommercialmarket
andpropertyvalues,landuse,andtheenvironment.Severalstudiesmentionedinthisreportshowedthat
thecrashrateisreducedasthenumberofaccesspointspermileisreduced,whenthereisaraisedmedian,
andwhenU‐turnsareaccommodatedinsteadofdirectleftturns.Thismanualalsoincludesasummaryof
researchonthesafetyandoperationaleffectsofAccessManagementTechniques(TRB,2003,p.19).
Furthermore,itshowsthatbusinessowners’concernsabouteconomicdownturnareinsignificant,since
left‐turnrestrictionsinTexasandmedianchangesinFloridadidnotaffectthebehaviorofregular
customers.Accessmanagementmayinfluencethesurroundingmarketareasandpropertyvalues.Even
commercialstripswithoutproperaccessmanagementmayincreaseinpropertyvalue.Furthermore,
accessmanagementmayhelptosustaineconomicdevelopmentinanarea.Nevertheless,thesamearea
mayexperienceeconomicdeclineifpooraccessmanagementisemployed.Lastly,landuseand
environmentaleffectsofaccessmanagementincludeaesthetics,unificationofactivitycenters,maintaining
thecapacityofavailableroadways,minimizingtheenvironmentalimpactofindividualaccessroads,and
moreefficientfuelconsumption.
Threebasicstepsinimplementingaccessmanagementtoaroadwayaredefiningaccesscategories,
establishingaccessmanagementstandards,andassigningcategoriestotheroadwaysorroadway
segments.Initialfactorstobeconsideredarethedegreeofroadwayimportance,roadwaycharacteristics,
landuseandgrowthmanagementobjectives;andthecurrentandpredictedflowsofgeneraltransitaswell
aspedestrianandbicycletraffic.Fourgeneralaspectsofdevelopingaccessmanagementstandardsinclude
medians,degreeofurbanization,speed,andsafety(TRB,2003,p.71).Finally,theassignmentofcategories
inroadwaysystemsneedstotakeintoaccountthefollowingfactors(p.77):
 Theintendedfunctionoftheroadwayasacomponentofacompletetransportationsystem
network;
 Theroadwaysegment’senvironment(ruralandundeveloped,urbanfringe,sub‐urban,urban,and
denselydevelopedorurbancore);
 Theavailabilityofasupportingroadwaysystemtosupplyalternativeaccess;and
 Thedesiredorappropriatebalancebetweensafetyandfrequencyofaccess.
4.1.1.8NCHRPSynthesisofHighwayPractice332:AccessManagementonCrossroadsintheVicinity
ofInterchanges(ButoracandWen,2002).Thisdocumentreviewscurrentpracticesinaccesslocation
anddesignofcrossroadsinthevicinityofinterchanges.Eightcasestudieswereselected—threefornew
interchangesandfiveforretrofitinterchanges.Varyingdegreesofaccessmanagementonthecrossroadsin
thevicinityofinterchangesareemployedbystateandprovincialagencies.Therespondingagenciesinnine
outof36stateshavelegislativesupportfortheaccessspacingstandard,byadoptingthoseintoregulations.
Inthisdocument,itismentionedthateventhoughagenciescouldusedifferentfactorsindetermining
accessspacingrequirements,anumberofthemwereestablishingaspacingof100ft.forurbanand300ft.
forruralinterchangesfollowingthe1991AASHTOrecommendations.Inpractice,theaccessspacing
standardsforcrossroadsrangefromzeroto1,320ft.,withonlyhalfoftheagencieshavingdetailed
methodologyforcalculatingtheactualdistance.Agenciesusefourdifferentreferencepointstomeasure
theaccessspacingdistancetothenearestdownstreamintersection.
Importantfactorsthatcontributetothespacingdistanceandappropriatecrossroadlocationsare:turning
movementcomplexity,designspeed,surroundinglanduseandenvironment,crossroadclassification,and
levelofinterchange.Otherfindingsarerelatedtoissuesonputtingaccessmanagementintopractice.
Barrierstoaccessmanagementimplementationcouldbeconqueredbyhavingconsistentaccess
managementpolicies,integratingtheprocessofplanning,designing,andoperating,aswellasreservingthe
interchangefacilitiesandthedownstreamaccesslocationpointsonthecrossroads.
4.1.1.9NCHRPSynthesis304:DrivewayRegulationPractices(Williams,2002).Thisresearch
examinesstateandlocalagencies’surveysfortheirdrivewaypolicies.Alongwithaliteraturereviewabout
Roundabouts and Access Management
Page 59
Chapter4ReviewofFederalandStatePractices
driveways,thefollowingobjectivesarepresented:(1)reviewthecurrentpracticeofdrivewaysregulations,
(2)presentstateandlocalpracticeregardingdrivewayregulations,(3)determinetheimpactofthe
drivewayregulations,and(4)findtheissuesandlessonslearnedfromthecases.Suggestionsforeffective
drivewayregulationsincludehavingconsistentdecisionsandenforcement,apre‐applicationprocess,
strongstatutoryauthority,up‐to‐datedesignstandards,andfieldreviews.Otherimportantaspectsare
stakeholders’activecommunicationsandcoordination,competentstaffs,andpubliceducationofdriveway
regulations.
InNCHRPSynthesis304,specificdistancesfordrivewaysareprovidedforSouthCarolina.More
specifically,atSouthCarolina,theaccessspacingstandardsdependontheoperatingspeed.Thespace
betweentwodrivewaysissettoaminimumof100ft.foroperatingspeedsof30mphorlessandtoa
minimumof350ft.betweendrivewaysonroadswithspeedsof55mphormore.Thesestandardsmaybe
modifiedtoaccommodateuniquecasesbutspacelessthan40ft.betweentwoone‐waydrivewaysis
nowhereallowed.ThisdocumentreferstodrivewaywidthfortheWashingtoncountyinOregonwherea
residentialdrivewaymustbebetween12and24ft.wide,unlessspecialpermissionisobtainedfor
increasingthewidthandacommercialdrivewayshouldbebetween15and40ft.wide.
4.1.1.10 NCHRPReport420:ImpactsofAccessManagementTechniques(Gluck,Levinson,and
Stover,1999).Thisreportfocusesonthemethodsforevaluatingparticularaccessmanagement
techniquesintermsofsafetyandtrafficoperations.Thisresearchidentifiesavailabletechniques,and
collectsandanalyzesthemethodsanddatafromvarioussources.Theprioritiesforaccessmanagement
analysisare:
1. Trafficsignalspacing
2. Unsignalizedaccessspacing
3. Cornerclearancecriteria
4. Accessseparationatinterchanges
5. Medianalternatives
6. Left‐turnlanes
7. U‐Turnsasalternativestodirectleftturns
8. Right‐turnlanes
9. Typesofdriveways
10. Frontageroads
Thisreportreachesseveralconclusions.Crashratesarehigherwheresignaldensityishigher,orwhere
un‐signalizedintersectionsaremorecloselyspaced.Safetyandoperationsaspectsarebetterifthereis
morecornerclearance.Safetyisalsoassociatedwithraisedmedians.Left‐turnstoragelanesupgrade
safetyandcapacitybyprovidingspacesforturningvehicles.Indirectleft‐turnsorU‐turnsmayimprove
safety,capacityandtraveltime.Problemscanexistiffrontageroadsarelocatedtooclosetotheramp
terminal.Frontageroadsalongfreewaysmayneedtobeallocatedproperlytodecreasearterialleftturns,
weavingmovements,andenhancetheaccess.
4.1.1.11NCHRPReport395:CapacityandOperationalEffectsofMidblockLeft‐TurnLanes
(BonnesonandMcCoy,1997).Thisresearchprovidesamethodologytoevaluatemidblockleft‐turn
treatmentsandtheguidelinestoselecttheappropriateraised‐curbmedians,two‐wayleft‐turnlanes,and
undividedcrosssectionsalternativesforintersections.Threemodelswereevaluated:theoperationmodel,
safetymodel,andaccessimpactmodel.Datatobuildthemodelscamefrom32fieldstudiesineightcities
andfourstates,alongwithinformationobtainedfromtheinterviewsof165businessownersandmanagers
withbusinessesalongfourarterialsinfourcitiesandthreestatesand117additionaltrafficsimulationruns
toobtainmoretrafficdata.Whilethisresearchwascompletedneartraditionalsignalizedandunsignalized
Roundabouts and Access Management
Page 60
Chapter4ReviewofFederalandStatePractices
intersections,theconcernsraisedheremaybeapplicabletomid‐blockleft‐turntreatmentsnear
roundabouts.
Thisresearchfocusesonthetwotreatments—anundividedcrosssectionandtwo‐way‐left‐turnlanes
(TWLTL).Importantfindingsfromthisresearchinclude:(1)decreasingperformanceofunsignalized
intersectionswhentheproximitybetweenintersectionsiscloser,(2)anundividedcrosssectionmaygive
moredelaythantheraised‐curbmedianandTWLTL,(3)whenthedemandis40,000vehiclesperdayor
less,anyoftheleft‐turntreatmenttypesperformswithoutcongestion,(4)safetyanalysisshowshigher
frequencyofcrashesonstreetsegmentswithhighertrafficdemandsanddenserdrivewaysandpublic
streets,(5)fieldstudiesshownochangeintheprovidedaccesstoadjacentpropertiesaftertheretrofitof
left‐turntreatment,(6)businessownersbelievethatchangingfromanundividedcrosssectiontoeither
330‐ft‐openingsofraised‐curbmedianorTWLTLmayenhancebusinessandtrafficconditions;meanwhile,
theyalsobelievethat660‐ft‐openingsmaynotimprovethoseconditionsifthechangingoccursfrom330‐
ft‐openingsofraised‐curbmedianorTWLTL,and(7)businessownersconsiderthatcustomershold
serviceorqualitytobemoreimportantthanpropertyaccess.
4.1.1.12NCHRPReport348:AccessManagementGuidelinesforActivityCenters(Koepkeand
Levinson,1992).Thisreportprovidestheaccessmanagementguidelinesforactivitycenter.Althoughit
focusesonaccessmanagementnearactivitycenters,theprinciplesdiscussedinthisdocumentcanbemore
generallyapplicabletotheuseofaccessmanagementinothercontexts.Overall,thepurposeofaccess
managementis“topreservethefunctionalintegrityandoperationalviabilityoftheroadsystem(p.1)”.
Takingthemaindefinitionofaccessmanagementas“theprocessthatprovidesormanagesaccesstoland
developmentwhilesimultaneouslypreservingtheflowoftrafficonthesurroundingroadsysteminterms
ofsafety,capacityneeds,andspeed”(KoepkeandLevinson,1992,p.1),thisdocumentconsidersthreekey
elementsforaccessmanagement:(1)specifyingthecontrolaccesswithvariousroadwayclassifications,(2)
identifyingamethodtohavespecialpermissiononceitwasdeterminedthatproperaccesscouldnotbe
built,and(3)findingwaystoimplementthestandards.
Thedocumentpresentstherevisedguidelinesformanagingaccessonstreetsandhighwaysinthevicinity
ofactivitycenters.Theinformationprovidedwasobtainedbyinterviewingstateandlocalgovernment
officials,aswellasactivitycenterdevelopersandmanagers.Thisreportdiscussesthebenefitsofaccess
managementincludingreducingdevelopmentcostsandincreasingsafety.Thetenchaptersofthis
documentfocusonthebroadguidelinesforbuildingupaccessmanagementprograms.Intheend,this
documentproposesthatprogramsshouldhaveproperaccessmanagementcodesthatincludeaccess
controlandspacingcriteria;designstandards;andtrafficpermitproceduresandrequirements.
4.1.2States’GuidanceforAccessManagement
StateDocumentsthatrefertoaccesselementsareroadwayorhighwaydesign/manuals,access
managementmanualsanddrivewaymanuals.Thelisteddocumentscanbefoundanddownloadedfrom
stateDOTwebsitesaboutAccessManagementandfromNCHRPSynthesis404,StateofPracticeinHighway
AccessManagement(GluckandLorenz,2010),whichincludesinformationonwheretofindeachstate
documentonaccessmanagement.Twenty‐oneDOTsincludeaccessmanagementdocumentsontheir
websites.ThecompletelistandstateDOTwebsitelinkscanbefoundinAppendixB.Mostwebpages
containinformationaboutaccessmanagement,andtheaspectsthatshouldbeconsidered.Thewebsites
alsoincludelinkstodesignmanualsandotherrelateddocuments.
Roundabouts and Access Management
Page 61
Chapter4ReviewofFederalandStatePractices
4.1.2.1 AccessManagementGuidelines.Table10showsthatstateDOTshavevarioustypesof
documentsmentioningaccessmanagement.Forty‐threestates,includingtheDistrictofColumbia,have
incorporatedaccessand/oraccessmanagementintotheirplanninganddesignpolicies.Morespecifically,
19stateshaveaccessmanagementmanuals,separatefromgeneraldesignmanuals.ElevenstateDOTs
mentionaccessmanagementordesignmanuals,whileanother16DOTshaveotherrelateddocumentswith
othernames.ThelinkstothosedocumentscanbefoundinAppendixB.
Table10.MainDocumentsoftheAccessManagement‐RelatedStateDOTsGuidebooks
AccessManagement
Manual/Guidebook
Alabama(2013)
Florida(2009)
Idaho(2001)
Indiana(2009)
Iowa(IowaDOT,2012)
Kansas(2013)
Michigan(2001)
Minnesota(2008)
Mississippi(2012)
Missouri(2003)
Nevada(1999)
NewJersey(2013)
NewMexico(2001)
Ohio(2001)
Oregon(2012)
SouthCarolina(2008)
Texas(2011)
Vermont(1999)
Virginia(2007)
Roadway/HighwayDesign
Manual
Arizona(2012)
California(2012)
Connecticut(2012)
Illinois(2010)
Massachusetts(2006)
Montana(2007)
NewYork(2002)
Utah(2007);
NorthDakota(2009)
SouthDakota(web,2013)
Washington(2012)
OtherRelatedDocuments
StateHighwayAccessCode/Manual:
Colorado(1998)
Delaware(2011)
DistrictofColumbia(2010)
Maryland(2004)
Wyoming(2005)
DrivewayManualor/andEncroachment
Control:
Georgia(2009)
WestVirginia(2004)
AccessConnectionPolicy/Rules:
Louisiana(2012)
Maine(2005)
AccessControlPolicy:
Nebraska(2006)
Washington(2009)
Wisconsin(FDM,2011)
RightofWayManual:
Utah(2006)
Montana(2007)
DrivewayPermit/Access:
NewHampshire(2000)
NorthCarolina(2003)
Source:CompilationfromDOTwebsites
TheformatofthesemanualsandguidebooksissimilartotheNCHRPSynthesis404,StateofPracticein
HighwayAccessManagement.However,thisreportupdatestheNCHRPSynthesisreport,whichwas
completedin2010,becausemanystatespreparedorrevisedtheirguidelinesaftertheNCHRPstudy.Ofthe
43statesthathaveaccessmanagement‐relateddocuments,16stateguidelines,includingWashingtonDC,
weredevelopedduringorafter2009.Asahighlight,StateofPracticeconductedsurveysofall50statesand
obtainedcomprehensiveinformationaboutthestateDOTprogramelements.Thesurveyresponsesare
showninAppendixC(GluckandLorenz,2010,p.47).Incontrast,thisresearchexploresDOTwebsitesand
locatesaccessmanagementdocumentsandresourcesonthosesites.
4.1.3NationalandStateGuidebooksforRoundabouts
NationalGuidebooks.Severalnationalguidebookswerewrittenaboutroundaboutsastheybecamemore
popularandgainedsupportfromdesignersandcommunitiesaroundthecountry.Thefirsthighwayguide
forroundaboutswaswrittenbyFHWAinthelate1990s.BoththeAASHTOPolicyonGeometricDesignof
HighwaysandStreets(2011)andtheFHWARoundabouts,AnInformationalGuide(Robinsonetal.,2000)
providethecurrentnationalstandardondesignguidelinesforroundabouts,aswellasallothertraffic
Roundabouts and Access Management
Page 62
Chapter4ReviewofFederalandStatePractices
engineeringanddesignaspectsacrossthecountry.Othernationalguidebooksandreportsthatgovern
roundaboutdesignintheUnitedStatesincludethefollowingNCHRPreports:
 NCHRPReport672:Roundabouts:aninformationalguide.Vol.672,(Rodegerdtsetal.,2010).
 NCHRPReport674:CrossingSolutionsatRoundaboutsandChannelizedTurnLanesforPedestrians
withVisionDisabilities,(Schroederetal.,2008).
 NCHRPReport572:RoundaboutsintheUnitedStates.Report572,(Rodegerdtsetal.,2007).
 NCHRPSynthesis264:ModernroundaboutpracticeintheUnitedStates,(Jacquemart,1998).
4.1.3.1NCHRPReport672:Roundabouts:aninformationalguide.Vol.672,(Rodegerdtsetal.,
2010).Thissecondeditionoftheroundaboutguideiscomprehensive,coveringplanning,operation,safety,
geometricdesign,trafficdesignlandscaping,andsystemconsiderations.Inonesectiononplanning,this
documentcomparesoperationalperformancefromtheroundaboutswithintersectioncontrols,suchas
TWSC,AWSC,andsignalcontrol.Theoperationsectionincludescapacityandperformanceanalysisof
trafficoperation,e.g.degreeofsaturation,delay,queuelength,andfieldobservation.Specificallyfor
geometricdesign,thisdocumentexplainshowtodesignroundaboutswith:
 Designspeed;
 Vehiclepaths;
 Inscribedcirclediameter;
 Designvehicle;
 Non‐motorizeddesignusers,entrywidth(tapperlength,additionallanelength,andflarelength);
 Circulatoryroadwaywidth;
 Centralisland;
 Entrycurvesandexitcurves;
 Pedestriancrossinglocationandtreatment;
 Splitterisland;
 Stoppingsightdistance(SSD);
 Intersectionssightdistance;
 Verticalconsideration(profiles,super‐elevation,anddrainage);
 Bicycleprovisions;
 Parkingandbusstoplocations;and
 Right‐turnbypasslanes.
Thesedesignstandardsarespecifiedfordouble‐laneroundaboutsandruralroundabouts.Specificdesigns
includeentrycurves,andexitcurvestoavoidpathoverlapindouble‐laneroundabouts;visibility,curbing,
splitterisland,andapproachcurvesforruralroundabouts.Additionally,theseguidelinesexploremini‐
roundabouts,whicharenotincludedinthisresearch.
Inthesafetysection,thisdocumentreviewsconflictpointsfordifferentusers,andcommoncrashtypesin
roundabouts.Signage,pavementmarkings,illumination,workzonetrafficcontrol,andlandscapingare
exploredinthesectionontrafficdesignandlandscaping.Inthelastsection,systemconsiderationsfocuson
trafficsignalsatroundabouts,at‐graderailcrossings,closelyspacedroundabouts,roundabout
interchanges,roundaboutsinanarterialnetwork,andmicroscopicsimulation.
However,thisdocumentdoesnotexplorehowroundaboutscanaccommodatelargevehiclesorhowto
designthemwithmorethantwoentrylanes.Itdoesnotincludeinformationaboutspecific“legalorpolicy
requirementsandlanguage.”ThisreportistheonemostfrequentlyadoptedbystateDOTsfortheir
roundaboutdesignorguidedocuments.
4.1.3.2NCHRPReport674:CrossingSolutionsatRoundaboutsandChannelizedTurnLanesfor
PedestrianswithVisionDisabilities,(Schroederetal.,2008).Thisdocumentdiscussesthesafetyof
Roundabouts and Access Management
Page 63
Chapter4ReviewofFederalandStatePractices
roundaboutsandchannelizedintersectionsforpedestrianswithvisiondisability.Theauthorsconducted
thestudyusinganexperimentaldesign(beforeandafter)fortreatmentinstallations,pedestrianmodels,
andsimulation.Treatmentsforpedestriansincludedthepedestrian‐actuated,flashing‐yellowbeacon,and
on‐pavementsoundstripsforvisually‐impairedpedestrians.Thestudytookplaceonsingle‐laneand
double‐laneroundabouts.TheformerwereinCharlotte,NC;Raleigh,NC;andGolden,CO,andthelatterin
Golden,CO.Thestudyincludesmeasuresforcrossingopportunity,utilizationofcrossingopportunity,
delay,andsafety.Oneoftheconclusionsisthatdelayisreducedafterthetreatmentforsingle‐lane
roundabouts.Inotherwords,accessibilityforpedestriansisimproved.However,thetwo‐laneroundabout
ischallengingandmaynotbeaccessibleforpedestrianswithvisiondisability.
4.1.3.3NCHRPReport572:RoundaboutsintheUnitedStates.Report572,(Rodegerdtsetal.,2007).
Themainpurposeofthisresearchwastodescribethemethodsofpredictingsafetyandoperationalaspects
ofroundabouts.Inaddition,thisreportalsomodifiedthedesigncriteriarelatedtothesafetyand
operationsofroundabouts.Thedocumentincludesfourmainsections:safetyperformance,operational
performance,geometricdesign,andpedestrianandbicyclistobservation.
InadditiontoanalyzingtheapplicabilityofvariouscrashpredictionmodelstotheUnitedStates,this
documentinvestigatessafetyperformanceofroundaboutsusinganempiricalBayesbefore‐after
procedure.ThisstudyfoundlargesafetyimprovementsfromconvertingTWSCandsignalizedintersections
intoroundabouts,butfoundnosafetyimprovementcomparedtoAWSCintersections.Additionally,safety
improvementsforsinglelaneroundaboutsweregreaterthanmulti‐laneroundabouts.Thisstudyalso
foundthatruralroundaboutshadgreatersafetyperformancethanurbanorsuburbaninstillationsandthat
anysafetybenefitdeclinedwithincreasesinAADT.
Next,theoperationalperformancereviewincludedentrycapacityandcontroldelaymodelsforone‐lane
andmultilaneroundabouts.Ingeneral,thisstudyfoundthatexistingmodelsdoapoorjobofestimatingthe
capacityforroundabouts.Tocorrectfortheseerrors,theauthorsproposeaseriesofcapacitymodelsthat
aremoreeffectivethanexistingmodelswithcalibration.However,controldelaymodelswerefoundtobe
effective.ThisstudyconcludesthatLOScriteriaforroundaboutsaresimilartothoseatunsignalized
intersections.
Furthermore,aspectsofdesignthatmaybeimportanttoconsiderare:accelerationanddecelerationeffects
onspeeds,ISD,anddesigndetailonmultilaneroundaboutssuchasvehiclepathalignment,lanewidth,and
driverinformationregardinghowtouselanemarkings.Moreover,thisstudydidnotfindanysignificant
effectsofsafetyforpedestriansandbicyclists.Inaddition,thereisconcernaboutthedesignofexitlanesto
increasetheawarenessofpedestriansincrosswalks.Multilaneroundaboutdesignshouldcarefullyavoid
pathoverlap,andcrosswalkvisibilityneedstobecarefullydesignedtoaddressthereducedtendencyof
driversinmultilaneroundaboutstoyieldtopedestrians.
4.1.3.4NCHRPSynthesis264:ModernroundaboutpracticeintheUnitedStates,(Jacquemart,1998).
Thisreportpre‐datesothernationalresearchonroundabouts.ThereportexploredNorthAmerican(i.e.,
U.S.andCanadian)practicesatthetimeitwasdeveloped(1998).Italsoprovidesexamplesofguidelines
fromAustralia,theUnitedKingdom,France,SwitzerlandandGermany.Specifictopicsaddressedinclude
safety,capacityanddelay,issuesofroundaboutsforvarioususers,locationcriteriaforroundabouts,and
examplesoftheuseofroundaboutsintheUnitedStates.
ThissynthesisincludestheresultsofasurveyconductedamongallstateDOTsintheUnitedStatesaswell
astheircounterpartsintheCanadianprovinces.Thesurveyincorporatedtheresponsesofthosestate
DOTsregardingthewillingnesstobuildmoreroundaboutsintheirjurisdiction,anddesignguidelinesfrom
othercountriesorstatesthattheyusedasprecedence.Specifically,formakingasafetyanalysisfield
study,thisresearchincludedasafetyanalysisthatexaminedbeforeandafterscenariosof11roundabout
Roundabouts and Access Management
Page 64
Chapter4ReviewofFederalandStatePractices
sitesintheUnitedStates.Afterroundaboutswereinstalled,thetotalnumberofcrasheswasreducedby
37%atthese11sites.
Theauthorsfoundthatthesizeofroundaboutdiametersaffectthenumberoftotalcrashesandinjury
crashes,assmallerdiametersof37m.or121ft.showa53%decreaseintotalcrashesanda73%dropin
injurycrashes.Overall,thesamplesofthisstudyshowedadecreaseindelaysofabout75%withthe
roundaboutscomparedtopriortrafficcontrolmethodsatintersections.Issuesconcerningpedestriansand
bicyclistswererelatedto“theabsenceofclearright‐of‐waycontrol(p.2).Inthecaseofone‐laneandlow‐
speedroundabouts,itwassuggestedthebicyclelaneshouldmergeintotheroundaboutandthebicyclist
shouldsharethelanewiththecars.Formulti‐laneroundabouts,itwasrecommendedthatbicyclistsshould
haveseparatebikepaths,beassignedtoasharedpathwithpedestrians,orbererouted.
Thissynthesisshowsthemarkedbenefitsofroundaboutsregardingsafety,delay,andcapacity.Inaddition,
thisresearchagreesthatroundaboutsprovideaestheticandurbandesignbenefits.
4.1.4StateGuidanceforRoundabouts
ThestateguidebooksareusuallymentionedonstateDOTwebsites.Twenty‐sixstateshaveroundabout
websiteswithvaryingdegreesofinformation.Linkstootherstates’roundaboutwebsitesandnational
guidelinesarealsofoundonmostofthosewebsites.
Inadditiontonationalguidanceonroundabouts,accessmanagement,safety,andcapacity,ahandfulof
statesareleadingthewayinprovidingstatewideguidancethatsupplementsthenationalguidance.Those
statessupplementthenationalguidancewithvarioustypesofstate‐leveldocuments.Forexample,many
includedtheroundaboutdesignontheroadwaymanual.Somestateshavespecificlinkstothedesignof
roundabouts.Furthermore,VirginiaDOTplacedtheroundaboutdesignintheaccessmanagement
guidance,whichrelatestothepurposeofthisproject.TheactivitiesoffourteenstatesincludingArizona,
California,Iowa,Kansas,Kentucky,Maryland,Michigan,Minnesota,NewHampshire,Pennsylvania,
Virginia,Washington,andWisconsinwereselectedforfurtherexaminationbecausetheyhaveadditional
guidancebeyondthatprovidedinnationaldocuments.Thesearedescribedindetailbelow.Roundabout
guidanceinFloridaisalsoreviewedingreatdetaillaterinthischapter.Thisreviewincludestheextentof
roundaboutinformation,roundaboutusers’guide(s),existingroundaboutdesignguidance,access
managementguidance,anddrivewayspacinganddesignguidance.Severalofthestateguidebooksbase
theirguidanceontheFHWARoundabouts:AnInformationalGuide(Robinsonetal.,2000)andNCHRP
Report672:Roundabouts,AnInformationalGuide,SecondEdition,(Rodegerdtsetal.,2010).Particular
attentionisgiventostateguidanceonaccessmanagement,driveways,safety,androundaboutcapacityas
theyapplytoroundabouts.
Table11.RoundaboutStates’DesignGuidebooksReviewedinthisDocument
RoundaboutGuide
Facility
Access
RoadwayorHighway
DesignManual
Document
Development
Management
Manual
DesignStandard
Florida(1996,2000,2012)
Wisconsin(2011)
Virginia(2007)
NewHampshire(2007)
Arizona*(2003)
Iowa(2009)
Kansas(2003)
Minnesota(2009)
Pennsylvania(2007)
Kentucky(2010)
California(2007)
Maryland(2011)
Iowa(2008)
Washington(2011)
Michigan(2011)
Arizona(2012)
Maryland(2012)
*‐cannotbeaccessedonline
Roundabouts and Access Management
Page 65
Chapter4ReviewofFederalandStatePractices
Arizona.Roundabouts:AnArizonaCaseStudyandDesignGuidelines(Leeetal.,2003)andRoadwayDesign
Guidelines,Section403(AzDOT,2012)aretwodocumentsfromArizonaDOT(AzDOT).Thefirstisa260‐
pagedocumentthatdiscussesthecasestudiesofroundaboutsinArizona.Thesecondincludesasix‐page
sectiononroundaboutdesign.Bothdesignmanualsfollowthenationalguidelinesaboutroundabouts
California.ThemaindocumentaboutroundaboutsinCaliforniaisRoundaboutGeometricDesignGuidance
(Caltrans,2007).This113‐pagedocumenthasthreemainchapters:vehicleoperationsassessment,
pedestrianandbicycleconsiderations,andgeometricdesignconsiderations.Theresearchestablishes
policiesandstandardsforCaltransroundabouts.Theresearchfoundthatthesuccessfulperformanceofa
roundaboutismorearesultofoutputs(operationalandsafetyperformance,andaccommodationofusers)
thaninputs(individualdesigndimensions).ThisdocumentrecommendedmodificationofRoundabouts:An
InformationalGuide(Robinsonetal.,2000)inregardtoaccelerationanddecelerationeffects.
Iowa.ThePlanning‐LevelGuidelinesforModernRoundabouts,TechnicalMemorandum(Hallmarkand
Isebrands,2008)andDesignManualChapter6,GeometricDesign,6A‐3ModernRoundabout‐General
Guidance(IowaDOT,2009)arethetwoguidancedocumentsusedforroundaboutsinIowa.Thefirstisa32‐
pagedocumentthatprovidestheIowaDOTwithinformationandguidanceonroundaboutpolicies,design
guidelines,andpubliceducation.Itdevelopsaroundabouttaskforce,documentsbestpracticesofstates
withsuccessfulroundaboutprograms,developsimplementationguidelines,developsdraftroundabout
policies,andassistsinpubliceducationaboutroundabouts.Theseconddocument,writtenbytheIowa
DOT,isaseparatechapteroftheGeometricDesignmanual.Asectionofthechapter(16pageslong)focuses
onmodernroundaboutsforIowa.
Kansas.KansasRoundaboutGuide,ASupplementtoFHWA’sRoundabouts,AnInformationalGuide
(Kittelson&Associates,andTransystemCorporation,2003)isa176‐pagedocumentthatshows
supplementalaspects,suchasdifferentiatingtrafficcirclesfromroundabouts,anddetailingroundabout
selectioncriteria.Thisincludesaddingroundaboutcategoriesonthedesigncharacteristictable(whether
urbanorruralroundaboutsandwhethersingleordoublelane),aswellasdetailsofthedesignprocess.The
guidehighlightsfiveprojectsinKansaswithrespecttocurbandpavementdesign,signageonurban,
suburban,multilaneroundabouts,luminanceforintersectionsbasedonpavementclassification(the
Portlandcementconcretesurfaceandtypicalasphaltsurface),androadwayclassification.
Kentucky.KentuckyTransportationCabinet(KYTC)hasDesignGuidanceforRoundaboutIntersection
(KYTC,2010)toprovidespecificexplanationsofhowKentuckymayreviewandapproveroundabouts.This
documentalsolooksatwarrantanalysisandoperationalanalysisfortrafficdynamics.Theoperational
analysistakesintoaccounttheaspectsthatimpactroundaboutcapacity,suchasgeometricdesign,and
criticalheadway.
Maryland.TwodocumentsfromMarylandDOTare:Chapter3C—RoundaboutMarkings(Roundabout
DesignGuidelines,2011),andRoundaboutDesignGuidelines(MarylandStateHighwayAdministration,
2012).Thefirstdocumentincludesmarkingsforone‐,two‐,andthree‐laneroundabouts,aswellas
crosswalk,pedestrian,andbicyclistmarkingsinroundabouts.Theseconddocumentcoversdesignand
operationsaspectsforroundabouts.
Michigan.ThefirstdocumentaboutroundaboutsinthestateofMichiganisEvaluatingthePerformance
andSafetyEffectivenessofRoundabouts(Bagdade,etal.MichiganDepartmentofTransportation,2011).
ThisdocumentcompilesthegeometricfeaturesandcrashhistoryofroundaboutswithinMichiganandalso
presentstheSafetyPerformanceFunctions(SPFs)andCrashModificationFactors(CMFs)forroundabouts
inthestate.
Roundabouts and Access Management
Page 66
Chapter4ReviewofFederalandStatePractices
Minnesota.MnDOThasroundaboutdesignguidelinesintheRoadDesignManual:Chapter12—Design
GuidelinesforModernRoundabouts(MnDOT,2009).Itshowstheenhancementtableoftypicalinscribed
circlediameterswithdailyservicevolumes,intersectioncontrolevaluationpolicy,asiterequirement
section,andspecialdesignfeaturestoaccommodatespecificlanduses.Additionally,thisdocument
suggestsRODELandAssessmentofRoundaboutCapacityandDelay(ARCADY)astoolsforintersection
controlevaluations.
NewHampshire.NHDOThasSupplementalDesignCriteria(NHDOT,2009).Thisisafive‐pagedocument
thatsupplementstheFHWARoundabouts:AnInformationalGuide(Robinsonetal.,2000)guidelinesfor
roundaboutdesignonNewHampshirestate‐maintainedroadways.Itmentionsconsiderationsfor
roundaboutdesign,includingoperations(withattachedcapacityworksheet,andRODELsetting),and
geometricdesign.
Pennsylvania.ThemaindocumentaboutroundaboutsinPennsylvaniaistheGuidetoRoundabouts:
PublicationNo.414(PennDOT,2007).This236‐pagedocumentsupplementsthepedestrianprovisionsof
FHWA'sRoundabouts,AnInformationalGuide(Robinsonetal.,2000)andprovidesconsistentinformation
regardingtheplanning,design,construction,maintenanceandoperationofroundaboutsinPennsylvania.
Thisdocumentalsopresentsdetailedrequirementsfordetectablewarningsurfacesandotherpedestrian
features.
Virginia.Virginia’saccessmanagementdocument,AccessManagementDesignStandardsforEntrancesand
Intersection(VDOT,2007),includesinformationaboutroundaboutinChapterF‐40Section2,Intersection
Design;SpacingStandard.This115‐pagedocumentexplainstheprocessofroundaboutdesigninVirginia,
accessmanagementforhighways,andpedestrian/bicyclistsafety,bymanagingthenumberofentrances
andrestrictingaccessfromoneormoredirections.Thestatehasadoptedapolicyonintersectiondesign
thatincludesthefollowingprinciples:limitthenumberofconflictpoints,coordinatedesignandtraffic
control,avoidcomplexmaneuvers,separateconflictpoints,favormajorflows,segregatemovements,
accommodatepedestriansandbicyclists,considerthedesignvehicle,andconsideraroundaboutdesign.
Washington.TheWSDOTDesignManual—Chapter1320Roundabout(WSDOT,2011)istheprincipal
documentaboutroundabouts.A50‐pagesectiongivesinformationaboutprocedurestodesigna
roundaboutinthestateofWashington.Section1320.11referstoaccess,parking,andtransitfacilities
around.Roundabouts.Morespecifically,thechapterincludesinformationrelatedtocornerclearance,
parallelroundabouts,U‐turns,parking,andtransitstopsinthevicinityofroundabouts.Thisguidance
indicatedthatnoroadapproachconnectionstothecirculatingroadwayareallowedatroundaboutsunless
theyaredesignedaslegstotheroundabout(WSDOT,2011).Fordrivewaysclosetoroundabout,this
guidancesuggestedthatitisdesirablethatroadapproachesnotbelocatedontheapproachordeparture
legswithinthelengthofthesplitterisland(WSDOT,2011).Theminimumdistancefromthecirculating
roadwaytoaroadapproachiscontrolledbycornerclearanceusingtheoutsideedgeofthecirculating
roadwayasthecrossroad(WSDOT,2011).Right‐in/right‐outdrivewaysarealsopreferredwhendesigning
drivewayclosetoroundabout.
Wisconsin.ThemaindocumentforroundaboutguidelinesinWisconsinisChapter11,Section26:
Roundabouts(WisDOT,2013).This79‐pagereportprovidesthegeneralguidelinefordesignand
constructionofroundabouts.Italsoprovidesthefirstsupplementaryguidanceforshared‐usepathsfor
bicyclists.Thisguidelineconsidersthreeaspectsrelatedtothelocationofdrivewaysontheroundabout
entryorexit:volumeofdriveways,operationalimpact,andsightdistancebetweenusers.Inaddition,the
chapterexplainstheRODELsoftwareindetail.ThischapteriscurrentlybeingupdatedandHCM2010,
usinglocallydevelopedgapparameters,willreplaceRODELasthesoftwaretooltoanalyzeroundabout
capacityandoperations(PatrickFlemming,PersonalCommunication,June25,2013).
Roundabouts and Access Management
Page 67
Chapter4ReviewofFederalandStatePractices
4.2StateofFloridaGuidance
4.2.1AccessManagementGuidanceinFlorida.
TheFDOTSystemsPlanningwebsite(FDOT,2014)doesnotspecificallyaddressplanningforroundabouts.
However,when‘roundabout’wasusedasthekeywordonthesearchengine,severalinformational
documentsappear.TheFloridaDOT’sAccessManagementsiteprovidesdefinitionsandcontains
informationaboutpermits,training,anddocumentsforaccessmanagement,butdoesnotprovidespecific
guidanceonaccessmanagementnearroundabouts.
Floridahastwomajorhandbooksrelatedtoaccessmanagement.Thefirst,theFDOTMedianHandbook
(2006)isan81‐pagereportthataddressesseveraldesignconsiderationsrelatedtoroundabouts.However,
itdoesnotexplicitlydetailanythingaboutroundaboutdesignoraccessmanagement.TheFDOTDriveway
InformationGuide(2008)isa94‐pagereportthataddressesseveralguidelinesfordrivewaydesignin
Florida,suchassightdistanceatdriveways,drivewaylocation,andpedestrianfactors,butdoesnotmake
anyreferencetoroundabouts.
ThefollowingsectionsreviewaccessmanagementtechniquesinFlorida.Theseincluderoadway
classification,drivewaydesignandspacing,cornerclearance,medianopeningdesign/spacing,sight
distance,turn‐lanelocationanddesign,andauxiliarylaneanddesign.
RoadwayClassification.FDOT’sStateHighwayAccessManagementClassificationSystemandStandards
(FDOT,2010)containsroadwayclassificationsbasedonaccessclass,segmentlocationandapplicable
spacingstandards.FDOTsegmentsaccessintosevenclasses:(1)Accessclass1isforlimitedaccess
facilitiesthataredesignedforhighspeedandhighvolumetraffic(e.g.,interstatehighwaysandFlorida’s
Turnpike;(2)accessclass2roadwaysarehighlycontrolledaccessfacilitiesdistinguishedbytheabilityto
servehighspeedandhighvolumetrafficoverlongdistancesinasafeandefficientmanner;(3)accessclass
3roadwaysarecontrolledaccessfacilitieswheredirectaccesstoabuttinglandiscontrolledtomaximize
theoperationofthethroughtrafficmovement;(4)accessclass4roadwaysarecontrolledaccessfacilities
wheredirectaccesstoabuttinglandiscontrolledtomaximizetheoperationofthethroughtraffic
movement;(5)accessclass5roadwaysarecontrolledaccessfacilitieswhereadjacentlandhasbeen
extensivelydevelopedandwheretheprobabilityofmajorlandusechangeisnothigh;(6)accessclass6
roadwaysarecontrolledaccessfacilitieswhereadjacentlandhasbeenextensivelydeveloped,andthe
probabilityofmajorlandusechangeisnothigh;and(7)accessclass7roadwaysarecontrolledaccess
facilitieswhereadjacentlandisgenerallydevelopedtothemaximumfeasibleintensityandroadway
wideningpotentialislimited.
AvisualdepictionofhowFlorida’sroadwaysystemfitsinwiththeaccessmanagementclassificationsis
showninFigure20:
Roundabouts and Access Management
Page 68
Chapter4ReviewofFederalandStatePractices
Figure20.RoadwayFunctionClassificationinFlorida(FDOT,2010,p.24)
Eachoftheseroadwayclassificationshasasetofspacingstandardsandotherassociatedaccess
managementcategories.Forclass1roadways,decisionsonspacingarebaseduponwhetherasegmentis
locatedwithinaCentralBusinessDistrict(CBD)orCBDfringeforcitiesinurbanizedareas.Thespacingis
onemileintheexistingurbanizedareasotherthantype1;2milesinthetransitioningurbanizedareas;3
milesinurbanareasotherthanareas1and2;and6milesinruralareas,respectively.Otherclasseshave
connectionspacingstandardsbasedonthepostedspeedlimit.Class2toClass7aredefinedasfollows,
accordingtotheirrestrictivenessfromthemosttotheleastrespectively(FDOT,2010,p.67).Accessclass
2isfurtherdistinguishedbyahighlycontrolled,limitednumberofconnectionsandmedianopenings,and
infrequenttrafficsignals.Thelandadjacenttoaccessclass3and4roadwaysisgenerallynotextensively
developedand/ortheprobabilityofsignificantlandusechangeexists.Theseroadwaysaredistinguished
byexistingorplannedrestrictivemedians.Accessclass5roadwaysarealsodistinguishedbyexistingor
plannedrestrictivemedians.Accessclass6roadwaysaredistinguishedbyexistingorplannednon‐
restrictivemediansorcenterlines.Accessclass7includesonlyroadwaysegmentswherethereislittle
intentoropportunitytoprovidehigh‐speedtravel.Exceptionstoaccessmanagementstandardsinthis
accessclassmaybeallowedifthelandownersubstantiallyreducesthenumberofconnectionscomparedto
existingconditions.Theseroadwayscanhaveeitherrestrictiveornon‐restrictivemedians(FDOT,2010).
DrivewayDesignandSpacing.Inexplainingthedrivewaydesign,FDOTprovidesthefollowingfigureto
understandtheelementsofdrivewaylocation.
Figure21.DrivewayDesignandSpacing(FDOT,2008,p.9)
Roundabouts and Access Management
Page 69
Chapter4ReviewofFederalandStatePractices
ThedrivewayfeaturesillustratedinFigure21aredescribedindetailinpage9oftheDrivewayInformation
Guide(FDOT,2008)andarealsoprovidedbelow:
 Radius(R)–sizeofcurvedapproach/exitofdriveway
 Flare(F)–sizeofangledapproach/exitofdriveway
 Width(W)–spaceforvehiclesoperatingondriveway
 DrivewayDistance(D)–orspacingbetweendriveways
 CornerClearance(C)–similarto(D)butmeasuredfromamajorintersection
 Angle(Y)–angleofdriveway
 Setback(G)–distancefrompublicrightofwaytothecloseststructure
 SightDistance–lengthofroadvisibletothedriverrequiredforvehiclestomakesafemovements
 DrivewayLocation–positionofdrivewayinrelationtoothertrafficfeaturessuchasintersections,
neighboringdriveways,andmedianopenings
 DrivewayLength–(alsocalled“throatlength”)distanceneededintositetotransitionvehiclesto
theinternalcirculationsystemofthesite
 Grade–slopeofdriveway
 DrivewayTrafficSeparators/ChannelizingIslands–sizeandpositionofbarrierseparating
trafficmovementsonthedriveway
 RightTurnLanes–separatelanesonroadwaytofacilitaterightturnsintodriveway
 Structure–Building,GasIsland,Gate,etc.
FollowingNCHRPReport548AGuidebookforIncludingAccessManagementinTransportationPlanning
(Roseetal.,2005,p.40),FDOT’sDesignStandardsclassifiesdrivewaysbasedontheexpectedvolumeand
thetypeoftraffic.ThedesignstandardsfordrivewaysarefoundinStandardIndex515(FDOT,2010).
Additionally,FDOTgiveslanduseexamplesofeachcategory.Forinstance:thefirstcategoryhasexamples
ofoneortwosingle‐familyhomes;thesecondcategoryhasthreeto60housingorapartmentunits,small
officesinconvertedhomes,or“momandpop”businesses;thethirdcategoryhassmallstripshopping
centers,andgasstation/conveniencemarkets;andthelastcategoryhasanexampleofa150,000‐ft
shoppingcenter,grocery/drugstorewithtento15smallerstores.
FDOTshowstheconstructiondesignsfortwoprimaryshapes:“curbedflareddrivewayorthedropped
curb”andthe“radialreturn.”Unlessthedrivewaysarehighervolume,thestandardsfor“curbedflared
driveway”arepredominantinurbanroadways.However,afewruralroadwaysmayhavecurbsand
gutters.Forruralroadways,FDOTsuggestsfollowingtheroundedradialreturndesign.
TheDrivewayInformationGuidealsoexplainshowthedrivewayshouldintersectwithon‐streetparkingor
bike‐lanes,andwheretheeffectiveturningradiusshouldbeincreasedfromaround6to14ft.Thecurband
effectiveradiusaredisplayedinFigure22.
Figure22.EffectiveRadiusandCurbRadius(FDOT,2008)
Roundabouts and Access Management
Page 70
Chapter4ReviewofFederalandStatePractices
Additionally,thedrivewaydesigncriteriaforseverallanduses,suchasshoppingcenter,officecomplex,
andconveniencestoresaresuggested.ThisstandardisadaptedfromTransportationandLandDevelopment
(2002)(seeAppendixB,otherstaterelateddocuments,andFloridaMedianHandbook(FDOT,2006)).
Ramp design spacing is also explained in this document. It is based on area types, such as urbanized,
transitioning,andrural,aswellasassumedpostedspeed.FDOThastherecommendedminimumspacing.
Thedimensionoframpdesignspacingiscalculatedfromonoroff‐ramp,asdisplayedinFigure23.FDOT
refers to the NCHRP Report 420 Impacts of Access Management Techniques for minimum ramp spacing
(FDOT, 2008, p. 78). Under the circumstances when roundabouts are located close to highway
interchanges, ramp design spacing must be considered. Small spacing between roundabout and
interchanges could potentially compromise the operation of both roundabout functional area and ramps
thatenter/exitroundabout.
Figure23.RampSpacing(FDOT,2008,p.78)
Roundabouts and Access Management
Page 71
Chapter4ReviewofFederalandStatePractices
Figure24.RoundaboutatanInterchange(FHWA,2006,p.8)
CornerClearance.AccordingtotheAASHTOGreenBook,cornerclearancemeansproperdriveway
placementsothatadrivewayisnotwithintheinfluencingareaofanotherdriveway.FDOT’sDriveway
InformationGuidedisplaysthefigure(Figure25)ofadrivewaywithanimproperlocationtoillustrate
cornerclearance.
Figure25.CornerClearance(FDOT,2008,p.73)
Roadwayclassificationdeterminesthespacingforcornerclearance,alongwiththespeedlimitonthe
roadway.FDOTalsodetailsthedownstreamcornerclearancestandardforaminorsidestreet.Figure26
illustratesthedownstreamcornerclearance.
Roundabouts and Access Management
Page 72
Chapter4ReviewofFederalandStatePractices
Figure26.CornerClearanceforDownstream(FDOT,2008,p.76)
Thestandardfordownstreamcornerclearanceisalsodefinedbywhethertheintersectionischannelized,
(witharadiusof50ft).Foraradiusofmorethan50ft,thestandardappliesforchannelization
downstream.
MedianOpeningDesign/Spacing.FDOTappliesthemedianopeningstandardbasedonthepostedspeeds
andonthephysicalcharacteristics—whethertheopeningisfullordirectional.Medianopeningdistances
rangefrom330to2,640ft.dependingonopeningtype,designspeedandroadwayclassification,asseenin
Table12.AccessManagementStandardsfromRule14‐97(FDOT,2006,p.15).
Table12.AccessManagementStandardsfromRule14‐97(FDOT,2006,p.15)
Class
2
3
4
5
Medians
MedianOpenings
Restrictive
w/ServiceRoads
Restrictive
Non‐Restrictive
Restrictive
Full
Directional
2,640
1,320
2,640
2,640
1,320
atgreaterthan45
mphPosted
Speed
6
7
660
Signal
45mph and
lessPosted
Speed
2,640
Morethan
45mph
Posted
Speed
1,320
2,640
2,640
2,640
660
660
440
440
440
245
440
125
245
125
660
atgreaterthan45
mphPosted
Speed
1,320
1,320
at45mphorless
PostedSpeed
at45mphorless
PostedSpeed
660
1,320
1,320
Non‐Restrictive
BothMedianTypes
Connection
330
SightDistance.Thisguidanceisneededtoimprovesafety.ThesightdistancestandardsincludetheSSD,
thedistancenecessarytostop,andISD.FDOTsets14.5ft.astheminimumdrivereyesetback.Fornew
developments,thedistanceforSSDshouldfollowthestandardbasedonthedesignspeedoftheroadway.
Roundabouts and Access Management
Page 73
Chapter4ReviewofFederalandStatePractices
Figure27.SightDistanceandDriverEyeSetbackDrivewayInformationGuide(FDOT,2008,p.62)
OtherthanSSDandISD,FDOThassightdistancestandardsforroadwaysupstreamanddownstreamthat
haveon‐streetparking.Foraspeedof0to30mph,itissuggestedthattheupstreamlanesbeatleast85ft.
andthedownstreamtwolanes,atleast60ft.Withfourlanesthedistanceshouldbe45ft.Foraspeedof35
mph,upstreamisatleast100ft.downstreamfortwolanes,andatleast70ft.andfourlanesat50ft.
Turn‐LaneLocationandDesign.FDOTsuggeststhestandardforaradialreturndesignisusedforan
exclusiveright‐turnlane.Meanwhile,theflaredrivewaystandardisforlowvolumedriveways.The
guidelinegivesclassificationofroadwaysbasedonthepostedspeedlimit,andthenumberofrightturns
perhour,i.e.45mphorlesswith80‐125vehicles,andover45mphwith35‐55vehicles.FDOTsuggests
havingnomedianopeningsacrosstheleft‐turnlane(FDOT,2008,p.77).Thedrivewayshouldbelocatedat
least100ft.fromtheoppositemedianopening.Thisdocumentalsosuggestshavinganadditional
pavementacrossthemedianopeningbecauseitmaysupporttheU‐turnmovement.FDOTsuggests
permittingleft‐turnsacrosshighvolumeroads,whenjointandcrossaccessexist.Figure28showsan
exampleofjointandcrossaccess.
Figure28.JointandCrossAccess(FDOT,2008,p.86)
Foranotherjointandcrossaccess,theFDOTreferstothedocumentManagingCorridorDevelopment,A
MunicipalHandbook(WilliamsandMarshall,1996),forthefollowinginformation.
Roundabouts and Access Management
Page 74
Chapter4ReviewofFederalandStatePractices
Auxiliary‐LaneLocationandDesign.FDOThasthestandardforintersectionchannelizationdesign.First,
thestandardchannelizesdivisionalislands,includingpedestrianrefugeislands,trafficseparation,and
trafficflowseparation.AccordingtoStandardIndex515,theminimumwidthforadrivewaydivisional
islandis4ft.andthemaximumis22ft.However,ifthedrivewaysarenotincludedinthestandardindex,
theminimumis6ft.andthemaximumis16ft.
TheDrivewayInformationGuiderecommendsthelengthfordrivewaysthathaveparkingmovements
shouldbeatleast50ft.togivespaceforonevehicletoenter(fromthesidewalk).Thepreferreddistance
forparkingmovementsisequaltoorgreaterthan30ft.fromtheroadway,andmorethanorequalto20ft.
fromthesidewalk.
Thislengthisdifferentforlanduseswithadrive‐through.Thisdocumentalsosuggeststhespacesallow
vehiclequeuesatfast‐foodestablishments,banks,carwashes,daycarefacilities,drycleaners,anddrive‐
throughstand‐alonedrugstores.FDOTalsosuggestsmaximumqueuesforschoolbusstops,anddriveways
forstaff,parentsandstudents.Thisstandardisbasedoncriticalpeakmorningandafternoonhours.
Inadditiontothosestandards,FDOTalsomakessuggestionsfordrivewaysnearbusstopsandtransit
facilities.Theoppositesidesofaroadwaymayresultinjogmaneuvers(forundividedroadwaysorthose
withtwo‐wayleft‐turnlanes(TWLTL)(FDOT,2008,p79).Asaconsequence,FDOTrecommendsthe
roadwayoffsetdistancesadaptedfromDOT.
4.2.2RoundaboutsGuidanceforFlorida
SeveraldocumentsareidentifiedasroundaboutguidelinesatFDOT.TheseincludeFloridaRoundabout
Guide(FDOT,1996),RoundaboutJustificationStudy(Chapter16inManualonUniformTrafficStudies,
FDOT,2000),FloridaIntersectionDesignGuide2013(FDOT,2007)andBicycleandPedestrian
ConsiderationsatRoundabouts(Shenetal.,2000).
The109‐pageFloridaRoundaboutGuide(FDOT,1996),whichdetailsroundaboutdesignandguidancein
thestate,waspublishedearlierthanFHWA'sRoundabouts,AnInformationalGuide(Robinsonetal.,2000).
TheFloridaguideincludesprocedurestojustifytheneedtobuildaroundabout,whiletheFHWAdocument
doesnot.Thisguideisintheprocessofbeingreplaced,withadditionalguidancebeingincorporatedinto
otherguidancedocuments;thestatehasofficiallyadoptedNCHRP672,Roundabouts,AnInformational
Guide(BansenandSullivan,2013).OthersupplementalaspectsoftheFloridaguideareexplanationsfor
usingtheSIDRAsoftware.Inaddition,thisdocumentalsoconsidersothersoftware,suchasARCADY,and
RODEL.TheFloridaguideincludesformstodeterminecapacityandotherrequiredmaterialstojustifythe
useofaroundabout;muchofthisguidancehasbeensunsettedwiththeadoptionofNCHRP672,
Roundabouts,AnInformationalGuideandtheinclusionofChapter7intotheState’sIntersectionDesign
Guide2013.
ThesecondroundaboutdocumentistheManualonUniformTrafficStudies,Chapter16‐Roundabout
JustificationStudy(2000).WrittenbyFDOTandpublishedin2000,this16‐pagereportisthelastchapter
intheFDOTManualonUniformTrafficStudies(MUTS).TheMUTSestablishesminimumstandardsfor
conductingtraffic‐engineeringstudiesonroadsunderthejurisdictionoftheFDOT.Thechapteron
roundaboutsjustifiestheiruseintheStateofFlorida,andcomparesthemtothreeotheralternativesto
intersectioncontrols–trafficsignals,TWSC,andAWSC.Thischaptercitesthe1996FDOTFlorida
RoundaboutGuideforspecificguidelinesonroundaboutlocation,design,andoperation.
ThethirddocumentthatprovidesinformationonroundaboutsistheFloridaIntersectionDesignGuide,
2013ForNewConstructionandMajorReconstructionofAt‐GradeIntersectionsontheStateHighway
System.This226‐pagedocumentincludeschaptersonintersectiondesignconcepts,geometricdesign,
Roundabouts and Access Management
Page 75
Chapter4ReviewofFederalandStatePractices
signalization,signsandmarkings,objectsandamenities,androundabouts.Itstatesthatmodern
roundaboutsshouldbeconsideredforanynewroadorreconstructionprojectastheyseemtoprovide
safetyandoperationaladvantages.Consistentwithotherstateguidance,theIntersectionDesignGuide
adoptsNCHRP672,Roundabouts,AnInformationalGuide(2010)asthemainguidefordesigning
roundaboutsinFlorida.Itmentionsthatroundaboutscontrolright‐of‐waysimilartosignalizationbutoffer
moreadvantagesthansignalizedintersections,suchasreducingtheconflictpointswithintheintersection,
reducingdelay,norequiredpowerortimingsuchaswithsignals,lesseningthenumberorturnlanes,
eliminatingtheneedforextraqueuingspace,andothers.Roundaboutscanalsoreduceright‐anglecrashes.
FDOTgenerallyrecommendsuptotwolanesinroundaboutsunlesstherearespecificneedsin
accommodatingmovementsinspiralor“Turbo”roundabouts.Inaddition,drivewaysshouldnotbeallowed
inthecirculatoryroadwayunlessthereisenoughdemandtosupporttheirconstructionasadditionallegs
oftheroundabout.
Regardingroundaboutsandaccessmanagement,thisdocumentacceptsthatroundaboutscanbeusedas
partofanaccessmanagementplanastheycontributetoreducingdownstreamleftturnsbecausevehicles
canperformU‐Turnswithintheroundaboutsandthenaccessanareabyturningright.Bicyclescanaccessa
roundaboutasvehiclesusingthecirculatoryroadwayoraspedestriantrafficusingthesidewalks.Bicycle
lanesshouldendatbypassrampstoallowbicyclestousethesidewalkiftheyprefer,alwaysyieldingto
pedestrians.Pedestriantreatmentsatroundaboutsarethesameasinotherintersectiontypes.Incaseof
busroutespassingthroughroundabouts,busbaysshouldbeplacedcarefullyonthenearsideofthe
roundaboutapproachsothatwillnotcreatevehiclequeuesthatspillbackintothecirculatoryroadway.
Busstopslocatedonthefarsideoftheroundaboutshouldhavepulloutsorbemovedfurtherdownstream
tothesplitterislandinordertoavoidinterruptingregulartraffic.
Asmentionedearlierinthisreport,adequateSSDhastobeprovidedatroundabouts.FloridaIntersection
DesignGuideadaptstheSSDformulaandtheISDrequirementsfromNCHRP672,Roundabouts,An
InformationalGuide(Equations6‐5‐6‐7,pp.6‐61‐6‐63inRodegerdtsetal.,2010).
ThefourthroundaboutdocumentisBicycleandPedestrianConsiderationsatRoundabouts(2000).Written
byFDOTandpublishedin2000,thisreportexaminestopicsofspecificconcerntobicyclistsand
pedestriansatroundabouts.Theconclusionsofthisstudyarethatifnotproperlydesigned,roundabouts
canhavehigherbicyclecrashratesthanthoseofvehiclesandpedestrians,andthemulti‐laneroundabouts
createmoretensionandarelesssafeforbicyclistsandpedestriansthanone‐laneroundabouts.Thereport
recommendstheuseofadditionalbicyclefacilitiesoutsidearoundaboutifspaceisavailable.Also
recommendedarecrossingprovisions,andpropersignage.
Inadditiontotheabovedocuments,FDOTpresentedaPowerPointpresentation—Roundabouts,Florida’s
ImplementationStrategy(PrytykaandSullivan,2012)atthe2012DesignTrainingExpo.Thispresentation
capturessupplementalaspectsfromFHWA'sRoundabouts,AnInformationalGuide(Robinsonetal.,2000),
especiallyonpedestrians,trucks,andpavementmarkinginformation.
4.3NationalGuidanceonAccessManagementintheContextofRoundabouts
Amongallthenationalguidancedocumentsonroundaboutsandthedocumentsonaccessmanagement,
onlyNCHRPReport672:Roundabouts,AnInformationalGuide(Rodegerdtsetal.,2010)referstothe
accessmanagementinthecontextofroundabouts.
NCHRPReport672.Roundabouts,AnInformationalGuide(Rodegerdtsetal.,2010).This
informationalguideonroundaboutsincludesaccessmanagementinformationinthecontextof
roundaboutsunderthegeneralcharacteristicsofroundaboutsaspartofthegeometricprocess(Sections
Roundabouts and Access Management
Page 76
Chapter4ReviewofFederalandStatePractices
2.2.5p.2.9and6.11,pp.6‐95to6‐98).Theinformationonaccessmanagementbuildsuponthe
informationprovidedintheKansasRoundaboutsGuide(Kittelson&Associates,Inc.andTranSystem
Corporation,2003).Animportantfactmentionedinthisdocumentisthat“Mostoftheprinciplesusedfor
accessmanagementatconventionalintersectionscanalsobeappliedatroundabouts”(p.2‐9).Thereport
alsostatesthat“Accessmanagementatroundaboutsfollowsmanyoftheprinciplesusedforaccess
managementatconventionalintersections”(p.6‐95).However,roundaboutsaredifferentfromothertypes
ofintersectionsbecausetheycanprovideU‐turnopportunitiesallowingforareductionoffullaccesspoints
alongaroadwaysegmentandthereforeenhanceaccessmanagement.
Publicandprivatepropertyaccesswithinthevicinityofaroundaboutshouldbecarefullyevaluatedand
thecasesof“accessintotheroundaboutitself”and“accessneartheroundabout”shouldbetakeninto
account.Drivewayslocatedintheroundaboutshouldbeavoidedbecausetheycancreateconflictsinthe
circulatoryroadway,includingaccelerationanddeceleration,eventhoughtherearecaseswheredirect
accessisgiventoresidencies.Inordertohaveadrivewaytakingdirectaccesstothecirculatoryroadwayof
aroundabout,noalternativeaccesspointsshouldbeavailable,lowtrafficvolumesshouldbepresentatthe
driveway,alownumberofunfamiliardriversshouldusetheroundabout,thedrivewayshouldbeproperly
designedtoallowvehiclestoturnaroundandexitfacingforward,andtheroundaboutshouldprovide
adequatesightdistanceandSSD.Wheredrivewaysarelocatedinorneararoundabout,thedesignshould
giveaclearvisualindicationthatprivatedrivewaysareadjacenttotheroundaboutandarenotforpublic
use.
Theabilitytoprovidepublicandprivateaccesspointsneararoundaboutisinfluencedbyanumberof
factorssuchasthecapacityoftheminormovementsattheaccesspoints,theneedtoprovideleft‐turn
storageonthemajorstreettoservetheaccesspoint,theavailablespacebetweentheaccesspointandthe
roundabout,andthesightdistanceneeds.Figure29showsthetypicaldimensionsforleft‐turnaccessnear
roundabouts.Theyincludeaminimumof50ft.toclearthemedian,aminimumof75ft.toallowfortheleft
turningmovement,and90ft.fordecelerating(oraccelerating)maneuveringandqueuingintheleftturn
lane.

Figure29.TypicalDimensionsforLeft‐turnAccessnearRoundabouts(Rodegerdtsetal.,2010,p.6‐98)
4.4States’GuidanceonAccessManagementintheContextofRoundabouts
Asmallnumberofstatesrefertoaccessmanagementwithinthecontextofroundabouts.Someincludesuch
informationintheirroundaboutsmanualsandsomeintheiraccessmanagementmanuals.Fromtheseven
Roundabouts and Access Management
Page 77
Chapter4ReviewofFederalandStatePractices
statesthatrefertoaccessmanagementinthecontextofroundabouts,onlythreeofthemsubstantially
supplementinformationfromthenationalguidance.ThesestatesincludeKansas,Virginia,andWisconsin.
AdditionalinformationisprovidedaboutaccessmanagementinCalifornia,Iowa,Michigan,and
Pennsylvaniaaccessmanagementguidancedocuments.Stateinformationisdescribedfortheseseven
statesinthefollowingsection.
California.TheCaltransRoundaboutGeometricDesignGuidance(Caltrans,2007)mentionsthatattention
shouldbepaidtoprovidingaccesstopedestrianswithvisualimpairmentsatroundaboutsand,more
particularly,atmultilaneroundabouts,asoften,conventionaldesignmaynotbesufficient.Also,Caltrans
DivisionofDesignandOfficeofGeometricDesignStandardsdevelopedtheDesignInformationBulletin
Number80‐01asasupplementtotheFHWAReport,Roundabouts:AnInformationalGuide.Oneofthe
additionsregardingaccessmanagementwasaccommodatingbicyclistsonthestatehighwaysystemby
providingrampstoentertheshared‐usepathforthosewhodonotwanttousethecirculatoryroadway.
Anotheradditionwastherecommendationofcrosswalkswith“zebra”longitudinallines,transverselines,
anduseofdetectablewarningsurfacesatallpedestriancrossings.
Iowa.IowaDOThassponsoreditsstateuniversitytodevelopPlanning‐LevelGuidelinesforModern
Roundabouts(HallmarkandIsebrands,2008).Thatguidebookstatesthataccesstopedestriansisonly
allowedacrosstheapproachlegs,andparkingisnotallowedwithinthecirculatingroadway,andthat
roundaboutscanbeconsideredincaseswherethereisneedforU‐turnsandwhereright‐in‐right‐out
restrictionsexist.Anoteinthedocumentmentionsthat“[a]ccessmanagementprinciplesalignwithhow
roundaboutsfunctionandoperate.Corridorsthatarehamperedwithnumerousaccesses,especiallythose
tobusinesses,canbenefitfromroundabouts.RoundaboutsfacilitatetheuseofU‐turnsatintersectionsand
allowforrightturnsintodrivewaysandparkinglotsratherthanleftturnsacrosstraffic.Theimpactsof
right‐in‐right‐outrestrictionsandclosedmediansbecomereducedwhenroundaboutsprovideanaturalU‐
turnatanadjacentintersection”(HallmarkandIsebrands,2008,p.17).
Kansas.AccessmanagementinthecontextofroundaboutsisreferredtointwoKansasDOT(KsDOT)
documents:KansasRoundaboutGuide:ASupplementtoFHWA’sRoundabouts(Kittelson&Associatesand
TransystemCorporation,2003);andKsDOTAccessManagementPolicy(KsDOT,2013).Thefirstdocument
includesalltheinformationonaccessmanagementthatNCHRPReport672,Roundabouts,AnInformational
Guideadapted.Thisinformationisdescribedabove,inSection4.3.Intheseconddocument,theaccess
spacingfromroundaboutintersectionsisdiscussed.KsDOT’sroundaboutaccessspacingtoanaccesspoint
onthehighwayisconsistentwithKsDOT’sunsignalizedaccessspacing.Thatspacingshouldbemeasured
fromtheendofthesplitterisland,leavingtheroundaboutasshowninFigure30.Theappropriatecorner
clearanceisthenprovidedbetweentheendofthesplitterislandandthefirstaccesspointalongthelocal
intersectingroadway.
Roundabouts and Access Management
Page 78
Chapter4ReviewofFederalandStatePractices
Figure30.MeasuredDistancefromSplitterIslandtoFirstAccessPoint(KsDOT,2013,p.4‐26)
Michigan.MDOThastwoguidebooksthatfocusonaccessmanagementwithinandnearroundabouts.In
EvaluatingthePerformanceandSafetyEffectivenessofRoundabouts(Bagdadeetal.,2011)theresearch
reportmentionsthatadditionalprovisionssuchaspedestrianhybridbeacons,flashingpedestrianbeacons,
andraisedsidewalksmaybeincludedintwo‐laneroundaboutstoenhancethesafetyofvisuallyimpaired
pedestrians.TheAccessManagementGuidebookstates(MDOT,2008)that“Drivewaysneedtobelocateda
safedistancefromaroundaboutwithadequatesignage.Drivewaysshouldnotbelocatedwithina
roundabout”(MDOT,2008,p.3‐29).
Pennsylvania.Pennsylvania’sGuidetoRoundaboutsnotes(PennDOT,2007)thataccessiblepedestrian
crossingshouldbeprovidedatallroundaboutsexceptruralroundaboutswithnonexistentpedestrian
activity.Pedestriancrossingsshouldbelocatedbackfromthecirculatoryroadwayandthesplitterisland
shouldbecuttoallowpedestrians,wheelchairs,strollers,andbicyclestopassthrough.Bicyclesshouldbe
giventheoptionoftravelingthroughtheroundabouteitherasavehicleorasapedestrian,basedonthe
bicyclist’slevelofcomfort.Inthecasewherebicyclistschoosetosharethesidewalkandtravelas
pedestrians,theyarerequiredtodismounttheirbikeandwalkwithit.PennDOT’sGuidetoRoundabouts
(PennDOT,2007)wasdevelopedbasedonKsDOT'sRoundaboutGuide,(Kittelson&Associatesand
TransystemCorporation,2003)anditincludesexactlythesameinformationonaccessmanagementinthe
contextofroundaboutsastheKsDOT’sRoundaboutGuide.
Virginia.AccessManagementDesignStandardsforEntrancesandIntersection(VirginiaDOT,2007,revised
2011),includesinformationaboutroundaboutsinAppendixF,Section2(VirginiaDOT,2007).Inthat
guide,roundaboutsareseparatedfromsignalizedandunsignalizedintersections/crossoversbythe
unsignalizedintersectionspacingstandard(e.g.,secondcolumninFigure31).Theyarealsoseparatedfrom
otherroundaboutsbythepartialaccessentrancespacingstandard(i.e.,thelastcolumninFigure31);
partialaccessentrancereferstoroadwaysthathaveaccessmanagementtechniquestopreventleft‐turn
ingressandegressmovementsandfacilitateright‐inandright‐outmovements.Thespacingismeasured
fromtheouteredgeofthenearestinscribeddiameter,notthecenterline.Thespacingstandardsusedare
showninFigure31.Inaddition,designguidelinesregardingpedestrianandbicycletreatmentsshould
followNCHRPReport672,Roundabouts,AnInformationalGuide.
Roundabouts and Access Management
Page 79
Chapter4ReviewofFederalandStatePractices
Figure31.MinimumSpacingStandardsforCommercialEntrances,Intersections,andCrossovers(VDOT,
2007,p.F‐23)
Wisconsin.Wisconsin’sRoundaboutGuide(WisDOT,2011)includesinformationaboutaccesscontrolin
Chapter11,Section26.Thatchapterwasrecently(March4,2013)updated.Basedonthatguide,
roundaboutswouldfacilitateleftturnsandU‐turnstoaccesspropertiesontheoppositesideofthe
highway.Also,thepedestriancrossinglocationshouldbesetbackfromtheyieldline,typicallyonecar
length.Inaddition,connectingtworoundaboutswitharaisedmedianprecludesleftsin/outfromtheside
streetorbusinessaccesstoprotectmain‐linecapacity,althoughmajorcommercialdrivewaysmaybe
allowedasonelegoftheroundabout.Minorcommercialandresidentialdrivewaysarenotrecommended
alongthecirculatingroadwayexceptiftheyaredesignedasalegoftheroundabout,anddrivewaysshould
besetbacktopreventinterferencewithpedestrianmovementsincrosswalks(WisDOT,2011).Whenit
comestoaccessmanagement,theguidestates:
Retrofitofsuburbancommercialstripdevelopmenttoaccomplishaccessmanagementobjectivesof
minimizingconflictscanbeaparticularlygoodapplicationforroundabouts.Raisedmediansareoften
designedforStatearterialstominimizeleftturnconflicts;androundaboutsaccommodateU‐turns.Left‐
turnexitsfromdrivewaysontoanarterialthatmaycurrentlyexperiencelongdelaysandrequiretwo‐stage
left‐turnmovementscouldbereplacedwithasimplerrightturn,followedbyaU‐turnatthenext
roundabout.Again,apackageofimprovementswithdrivewayconsolidation,reversefrontage,and
interconnectedparkinglots,shouldbeplannedanddesignedwithcloselocalcollaboration.Also,a
roundaboutcanprovideeasyaccesstocornerpropertiesfromalldirections.(WisDOT,2011).
4.5RoundaboutLocationGuidelines
KansasDOTmentionedsiteswhereroundaboutsbringadvantages,andwheretheroundaboutshouldbe
builtcautiously.Intersectionsthatmayhavebenefitsinconvertingintoroundaboutsaretheoneswith
(Kittelson&AssociatesandTranSystemCorporation,2003,p.38):
Roundabouts and Access Management
Page 80
Chapter4ReviewofFederalandStatePractices













Historicalsafetyproblems;
Relativelybalancedtrafficvolumes;
Highpercentageofturningmovements;
Highvolumesatpeakhoursbutrelativelylowvolumesatnon‐peakhours;
Existingtwo‐waystop‐controlledthathavehighside‐streetdelay;
TherequirementstoaccommodateU‐turn;
Aroleasgatewayorentrypointtocampus,neighborhood,commercialdevelopment,orurbanarea;
Intersectionswhereacommunityenhancementmaybedesirable;
Intersectionswheretrafficcalmingisadesiredoutcomeoftheproject;
Intersectionswheregrowthisexpectedtobehighandfuturetrafficpatternsareuncertain;
Locationswherethespeedenvironmentoftheroadchanges;
Locationswithaneedtoprovideatransitionbetweenlanduseenvironments;and
Roadswithahistoricalproblemofexcessivespeeds.
However,thelocationsofroundaboutthathavethefollowingconditionsshouldreceiveextraattention:
 Intersectionincloseproximitytoasignalizedintersectionwherequeuesmayspillbackintothe
roundabout;
 Intersectionslocatedwithinacoordinatedarterialsignalsystem;
 Intersectionswithaheavyflowofthroughtrafficonthemajorroadopposedbyrelativelylight
trafficontheminorstreet;
 Intersectionswithphysicalorgeometriccomplications;
 Locationswithsteepgradesandunfavorabletopographythatmaylimitvisibilityandcomplicate
construction;
 Intersectionswithheavybicyclevolumes;and
 Intersectionswithheavypedestrianvolumes.
CloselySpacedRoundabout.WisconsinDOTconsidersroundaboutstobecloselyspacedwhenthe
distanceislessthan1,000ft.fromthecenterofeachroundabout.
4.6GeometryDesignGuidelines
Thisreviewhighlightsgeometricaspectsthatdifferamongstates’guidanceandNCHRPReport672,
Roundabouts,AnInformationalGuideorotherlistednationaldocuments.
WisDOTmentionedtheeffectsofdesignelementsonSafetyandOperationsandoutlinestrade‐offeffects
ontherelationshipbetweensafetyandcapacityasshowninFigure32.
Figure32.TheEffectofDesignElements(WisDOT,2011,p.38)
Roundabouts and Access Management
Page 81
Chapter4ReviewofFederalandStatePractices
Speed.KansasDOTprovidestheroundaboutdesignspeedbasedonsitecategories:mini‐roundabout,
urbancompact,urbansingle‐lane,ruralsingle‐lane,urbandouble‐lane,andruraldouble‐laneroundabout.
Table13showstheroundaboutdesignspeedthatKansasDOTapplied.
Table13.RoundaboutDesignSpeed
SiteCategory
MaximumEntry(R1)DesignSpeed
MiniRoundabout
20mi/h(32km/h)
UrbanCompactRoundabout
20mi/h(32km/h)
UrbanSingle‐LaneRoundabout 25mi/h(40km/h)
RuralSingle‐LaneRoundabout
25mi/h(40km/h)
UrbanDouble‐laneRoundabout 25mi/h(40km/h)
RuralDouble‐LaneRoundabout 30mi/h(48km/h)
Source:KansasDOT,p.67
Lanenumbersandarrangements.Indeterminingthese,Caltransusedcapacitymodelstakingcritical
headwayandfollow‐upheadwayspecificallyasfollows:single‐laneroundabout(4.8sand2.5s,
respectively);multilaneroundabouts,leftlane(4.7sand2.2s,respectively);andmultilaneroundabouts,
rightlane(4.4sand2.2s,respectively).HeadwayvaluesforWisDOTarepresentedinTable14.
Table14.RecommendedHeadwayValues(WisDOT,2011,p31)
Spacing.Caltransdevelopedastandardforspacingentriesandexitstominimizeexit‐circulatingconflicts.
Thespacingisconsideredimportantformultilane,morethanforfour‐legandskewed‐legroundabouts.As
aresponsetothecirculating‐exitingpathconflict(Figure33),Caltransofferedtwosolutions,asseenin
Figure34.
Roundabouts and Access Management
Page 82
Chapter4ReviewofFederalandStatePractices
Figure33.ExampleSolutionDesignwithCirculating‐ExitingPathConflict(Caltrans,2007,p.62)
(
(
Figure34.SolutionOptionsforCirculating‐ExitingPathConflict:(i)ModifyLaneConfiguration,and(ii)
RealignApproaches(Caltrans,2007,p.63‐64)
SightDistance.AzDOTrequiresthataroundaboutdesignmeettwosightdistancestandards:SSDandISD.
TheISDincludestheapproachanddeparturesighttriangles.Caltransfocusesonensuringpropersightto
theleft.Forsightdistancecalculations,“thecriticalheadwayof5.9sec.isrecommendedinsteadofthe6.5
sec.presentedinRoundabouts:AnInformationalGuide(Robinsonetal.,2000).Thismethodologyshouldbe
consideredinterimuntilastudyonroundaboutISDiscompleted”(p.viii).Fortheangleofvisibility,
CaltranscomparedAASHTO,TheCaltransHighwayDesignManual,andFHWAHighwayDesignHandbook
forOlderDriversandPedestrians,whichhadminimumanglesof60degrees,75degreesatgrade,and75
degrees,respectively.Figure35showsanexampleofanintersectionthathasaproblemwiththeangleof
visibility.KansasDOTreferstotheFHWAPublication(Robinsonetal.,2000)fortheISDandAASHTO
fourthedition.Thecalculationassumedacriticalgapof6.5s.andof4.6s.ifconstraintsfromtopographic
featuresorbuildingexist(similartothelowerboundoftheHCM2000(TRB,2000)).
(
(
Roundabouts and Access Management
Page 83
Chapter4ReviewofFederalandStatePractices
Figure35.AngleofVisibility:(i)theAngleisTooSevere(ii)RealignedRampTerminalApproachtoHave
BetterAngleofVisibility(Caltrans,2007,p.65)
KansasDOTdecidesthedesignspeedfromthecalculationofSSDandISD.First,SSDincludesthe
requirementsofapproachsightdistance,sightdistanceonthecirculatoryroadway,andsightdistanceto
crosswalkontheimmediatedownstreamexit.Also,KansasDOTmentionsthatsightdistancefor
landscapingmaterialshavelimitationof2ft.or600mm.height.
WisDOTspecifiestheguidanceforclosely‐spacedmultipleroundabouts.Inthedocument,WisDOTusesthe
minimumvisibilitydistanceshowninTable15.
Table15.WisconsinDOTMinimumVisibilityDistance
*MinimumVisibilityDistancesarefromSection2C.36oftheWisconsinSupplementtothe2009MUTCD
InscribedCircleDiameter(ICD).TheCaltranscomparedICDforFHWAstandard,Kansas,Arizona,and
Wisconsindependingontheroundaboutcategories.Table16displaystheICDforthesestates.
Table16.TypicalInscribedCircleDiameterRanges(Caltrans,2007,p.67)
Toupdatethosestandards,Caltransincorporateslanenumbersandarrangements,designvehicles,
numberoflegs,andapproachalignmentontheirstandards.Table17givesthecommonrangesofinscribed
circlediametersbasedontheaforementionedfactors.
Roundabouts and Access Management
Page 84
Chapter4ReviewofFederalandStatePractices
Table17.CommonRangesofInscribedCircleDiameters(Caltrans,2007,p.68)
Geometricdesignforusers.Theneedsofvarioususersareconsideredinthestateguidelines.For
example:designvehicle,pedestrians,bicyclists,andolderdrivers.First,AzDOTappliedspecial
considerationstoroundaboutsbyaddingatruckapron.Caltransusesthedesignvehicleasoneof
geometricdesignconsideration,coveringcarsweptpathfordifferenttypesofdesignvehicles.The
guidelinescomparisonfordesignvehiclesformultilaneroundaboutsispresentedinTable18.Inaddition,
Caltransprovidesdesignrecommendationsforpedestrians,includingcrossingtreatmentsand
methodologiesasinTCRPReport112andNCHRPReport562.
Roundabouts and Access Management
Page 85
Chapter4ReviewofFederalandStatePractices
Table18.TheGuidelinesComparisonforDesignVehiclesonMulti‐laneRoundabouts(Caltrans,2007).
WisDOThascompleteguidancefordesignvehiclesontwo‐laneroundabouts.Theguidebookexploresthree
designcategoriesforlegaltruckaccess(WisDOT,2013,p.47).Thefirstcaseiswhenroundaboutsallow
truckstoencroachintoadjacentlanesastheyapproach,enter,circulate,andexittheintersection.The
secondcaseiswhenroundaboutsallowtrucksin‐laneastheyapproachandentertheroundabout,butmay
requiretruckstoencroachintoadjacentlanesastheycirculateandexittheintersection.Thethirdcaseis
whenroundaboutsaccommodatetrucksin‐laneastheyapproachandtraversetheentireintersection.
Besidesdesignvehicles,thestates’roundaboutguidesaddressconcernsaboutpedestriansandbicycle
accommodations.KansasDOTfocusesongeometricelementsforpedestriancrossings,suchaslocation,
Roundabouts and Access Management
Page 86
Chapter4ReviewofFederalandStatePractices
crossingalignmentandsplitterislands.Inaddition,thedocumentoftheKansasDOTpaysattentionto
visuallyimpairedpedestrians,waystoavoidhavingthepedestriancrossthecentralisland,andto
providingmulti‐modalsidewalks.Detaileddesignsforpedestriansincludethefollowingaspects:(1)the
pedestriancrossingisexpectedtomaintainonevehiclelengthorabout25ft.awayfromtheroundabout
entrance;(2)curbrampsandpedestriancrossingsshouldbeavailableandbestraightandcontinuously
alignedontheroundabout;(3)way‐findingandgapdetectionmayneedtobeconsideredforvisually
impairedpedestrians;and(4)thedistanceofsidewalksfromthecirculatoryroadwayshouldbeatleast2
ft.,althoughtherecommendeddistanceis5ft.Furthermore,itisrecommendedthatthebikelanemerge
withsidewalksatleast100ft.(30m)upstreamoftheentranceline.
Toaccommodatepedestriansandbicyclists,WisDOTdescribesdesignguidanceforpedestrianfacilities,
bicyclemarkings,andbikerampentrancesandexits(WisDOT,2013,p.18).Thepedestrianfacilitiesinclude
thesidewalks,shared‐usepaths,androundaboutsidepaths.WisDOTfoundthatroundabouts,when
comparedtoothertypeofintersections,dohaveanadvantagewhenpedestrianandbicyclistsafetyis
concerned(WisDOT,2013,p.18).Thisisbecausethelowoperatingspeedsthroughroundaboutsandthere
arelessconflictpointbetweenpedestriansandvehicles.Forpedestriancrossingatroundabout,itis
importanttochooseacrosswalklocationthatcanbalancepedestriansafety,theirconvenienceandthe
operationofroundabouts.Forbicyclists,thebiggestchallengeisaccommodateturningmovementat
roundabouts.WisDOTrecommendedusingpedestrian‐bicyclepathseparatefromthecirculatoryroadway
toaccommodatebicyclistatroundabouts(WisDOT,2013,p.19).
Roundabouts and Access Management
Page 87
Chapter5SafetyAnalysis
ChapterFive:SafetyAnalysis
Thischapterincludesasafetyanalysisthatinvestigatespotentialsafetyconcernsassociatedwith
roundaboutsincommercialareasinFlorida.AsidentifiedinChapterThree,thepotentialsafetyconcerns
include:(1)impactofdrivewaycornerclearancesonroundaboutsafety;(2)safetyimpactofmedian
openingsinthevicinityofroundabouts;(3)safetyatroundaboutsthatprovidedirectaccesstoactivity
centers;and(4)safetyofvulnerableroadusersincludingpedestriansandbicyclists.
Generalstatisticsthatgiveanoverviewofthecrashesthatoccurredinthevicinityofallidentified
roundaboutsinFloridaareprovidedfirst.Ananalysisbasedoncrashdataanddetailedreviewofpolice
reportsisthenconductedtoaddresseachofthepreviouslylistedsafetyconcerns.Thechapterconcludes
withasummaryoffindingsandalistofspecificrecommendations.
5.1OverallCrashStatistics
AsindicatedinChapterThree,atotalof1,882crasheswerefoundtooccurduring2007‐2011within500ft.
of283roundabouts.Thissectionprovidesanoverallsummaryofthesecrashesinthefollowingorder:(1)
areatype;(2)crashtype;(3)crashseverity,and(4)numberofvehiclesinvolvedinacrash.
5.1.1AreaType
The283roundaboutswerecategorizedintotwodifferentareatypes:commercialandresidential.
Commercialroundaboutsarethosethatarelocatedincommercialareasthatservemostlycommercial
traffic.Similarly,residentialroundaboutsarethosethatarelocatedinmainlyresidentialareas.Mixed‐use
areas,whichincludebothcommercialandresidential,areincludedwithcommercialroundaboutsbecause
ofthetrafficassociatedwiththecommerciallanduse.Table19givesthetotalnumberofroundaboutsand
crashesineachareatype.Table19alsoprovidesthecrashstatisticsbyareatype.Overall,eachroundabout
experiencedanaverageof6.65crashesduringthefive‐yearanalysisperiod;withcommercialroundabouts
experiencing8.10crashesperroundaboutwhileresidentialroundaboutsexperienced5.40crashesper
roundabout.Thetablealsoshowsahigherstandarddeviationforthenumbersofcrashesforroundabouts
incommercialareas,indicatingthatthecrashfrequenciesvarymoreamongthecommercialroundabouts
thantheresidentialroundabouts.
Table19.StatisticsbyAreaType
AreaType
TotalCrashes
inFiveYears
(a)
Numberof
Roundabouts
(b)
Commercial
Residential
Total
1,061
821
1,882
131
152
283
Crashes
perRoundaboutin
FiveYears
(a/b)
8.10
5.40
6.65
Standard
Deviation
13.65
9.20
11.53
5.1.2CrashType
Table20givesthesummaryofcrashstatisticsbycrashtypeandareatype.Italsoprovidesthepercentof
nighttimecrashesbycrashtype.Figure36providesthepercentageoftotalcrashesandnighttimecrashes
bycrashtype.Collisionwithafixedobjectwasthemostfrequentcrashtype.Aboutaquarter(24.7%)ofall
crashesthatoccurredinthevicinityofroundaboutsresultedfromvehicleshittingafixedobject,mostly,
Roundabouts and Access Management
Page 88
Chapter5SafetyAnalysis
theroundaboutcenterisland.Also,abouttwo‐thirds(62.9%)ofthesecrashes(i.e.,collisionwithafixed
object)occurredatnight.Nexttothecollisionwithafixedobject,angleandrear‐endcrashesweremost
common,accountingfor21%and18.5%oftotalcrashes,respectively.Additionally,thedistributionof
crashtypeswasfoundtobesimilarincommercialandresidentialareas.
CrashesperRoundabout
in5years
9
8
7
6
5
4
3
2
1
0
8.10
6.65
5.40
Commercial
Residential
Total
Figure36.StatisticsbyAreaType
Table20.StatisticsbyCrashType
CrashType1
CommercialArea
ResidentialArea
Percent
Percent
Percentof
Percentof
No. ofTotal
No. ofTotal
Nighttime
Nighttime
(a) Crashes
(c) Crashes
Crashes
Crashes
(a/1,061)
(c/821)
188
17.7% 19.1%
161 19.6%
20.0%
20
1.9% 40.0%
15
1.8%
53.3%
217
20.5% 18.9%
179 21.8%
26.3%
29
2.7% 13.8%
12
1.5%
33.3%
37
3.5% 24.3%
14
1.7%
21.4%
55
5.2% 23.6%
41
5.0%
19.5%
16
1.5% 31.3%
15
1.8%
26.7%
TotalCrashes
Percentof
Percentof
No.
Total
Nighttime
(d) Crashes
Crashes
(d/1,882)
349
18.5%
19.5%
35
1.9%
45.7%
396
21.0%
22.2%
41
2.2%
19.5%
51
2.7%
23.5%
96
5.1%
21.9%
31
1.6%
29.0%
Rear‐end
Head‐on
Angle
Left‐turn
Right‐turn
Side‐swipe
BackedInto
Collisionwith
27
2.5% 29.6%
18
2.2%
50.0%
45
2.4%
37.8%
ParkedCar
Collisionwith
48
4.5% 20.8%
32
3.9%
34.4%
80
4.3%
26.3%
MotorVehicle
Collisionwith
14
1.3% 40.0%
4
0.5%
25.0%
18
1.0%
36.8%
Pedestrian
Collisionwith
35
3.3%
8.6%
16
1.9%
18.8%
51
2.7%
11.8%
Bicycle
Collisionwith
250
23.6% 63.6%
215 26.2%
62.1%
465
24.7%
62.9%
FixedObject
AllOther
125
11.8% 47.2%
99 12.1%
43.6%
224
11.9%
45.7%
Total
1,061 100.0% 34.0%
821 100.0%
37.4% 1,882 100.0%
35.5%
1 Thesestatisticsarebasedonthefirstharmfulevent(FHE)codedinthepolicereports.Notethatthese
numbersaredifferentfromthoseprovidedlaterinthechaptersincedetailedanalyseswerebasedonthe
reviewofpolicereports.
Roundabouts and Access Management
Page 89
Chapter5SafetyAnalysis
100
100%
PercentofTotalCrashes
90%
PercentofNighttimeCrashes
80%
PercentageofCrashes
70%
63
60%
50%
46
37
40%
29
30%
20%
10%
36
19
20
21
22
24
20
25
22
12
2
2
3
5
2
1
3
0%
CrashType
Figure37.TotalandNighttimeCrashStatisticsbyCrashType
5.1.3CrashSeverity
Figure38providesthenumberandpercentageofcrashesbycrashseverity.Table21summarizesthe
crashesbycrashseverityandareatype.Amajorityofcrashes(i.e.,over60%)thatoccurredatroundabouts
resultedinpropertydamageonly(PDO).Severeinjurycrashes(i.e.,fatalandincapacitatinginjurycrashes)
accountedforlessthan5%ofthetotalcrashes.Severeinjurycrashfrequencyperroundaboutwasslightly
higheratcommercialroundabouts(5.4%)comparedtoresidentialroundabouts(4.4%).However,the
overalldistributionsweresimilar.Also,crashseverityofseveralcrasheswasunknown;mostofwhich
werearesultofhit‐and‐run(i.e.,thedriverfledthecrashsitepriortothearrivalofthelawenforcement
officials).
Fatal
10(0.5%)
Injury
634
(33.7%)
PDO
1,150
(61.1%)
Figure38.StatisticsbyCrashSeverity
Roundabouts and Access Management
Page 90
Chapter5SafetyAnalysis
Table21.StatisticsbyCrashSeverityandAreaType
CommercialArea
ResidentialArea
TotalCrashes
No.
Percent
No.
Percent
No.
Percent
(a)
(a/1,061)
(c)
(c/821)
(d)
(d/1,882)
FatalInjury
4
0.4%
6
0.7%
10
0.5%
IncapacitatingInjury
53
5.0%
30
3.7%
83
4.4%
Non‐IncapacitatingInjury
152
14.3%
105
12.8%
257
13.7%
PossibleInjury
164
15.5%
130
15.8%
294
15.6%
PropertyDamageOnly
642
60.5%
508
61.9%
1,150
61.1%
Unknown1
46
4.3%
42
5.1%
88
4.7%
Total
1,061
100.0%
821
100.0% 1,882
100.0%
1 Theseverityofacrashisunknownwhenthedriverfledthecrashsitepriortothearrivaloflawenforcement
officials.
5.1.4NumberofVehiclesInvolved
Table22providessummarystatisticsofsingle‐vehicleandmulti‐vehiclecrashesbyareatype.Overall,
aboutone‐thirdofthetotalcrashesweresingle‐vehiclecrashes,whiletherestinvolvedmultiplevehicles.
Thetableshowsthattheproportionofsingle‐andmulti‐vehiclecrasheswasfoundtobeconsistentacross
areatypes.
CrashSeverity
Table22.StatisticsofSingle‐vehicleandMulti‐vehicleCrashesbyAreaType
CommercialArea
CrashType
Single‐vehicle
Multi‐vehicle
Total
No.
(a)
342
719
1,061
Percent
(a/1,061)
32.2%
67.8%
100.0%
ResidentialArea
No.
(c)
292
529
821
Percent
(c/821)
35.6%
64.4%
100.0%
TotalCrashes
No.
(d)
634
1,248
1,882
Percent
(d/1,882)
33.7%
66.3%
100.0%
Table23givesthesummaryofsingle‐vehicleandmulti‐vehiclecrashstatisticsbycrashseverity.Single‐
vehiclecrashes(8.9%)hadahigherproportionofsevereinjuriesthanmulti‐vehiclecrashes(2.9%).Also,a
greaterpercentageofsingle‐vehiclecrashesresultedininjuriescomparedtomulti‐vehiclecrashes;68.8%
ofmulti‐vehiclecrashesresultedinPDOcrashes,whileonly45.9%ofsingle‐vehiclecrasheswerePDOs.Of
thesixfatalsingle‐vehiclecrashes,fourinvolvedmotorcycles,andinallthesefourcrashes,themotorcyclist
wasfoundtobeatfault.Anotherfatalcrashinvolvedavehicleandanintoxicatedpedestrianwhoraninto
thepathofthevehicle.Twoofthefourfatalmulti‐vehiclecrashesinvolvedagolfcart.
5.2ImpactofDrivewayCornerClearancesonRoundaboutSafety
Drivewaycornerclearanceisdefinedinthecontextofthisstudyastheminimumdistancebetweena
roundaboutandanadjacentdrivewayalongeachapproachordepartureleg.AsshowninFigure39,the
upstreamdrivewaycornerclearanceismeasuredfromthefirstdrivewayupstreamoftheroundaboutto
theroundabout.Likewise,thedownstreamdrivewaycornerclearanceismeasuredfromtheroundaboutto
thefirstdrivewaydownstreamoftheroundabout.
Roundabouts and Access Management
Page 91
Chapter5SafetyAnalysis
Table23.StatisticsofSingle‐vehicleandMulti‐vehicleCrashesbyCrashSeverity
Single‐vehicleCrashes Multi‐vehicleCrashes
TotalCrashes
CrashSeverity
No.
Percent
No.
Percent
No.
Percent
(a)
(a/634)
(b)
(b/1,248)
(c)
(c/1,882)
FatalInjury
6
0.9%
4
0.3%
10
0.5%
IncapacitatingInjury
51
8.0%
32
2.6%
83
4.4%
Non‐IncapacitatingInjury
128
20.2%
129
10.3%
257
13.7%
PossibleInjury
91
14.4%
203
16.3%
294
15.6%
PropertyDamageOnly
291
45.9%
859
68.8%
1,150
61.1%
UnknownInjury1
67
10.6%
21
1.7%
88
4.7%
Total
634
100%
1,248
100.0%
1,882
100.0%
1
Theseverityofacrashisunknownwhenthedriverfledthecrashsitepriortothearrivaloflawenforcement
officials.
Thefocusofthissectionistoanalyzedriveway‐relatedcrashestoidentifytheimpactsofupstreamand
downstreamcornerclearancesonroundaboutsafety.Inthisanalysis,acrashisconsideredtobedriveway‐
relatedifoneofthevehiclesinvolvedinthecrashwasenteringorexitingadriveway.Particularly,crashes
involvingvehiclesturningfromadrivewayontoamainstreet,turningfromthemainstreetontoa
driveway,andbackingoutofadrivewayontoanapproachlegwereidentifiedasdriveway‐relatedcrashes.
Figure39.UpstreamandDownstreamDrivewayCornerClearances
Policereportsofallthe1,882crashesthatoccurredwithin500ft.oftheroundaboutswerereviewedto
identifydriveway‐relatedcrashes.Ofthe1,882crashesthatoccurredatroundaboutlegs,only74crashes
wereidentifiedtobedriveway‐related.Ofthese74driveway‐relatedcrashes,37crashes(50%)occurredat
thefirstdriveways(i.e.,thedrivewaythatdefinesthecornerclearance)whileanequalnumberoccurredon
allotherdriveways.
Roundabouts and Access Management
Page 92
Chapter5SafetyAnalysis
Howissafetyatroundaboutsaffectedbycornerclearances?Table24givesthesummarycrashstatisticsof
the37driveway‐relatedcrashesthatoccurredatthefirstdriveway.Ascanbeinferredfromthetable,
severalapproacheshaveshorterupstreamanddownstreamcornerclearances.Ofthe37crashes,18
occurredatthefirstupstreamdriveway,andtheremaining19occurredatthefirstdownstreamdriveway.
Sixof18crashes(33.3%)occurredwhentheupstreamcornerclearancewaslessthan250ft.Ontheother
hand,15of19crashes(78.9%)occurredwhenthedownstreamdrivewaycornerclearancewaslessthan
250ft.Intermsofcrashseverity,ofthe37crashes,nonewerefatal,tworesultedinincapacitatinginjuries,
eightwerenon‐incapacitatinginjurycrashes,andtheremaining27werePDOs.
Table24.Driveway‐relatedCrashesThatOccurredwithinUpstreamandDownstreamDrivewayCorner
Clearances
CornerClearance
(feet)
UpstreamofRoundabout
No.of
No.of
Crashes/
Crashes
Legs
100Legs
0‐49
50‐99
100‐149
150‐199
200‐249
250‐299
300‐349
350‐399
400‐449
450‐500
Nodrivewaywithin500ft.
Total
a
b
c
1
1
1
1
2
4
0
4
2
2
0
18
29
70
55
53
41
35
18
18
13
17
141
490a
3.4
1.4
1.8
1.9
4.9
11.4
0.0
22.2
15.4
11.8
0.0
5.2b
DownstreamofRoundabout
No.of
No.of
Crashes/
Crashes
Legs
100Legs
0
3
4
1
7
1
2
0
0
1
0
19
36
64
61
44
28
40
18
22
16
12
149
490a
0.0
4.7
6.6
2.3
25.0
2.5
11.1
0.0
0.0
8.3
0.0
5.6c
The131roundaboutshave490legs.
Thevaluedoesnotincludeapproacheswithnodrivewayswithin500ft.Itiscalculatedas(18×100)/(490‐141).
Thevaluedoesnotincludeapproacheswithnodrivewayswithin500ft.Itiscalculatedas(19×100)/(490‐149).
Theseabovestatisticsindicatethatthedownstreamdrivewaycornerclearancehasagreatersafetyimpact
thantheupstreamdrivewaycornerclearance.Thisresultisconsistentwiththefactthatvehiclesexitinga
downstreamdrivewayexperiencereducedgapsduetodispersedplatoonsfromtheupstreamroundabout.
Thisisfurtheraggravatedbythefactthatroundaboutsalsoprovidelargercornerturningradii,allowing
vehiclestoturnrightatahigherspeed.Atcornerswithreducedsightdistance,itfurtherreducesthetime
availablefordrivewayvehiclestocompletetheirmaneuvers.Figure40showsanexamplelocationthathas
adownstreamdrivewaycornerclearanceoflessthan150ft.andwithareducedsightdistanceduetosight
obstructions.
Roundabouts and Access Management
Page 93
Chapter5SafetyAnalysis
Figure40.RoundaboutonSRA1A,NassauCounty,FloridawithReducedSightDistanceatDownstream
CornerClearance
5.3SafetyImpactofMedianOpeningsintheVicinityofRoundabouts
Oncorridorswithraisedmedians,accesstoabuttinglanduseisoftenprovidedthroughmedianopenings.
Sinceroundaboutsdisperseplatoons,turningtrafficatmedianopeningsinthevicinityofroundabouts
experiencereducedvehiclegaps,whichcouldresultinmorecrashes.Thissectionexaminesifmedian
openingsinthevicinityofroundaboutsposeasafetyconcernandwhetherclosingthemedianopenings
andhavingvehiclesmakeU‐turnsatthedownstreamroundaboutwouldbebeneficial.
Figure41(a)showsacaseinwhichvehiclesfromthemainstreetturnleftatamedianopeningontoa
drivewaythatislocateddownstreamofaroundabout.Figure41(b)showsanalternativewithoutthe
medianopeningandrequirethevehiclestomakeU‐turnsattheroundaboutdownstreamandthenmakea
rightturnontothedriveway.Similarly,Figure42(a)showsasecondcaseinwhichvehiclesexitingfroma
drivewaylocatedupstreamofaroundaboutturnleftatamedianopeningontothemainstreet.Figure42
(b)showsanalternativewithoutthemedianopeningandrequirethevehiclestofirstturnrightandthen
makeaU‐turnattheroundabouttocompletetheleftturn.
Inbothoftheabovecases,thefirstquestioniswhethercrashstatisticsshowsignificantsafetyproblems
associatedwiththeleft‐turningvehicles,eitherontooroutofadriveway.Toanswerthisquestion,crashes
involvingvehiclesturningleftatmedianopenings(i.e.,vehiclesturningleftfromthemainstreetontoa
drivewayandvehiclesturningleftfromadrivewayontothemainstreet)wereidentifiedbyreviewingthe
policereports.The131roundaboutswerefoundtohaveatotalof157medianopeningswithin500ft.The
crashdatashowthat,during2007‐2011,arelativelylowtotalof15crashesoccurredatthese157median
openings.Ofthese15crashes,eightinvolvedvehiclesturningleftfromthemainstreetontoadrivewayand
seveninvolvedvehiclesturningleftfromadrivewayontothemainstreet.Figure43andFigure44give
examplesofthesetwoscenarios,respectively.Amongthecrashesinvolvingvehiclesturningleftfroma
driveway,onlyonecrashresultedinanon‐incapacitatinginjuryandtherestwerePDOs.Asshownin
Figure45,theonlycrashinvolvinganinjuryoccurredwhenavehicleturningleftfromadrivewayontothe
mainstreetcollidedwithabicyclist.Oftheeightcrashesthatinvolvedvehiclesturningleftfromthemain
streetontoadriveway,threeresultedininjuries,onewasapossibleinjury,andtheremainingfourwere
PDOs.
Roundabouts and Access Management
Page 94
Chapter5SafetyAnalysis
(a)PotentialSafetyProblem:VehiclesTurnLeftfromMainStreetontoaDrivewayatMedianOpeningwith
ReducedGaps
(b)Alternative:VehiclesfromMainStreetTurnontoaDrivewaybyMakingaU‐turn
atDownstreamRoundabout
Figure41.Case1‐VehiclesTurningontoaDrivewayDownstreamoftheRoundabout
Roundabouts and Access Management
Page 95
Chapter5SafetyAnalysis
(a)PotentialSafetyProblem:VehiclesfromDrivewayTurnLeftatMedianOpeningwithReducedGaps
(b)Alternative:VehiclesfromDrivewayTurnLeftbyMakingU‐turnsatDownstreamRoundabout
Figure42.Case2‐VehiclesTurningLeftfromaDrivewayUpstreamofaRoundabout
Roundabouts and Access Management
Page 96
Chapter5SafetyAnalysis
Figure43.AnExampleofaCrashataMedianOpeningInvolvingaVehicleTurningLeftFromtheMain
StreetOntoaDriveway(CrashID:820970050)
Figure44.AnExampleofaCrashataMedianOpeningInvolvingaVehicleTurningLeftFromaDriveway
OntotheMainStreet(CrashID:801477040)
Figure45.ANon‐incapacitatingInjuryInvolvingaVehicleTurningLeftfromDrivewayandaBicyclist
(CrashID:801468970)
Roundabouts and Access Management
Page 97
Chapter5SafetyAnalysis
Giventhatexistingmedianopeningsdidnotposesignificantsafetyproblemintermsofbothcrashnumber
andcrashseverity,thesecondquestioniswhetherclosingthemedianopeningcouldbebeneficial.Whileit
isuncertainhowmanyofthe15crashesrelatedtothemedianopeningscouldhavebeenpreventedby
requiringvehiclestomakeaU‐turnatroundabouts,theU‐turnalternativeisknowntoposetwopotential
trafficoperationalproblems.
First,theU‐turnalternativemayincreasethenumberofsideswipecrashesatroundaboutsespeciallyfor
largevehicles.Largetrucksandbusesoftenfinditdifficulttonegotiateasmallerroundabout.Particularly,
lackofadequatelateralclearancecouldresultinheavyvehiclessideswipingothervehiclesorbecoming
involvedinacollisionwithafixedobject,usuallywiththeroundaboutcenterisland.During2007‐2011,a
totalof18crashesinvolvedheavyvehiclesatthe131commercialroundabouts.Figure46showsexamples
ofthesecrashes.Vehiclehittingafixedobject,followedbyangleandsideswipecrasheswere
predominantlyobserved.AllofthesecrasheswerefoundtobePDOs.
(a)Fixed‐ObjectCrash
(b)SideswipeCrash
Figure46.ExamplesofCrashesInvolvingHeavyVehiclesatRoundabouts
Roundabouts and Access Management
Page 98
Chapter5SafetyAnalysis
Second,theU‐turnalternativepreventscertainturningmovements,whichmayresultincrasheselsewhere.
Closingthemedianopeningpreventsthefollowingtwoturningmovements:(1)itpreventsvehiclesfrom
turningleftfromadrivewayontothemainstreet;and(2)itpreventsvehiclesfromturningleftfromthe
mainstreetontoadriveway.Figure47andFigure48illustratethesetwoscenarios.AsshownFigure47,
thevehiclefromthedrivewaycannotturnleftontothemainstreetandthevehiclehastoturnrightand
makeaU‐turndownstream.Similarly,asshowninFigure48,thevehiclefromthemainstreetcannotturn
leftontothedrivewaywhenthemedianopeningisclosed.Thevehicleshastogostraight,makeaU‐turn
downstream,andthenturnrightatthedriveway.
Figure47.ClosingMedianOpeningsPreventVehiclesFromTurningLeftFromtheDriveway
OntotheMainStreet
Figure48.ClosingMedianOpeningsPreventVehiclesFromTurningLeftFromtheMainStreet
OntotheDriveway
Thislimitation,however,suggeststhatifthereisasecondadjacentroundaboutdownstream(i.e.,
roundaboutsinseries)tofacilitatetheU‐turns,closingthemedianopeningcouldbecomebeneficial,asit
Roundabouts and Access Management
Page 99
Chapter5SafetyAnalysis
couldpotentiallypreventsomeofthemedianopeningrelatedcrasheswithoutmakingsometurning
movementsdifficult.Figure49providesanexampleofacandidatelocationforconstructingraisedmedians
toeliminateleftturningmovementsinvolvingvehiclesenteringandexitingthedriveways.Again,this
alternativeisviableonlywithlowvolumeofheavyvehiclesorwithlargerroundaboutsthatcouldbetter
accommodatelargevehicles.
(a) Withoutraisedmedians(b)Withraisedmedians
Figure49.ACorridorwithTwoRoundaboutsonSegoviaStreet,MiamiDadeCounty,Florida
5.4SafetyatRoundaboutsThatProvideDirectAccesstoActivityCenters
Accesstomajoractivitycenters,suchasbigboxretailstores,shoppingcenters,andmalls,isoftenprovided
atmid‐blocklocationsonacorridor.Figure50givesanexampleofthisscenario.Suchaccesscreatesan
intersectionoramajordrivewaytothedetrimentoftrafficflowonthecorridor.Onealternative,asshown
inFigure51,istohavetheaccesspointconnecteddirectlytotheroundabout,sendingallaccesstraffic
throughtheroundaboutcirculationlane(s).Doroundaboutsthatprovidedirectaccesstoactivitycenters
throughadedicatedlegperformlessfavorablyinsafetythanotherroundabouts?
Roundabouts and Access Management
Page 100
Chapter5SafetyAnalysis
Figure50.AnActivityCenterwithAccessThroughaMajorDriveway
Figure51.AnActivityCenterwithDirectAccessfromaRoundabout
Ofthe131commercialroundaboutsinFlorida,19roundaboutswerefoundtoprovidedirectaccesstothe
activitycenters.Ofthese19roundabouts,15haveeitherthreeorfourlegs.Thecrashexperienceofthree‐
andfour‐leggedroundaboutswithandwithoutdirectaccesstotheactivitycenterswascomparedusingan
independentt‐testwiththefollowinghypothesis:
 H0:thereisnodifferenceinmeans(i.e.,averagecrashesperroundabout)betweentheroundabouts
withandwithoutdirectaccesstoactivitycenters(µ1=µ2),
 H1:therearedifferencesinmeans(i.e.,averagecrashesperroundabout)betweentheroundabouts
withandwithoutdirectaccesstoactivitycenters(µ1≠µ2).
Table25summarizestheseresults.Ata5%significancelevel,theperformanceofthethree‐legged
roundaboutswithandwithoutdirectaccesswasstatisticallyinsignificant,whiletheperformanceofthe
four‐leggedroundaboutswithandwithoutdirectaccesswasstatisticallysignificant.Overall,therewas
sufficientevidencetosupporttheconclusionthatata5%significancelevel,therewasnosignificant
differenceintheperformanceofthree‐andfour‐leggedroundaboutswithdirectaccesstoactivitycenters
andthosewithoutdirectaccess.
Roundabouts and Access Management
Page 101
Chapter5SafetyAnalysis
Table25.StatisticsofRoundaboutswithThreeandFourLegs
RoundaboutsWITHDirectAccess
toActivityCentersThrough
aDedicatedLeg
RoundaboutsWITHOUTDirect
AccesstoActivityCentersThrough
aDedicatedLeg
Ata5%
Significance
Level,Isthe
Performanceof
No.of Total
Roundabouts
Total
Legs Crashes Numberof Crashesper Crashes Numberof Crashesper
Withand
inFive Roundabouts Roundabout inFive Roundabouts Roundabout WithoutDirect
Access
Years
(b)
(a/b)
Years
(d)
(c/d)
Significantly
(a)
(c)
Different?1
No
3
23
5
4.6
163
39
4.2
(p‐value:0.925)
Yes
4
33
10
3.3
473
60
7.9
(p‐value:0.021)
No
3and4
56
15
3.7
636
99
6.4
(p‐value:0.145)
1
Ata5%significancelevel,ifP‐value<0.05,itisconcludedthatthereisasignificantdifferenceintheperformance
ofroundaboutswithdirectaccesstoactivitycentersandthosewithoutdirectaccess.Similarly,ifP‐value>0.05,it
isconcludedthatthereisnosignificantdifferenceintheperformanceofroundaboutswithdirectaccesstoactivity
centersandthosewithoutdirectaccess.
Thenextquestioniswhetheritwouldbebeneficialifprovidingdirectaccesstoactivitycentersresultsin
fiveormorelegsataroundabout,i.e.,morethanthetypicalroundaboutswiththreeorfourlegs.Table26
showsthecrashstatisticsofcommercialroundaboutswithfiveandsixlegs.Itcanbeseenthatthese
roundaboutsexperiencedasignificantlyhighernumberofcrashes,especiallyinthesix‐leggedcase,when
comparedwiththoseofthree‐andfour‐legged.Thesignificantincreaseintheaveragecrashesisexpected
astheadditionallegsquicklyincreasethenumberofconflictpointsinthecirculationlanesandbecome
confusingtothedrivers.Figure52givesexamplesoftwosix‐leggedroundaboutswhichcollectively
experienced154crashesduringthefive‐yearanalysisperiod.
Theabovecrashstatisticssuggestthatprovidingdirectaccesstoactivitycentersatroundaboutsis
desirable,butonlyifitdoesnotincreasethenumberofroundaboutlegsbeyondthestandardfourlegs.
Table26.StatisticsofRoundaboutswithFiveandSixLegs
No.ofLegs
5
6
5and6
TotalCrashesinFiveYears
(a)
157
213
370
Roundabouts and Access Management
Page 102
Numberof Roundabouts
(b)
10
4
14
CrashesperRoundabout
(a/b)
15.7
53.3
26.4
Chapter5SafetyAnalysis
(a)PonceDeLeon,MiamiDadeCounty,Florida
(b)MemorialCausewayBoulevard,PinellasCounty,Florida
Figure52.ExamplesofSix‐leggedRoundaboutsthatExperiencedHighCrashes
Roundabouts and Access Management
Page 103
Chapter5SafetyAnalysis
5.5SafetyofVulnerableRoadUsers
Thissectionfocusesonevaluatingthesafetyofvulnerableroadusers(i.e.,pedestriansandbicyclists)inthe
vicinityofroundabouts.
5.5.1Pedestrians
During2007‐2011,the131roundaboutsincommercialareasexperiencedatotalof20pedestriancrashes,
constituting1.06%ofthetotalcrashes.Ofthe20pedestriancrashes,onewasfatalandtworesultedin
severeinjuries.Thefatalcrashinvolvedapedestrianwhowasintoxicated.Figure53givestheillustrative
sketchofthecrash.Besidesthisfatalcrash,apedestrianwasfoundtobeintoxicatedinoneothercrash,
whichresultedinanon‐incapacitatinginjury.
Illustrativesketchesanddescriptionsofthe20pedestriancrasheswerereviewedindetailtodetermine
theat‐faultroaduser.Ofthe20pedestriancrashes,10(i.e.,50%)occurredduetodriverfault,andthe
pedestrianwasfoundtobeatfaultinsevencrashes(i.e.,35%).Forthreecrashes,identifyingtheat‐fault
roaduserwasnotpossibleduetoinconclusiveinformationinthepolicereports.Whenthepedestrianwas
foundtobeatfault,thefollowingwerethemostfrequentcontributingcauses(numberinparentheses
indicatesthenumberofrelatedcrashes):
 pedestrianobstructedthepathofvehicles(3),
 pedestrianfailedtoyieldright‐of‐waytothevehicle(2),and
 pedestrianwasundertheinfluenceofalcoholand/ordrugs(2).
Figure53.FatalCrashInvolvingaPedestrian(CrashID:772427040)
Whenthedriverwasfoundtobeatfault,themostfrequentcontributingcauseswere:
 carelessdriving(5),
 driverfailedtoyieldright‐of‐waytothepedestrian(4),and
 driverdisregardedtrafficsignalorothertrafficcontrol(1).
Ofthe20pedestriancrashes,onlytwocrashesoccurredatroundabouts,andtheremaining18crashes
occurredontheapproachlegs.Crashesthatoccurredontheroundaboutlegswerereviewedindetailto
Roundabouts and Access Management
Page 104
Chapter5SafetyAnalysis
identifyanyspecificcontributingfactors.Table27providespedestriancrashstatisticsbymediantype.Of
the18pedestriancrashesthatoccurredontheroundaboutlegs,11crashes(61.1%)occurredatraised
medians,oneoccurredonalegwithTWLTL,whiletheremainingsixoccurredonundividedsections.From
thetable,itisalsoclearthatthenumberofpedestriancrashesper100legswashighestforraisedmedians
at6.40pedestriancrashesper100legs.Further,itwasfoundthatallthreesevereinjurypedestrian
crashesoccurredonapproacheswithapostedspeedgreaterthan30mph,andlow‐speedcorridors(i.e.,
postedspeedlimit≤30mph)didnotexperienceseriousinjuries.
Table27.PedestrianCrashStatisticsbyMedianType
MedianType
RaisedMedian
TWLTL
UndividedSections
Other
Total
Numberof
PedestrianCrashes
(a)
11
1
6
0
18
Numberof
ApproachLegs
(b)
172
18
281
19
490
NumberofPedestrianCrashes
per100ApproachLegs
(a)/(b)
6.40
5.56
2.14
0.00
3.67
5.5.2Bicyclists
During2007‐2011,atotalof47bicycle‐vehiclecrashesoccurredinthevicinityofthe131roundabouts.
Althoughnoneofthecrasheswerefatal,amajorityofthecrashesresultedinaninjury.Asitcanbeinferred
fromthetable,48.9%ofbicyclecrasheswerearesultofdrivererrorwhile40.4%ofthecrashesweredue
tobicyclisterror.Whenthebicyclistwasfoundtobeatfault,thefollowingwerethemostfrequent
contributingcauses(numberinparenthesesindicatesthenumberofrelatedcrashes):
1. bicyclistfailedtoyieldright‐of‐waytothedriver(7),
2. bicyclistobstructedvehicles’pathbyeitherfallingoffthebikeorlosingcontrolofthebikeintothe
pathofthevehicle(6),and
3. bicyclistrodeintoastoppedvehicle(3).
Whenadriverwasfoundtobeatfault,themostfrequentcontributingcauseswere:
 driverfailedtoyieldright‐of‐waytothebicyclist(13)and
 carelessdriving(9).
ThecorridoronSWSecondAvenueinGainesvillehasthreeroundaboutsandhad12bicyclecrashes(i.e.,
25.5%oftotalbicyclecrashes)duringthefive‐yearanalysisperiod.Figure54showstheaerialviewofthis
corridor,whichisneartheUniversityofFlorida(UF)mainentrance.Thiscorridorwasfoundtohavea
significantamountofbicycletraffic;thisdisproportionatelyhighexposureresultedinahighnumberof
bicyclecrashes.
Table28givesbicyclecrashstatisticsbasedonwherethecrashhadoccurred(i.e.,eitheratthe
roundaboutoronanapproachleg).The131roundaboutshave490legs;86ofthesehavedesignatedbike
lanes.During2007‐2011,these86legsexperiencedeightbicyclecrashes,whiletheremaining404legs
withoutdesignatedbikelanesexperienced20bicyclecrashes.However,thesestatisticsdonottakeinto
accountbicycleexposuredata.Inotherwords,locationswithdesignatedbikelanesmightexperiencemore
bicyclecrashessimplybecausemorebicyclistsusethefacility.
Roundabouts and Access Management
Page 105
Chapter5SafetyAnalysis
ThecorridoronSWSecondAvenueinGainesvillehasthreeroundaboutsandhad12bicyclecrashes(i.e.,
25.5%oftotalbicyclecrashes)duringthefive‐yearanalysisperiod.Figure54showstheaerialviewofthis
corridor,whichisneartheUniversityofFlorida(UF)mainentrance.Thiscorridorwasfoundtohavea
significantamountofbicycletraffic;thisdisproportionatelyhighexposureresultedinahighnumberof
bicyclecrashes.
Table28.BicycleCrashStatisticsbyLocationandCrashSeverity
CrashSeverity
CrashesatRoundabout CrashesonApproachLeg TotalBicycleCrashes
FatalInjury
IncapacitatingInjury
Non‐IncapacitatingInjury
PossibleInjury
PropertyDamageOnly
TotalCrashes
0
1
12
3
3
19
0
4
11
9
4
28
0
5
23
12
7
47
Figure54.CorridoronSW2ndAvenue,Gainesville,AlachuaCounty,Florida
5.6SummaryofFindings
Atotalof283roundaboutsinFloridawereincludedintheanalysis.During2007‐2011,1,882crashes
occurredwithin500ft.oftheseroundabouts.Policereportsofthesecrasheswerereviewedindetailto
investigatethefollowingpotentialsafetyconcernsassociatedwithroundaboutsincommercialareas:
 Impactofdrivewaycornerclearancesonroundaboutsafety.
 Safetyimpactofmedianopeningsinthevicinityofroundabouts.
 Safetyatroundaboutsthatprovidedirectaccesstoactivitycenters.
 Safetyofvulnerableroadusersincludingpedestriansandbicyclists.
Onaverage,eachroundaboutexperienced6.65crashesduringthefive‐yearanalysisperiod.Amajorityof
crasheswerefoundtobePDOs.Lessthan5%ofcrashesresultedinsevereinjuries(i.e.,fatalinjuryand
incapacitatinginjury).Intermsofcrashtype,collisionwithafixedobject,anglecrashesandrear‐end
crasheswerepredominant,constitutingover60%oftotalcrashes.
Roundabouts and Access Management
Page 106
Chapter5SafetyAnalysis
During2007‐2011,the131commercialroundaboutsexperiencedatotalof74driveway‐relatedcrashes.Of
thesecrashes,37(50%)occurredatthefirstdriveway(i.e.,thedrivewaythatdefinesthecornerclearance),
including18thatoccurredattheupstreamdrivewaycornerclearance,and19thatoccurredatthe
downstreamdrivewaycornerclearance.Morecrasheswerefoundtooccuratthefirstdriveway
downstreamratherthanupstreamofroundabouts,indicatingthatdownstreamdrivewaycorner
clearanceshaveagreatersafetyimpactthanupstreamdrivewaycornerclearances.Thisresultisconsistent
withthefactthatvehiclesexitingadrivewaydownstreamofaroundaboutexperiencereducedgapsdueto
dispersedplatoonsfromtheupstreamroundabout.Further,largercornerturningradiitypicalof
roundaboutsincreasesvehicle‐turningspeed.Whencombinedwithreducedsightdistanceduetosight
obstructions,thetimeavailablefordrivewayvehiclestocompletetheirmaneuverscouldbesignificantly
reduced.
Athigh‐volumelocations,turningtrafficatmedianopeningsinthevicinityofroundaboutsexperience
reducedvehiclegaps,whichcouldresultinmorecrashes.Toaddressthispotentialsafetyconcern,crashes
atmedianopeningsinvolvingleft‐turningvehicleswereidentified.Arelativelylowtotalof15crasheswere
foundtoinvolveturningvehiclesatthemedianopenings,andamajorityofthesewerenotsevere.Crash
datadidnotindicateanyserioussafetyissueswithmedianopeningsinthevicinityofroundabouts.
Nonetheless,closingthemedianopeningsandhavingvehiclesmakeU‐turnsatthedownstream
roundaboutcouldpotentiallypreventsomeofthesecrashes.However,thisalternativewasfoundtopose
twotrafficoperationalproblems.First,thisalternativemayincreasethenumberofsideswipecrashesat
roundaboutsespeciallyforlargevehicles.Thesecondproblemwithclosingmedianopeningsisthatit
preventscertainturningmovements,whichmayresultinmigrationofcrashes.Thisproblemexists
becausethereisnotanotherroundaboutavailabletofacilitatealltheU‐turnsneededwhenmedian
openingsareclosed.Atlocationswithbothupstreamanddownstreamroundabouts(i.e.,roundaboutsin
series),closingthemedianopeningcouldbecomebeneficial,asitcouldpotentiallypreventsomeofthe
medianopeningrelatedcrasheswithoutmakingsometurningmovementsdifficult.
Accesstomajoractivitycentersisoftenprovidedatmid‐blocklocationsonacorridor.Onealternativeisto
havetheaccesspointconnecteddirectlytotheroundabout(i.e.,throughadedicatedleg).Ofthe131
commercialroundaboutsinFlorida,19roundaboutswerefoundtoprovidedirectaccesstotheactivity
centers.Averagecrashesperroundaboutatthree‐andfour‐leggedroundaboutswithandwithoutdirect
accesstotheactivitycenterswerecomparedusinganindependentt‐test.Ata5%significancelevel,there
wasnosignificantdifferenceintheperformanceofthree‐andfour‐leggedroundaboutswithdirectaccess
toactivitycentersandthosewithoutdirectaccess.Itwasalsofoundthatroundaboutswithmorethanfour
legsexperiencedasignificantlyhighernumberofcrashes.Thiswasexpectedastheadditionallegsincrease
thenumberofconflictpointswithinthecirculationlanesandbecomeconfusingtothedrivers.Overall,the
crashstatisticssuggestthatprovidingdirectaccesstoactivitycentersatroundaboutsisdesirable,butonly
ifitdoesnotincreasethenumberofroundaboutlegstobeyondthestandardfourlegs.
Safetyofvulnerableroadusers(i.e.,pedestriansandbicyclists)inthevicinityofroundaboutswas
evaluated.Duringthefive‐yearanalysisperiod,the131commercialroundaboutsexperienced20
pedestriancrashes.Ofthese20crashes,onlytwooccurredatroundabouts,whiletheremaining18
occurredontheroundaboutlegs.Comparedtopedestriancrashes,bicyclecrashesweremorefrequent;
during2007‐2011,47bicyclecrasheswerereported.Ofthese47,19occurredatroundaboutsandtherest
wereontheroundaboutlegs.Roundaboutlegswithdesignatedbikelanesresultedinaslightlygreater
proportionofbicyclecrashescomparedtothosewithoutbikelanes.However,thisobservationdidnottake
intoaccountbicycleexposuredata,whicharenotavailableforthisstudy.
Basedontheresultsfromthesafetyanalysis,thefollowinggeneralrecommendationsrelatedtotheaccess
featuresinthevicinityofroundaboutsaremade:
Roundabouts and Access Management
Page 107
Chapter5SafetyAnalysis



Crashdatashowthatdownstreamdrivewaycornerclearanceshaveagreatersafetyimpactthan
upstreamdrivewaycornerclearances.Longerdownstreamcornerclearancesaredesirableto
provideadditionaltimefordrivewayvehiclesthatexperiencereducedvehiclegapsandhigher
approachvehiclespeedfromupstreamroundabouts.
Crashdatadidnotindicateserioussafetyissueswithmedianopeningsinthevicinityof
roundabouts.However,closingmedianopeningslocatedbetweentwoadjacentroundaboutscould
preventsomeofthemedianopeningrelatedcrashesandisdesirableifthecorridorisdesignedto
servelowheavyvehiclevolumesoriftheroundaboutsaresufficientlylargetosafelyaccommodate
U‐turnsbyheavyvehicles.
Crashdatadidnotshowanincreasedsafetyhazardatroundaboutsthatprovidedirectaccessto
activitycenters.Providingdirectaccesstoactivitycentersthroughadedicatedlegisdesirableto
improvetrafficoperationsonthecorridoriftheprovisiondoesnotincreasethenumberof
roundaboutlegstobeyondthestandardfour.
Roundabouts and Access Management
Page 108
Chapter6OperationalAnalysis
ChapterSix:OperationalAnalysis
Thissectionpresentsthefindingsoftheoperationalanalysisofroundaboutsandaccessmanagement.
Analysisofconflicts(involvingvehicles,pedestriansandbicyclists,etc.),accesstodrivewaysandviolation
oftrafficrulesatroundaboutsareconductedtosummarizetheissuesrelatedtoaccessmanagement.
6.1OverviewofDataCollectionSites
Siteobservationsanddatacollectionwereconductedatthe13selectedsitesinFlorida.Detailsofthe
roundaboutsareincludedinAppendixD.Ingeneral,thesiteswereselectedbasedupontrafficvolume,
proximityorlocationonstatehighways,orincloseproximitytodriveways.Adiversityofconditionswere
selectedforthefollowingcharacteristics:singleandmultiplelanes,commercialandmixed‐laneuses
adjacenttotheroundabout,proximitytoparking,asingleisolatedroundaboutandaroundaboutcorridor.
All13roundaboutsselectedforobservationareconsideredtohaveatleastamoderatetrafficvolumelevel
duringpeakperiods.Sevensitesaresingle‐laneroundaboutsandfivearemulti‐laneroundabouts.The
otherisconsideredacomplexroundabout,whichincludespiralroundabouts,turboroundaboutsora
roundaboutthathasmultiplesliplanes.Ninesitesarelocatedincommercialareas;theremainingfourare
locatedinamixed‐usearea.Allofthesiteshaveadrivewaynearby.Allofthemhavedrivewaysnearboth
accessandegresslegsoftheroundabout.Sevenofthesiteshavedrivewaysnearboththeaccessandegress
approachesoftheroundabout.Twositeshavedrivewaysinthemiddleoftheroundabout.Allofthesites
arelocatednearstatehighways,andoneisonastatehighway.Twositesarelocatedonstreetswithon‐
streetparking,whereintheparkingmaneuveronthestreetcouldaffecttheoperatingspeed,safety,and
perhapsaccessoftheroundabout.Foursitesarelocatedinaseriesofseveralroundabouts.Theliterature
suggeststhataseriesofroundaboutsinacorridor,particularlyacommercialcorridor,canprovideamore
aestheticallypleasingarea,slowtraffic,andimproveaccessandsafety.Buildingaseriesofroundaboutscan
createavibrantbusinessarea.Therefore,itisdesirabletolookattheperformanceandaccessissuesofa
seriesofroundabouts.
6.2AnalysisofAccessManagementIssuesAffectingOperations
Duringthefieldobservations,severalaccessmanagementissueswereidentifiedattheroundabouts.These
include:(1)conflictataccesspointswithinthefunctionalarea,whichincludesintersectionsofadriveway
andapproachinglaneofaroundabout,andtheimpactofqueuingontheoperationofanearbystop‐
controlleddriveway;(2)conflictsatroundaboutsinvolvingpedestriansandbicyclists;and(3)violationof
trafficrulesanditsimpactontheroundaboutoperations.Eachoftheseissuesisaddressedseparately
below.
6.2.1ConflictsatAccessPointwithinRoundabout’sFunctionalArea
Ifanaccesspoint,suchasadrivewayoranotherintersection,islocatedwithintheroundaboutfunctional
area,vehicleconflictsmayoccurandcompromisetheoperationoftheroundabout.Theconflictbetweena
vehiclemakingaleftturnintoadrivewayandtheopposingtrafficflowenteringtheroundaboutwasa
commonfieldobservation.Figure55showsanexamplethatwasobservedatSW2ndAvenueandSW6th
StreetintheCityofGainesvilleinAlachuaCounty.Thedrivewayislocatedneararoundabout(60ft.).
Whenthequeuespillsbackattheleft‐turninglane,leftturningvehiclesfromtheexitinglaneintothe
drivewaycanbeblockedattheturningbay,causingaspillbackintotheroundabout,whichtheninterferes
withtheoperationoftheentireintersection.
Roundabouts and Access Management
Page 109
Chapter6OperationalAnalysis
Figure55.ConflictofLeft‐turnVehicleatRoundabout(SW2ndAvenueandSW6thinAlachuaCounty)
AnothercaseiswhenanAWSCdrivewayisclosetoaroundabout.Inthissituation,thetrafficenteringthe
roundaboutcanspillbackintothedriveway.Figure56showsaroundaboutinMiami‐Dadeinwhichthe
trafficspillsbackfromtheapproachinglaneandblockstheoperationoftheAWSCdriveway.Acertain
distanceisclearlyneededbetweentheroundaboutandthenearbyintersection.
Figure56.RoundaboutObservationonSpillBackofEnteringTrafficintoanAdjacentAWSCIntersection
(NE10thCt.andSW152ndAve.,Miami)
6.2.2ConflictswithPedestrians
Figure57showstheinteractionbetweenpedestriansandvehiclesatIndependentDriveandSouthLaura
Street,inJacksonville.Sincethisroundaboutislocatedinabusinessandcommercialarea,wecanobservea
relativelyhighflowofpedestriantraffic.Whenacarstopsforapedestrianatacrosswalk,thequeuebehind
thecarspillsbackintothecirculatinglane,andaffectstheoperationoftheroundabout.
Roundabouts and Access Management
Page 110
Chapter6OperationalAnalysis
Figure57.RoundaboutObservationwithPedestrianConflict(IndependentDr.andS.LauraSt.,Duval
County)
6.2.3ViolationofTrafficRules
Thereareseveralcaseswheredriversviolatetrafficrulesandstopinthemiddleofroundabouts.Figure58
showsacaseatIndependentDriveandSouthLauraStreet,inJacksonville,wheretheroundaboutisplaced
nearabusinessandshoppingcenterdowntown.Peopletendtopickuppeopleattheroundaboutandcause
aqueueback‐upinthecirculatinglane.
Figure58.RoundaboutObservationwithDriverViolationofTrafficRules(IndependentDr.andS.Laura
St.,DuvalCounty)
Anotherexampleofviolationoftrafficrulesiswhenvehiclesstopatthedrivewayandpickuppeople.The
queuespillsbackintothecirculatinglaneandcausesonelanetojam.Carsinthislanetrytochangetothe
othercirculatinglaneanddisrupttheoperationoftheroundabout.
Roundabouts and Access Management
Page 111
Chapter6OperationalAnalysis
Figure59.RoundaboutObservationwithSpillBackfromDrivewayintoCirculatingLanes(CausewayBlvd.
andMandalayAve.,PinellasCounty)
6.2.4SummaryofOperationalAnalysis
Inmostcases,roundaboutsoperateinamannersimilartoothertypesofintersections,suchasnon‐
signalizedintersections.Thus,fromanoperationalperspective,accessmanagement,shouldbemanagedin
awaythatissimilartootherintersections.Howeverthecombinationofroundaboutandaccess
managementdoeshavesomeuniquefeaturesforoperations.
Insummary,thefollowingsuggestionsaremadetocountertheproblemsfoundinthesiteobservations.
Beforethedesignandconstructionoftheintersection,thedistancebetweentheroundaboutandnearby
drivewaysshouldbecarefullyconsideredinordertokeepthedrivewayandroundaboutsinoperation.
Thedistancebetweentheroundaboutandthenearbyintersectionshouldalsobecarefullyconsideredand
enoughstoragecapacityshouldbeprovidedtokeeptheroundaboutandanyadjacentintersections
functioningproperly.Ifthetrafficvolumeismoderateandthepercentageofheavyvehiclesislow,whena
drivewayhastobelocatedclosetoaroundabout,amedianclosingshouldbeusedandanotherroundabout
atthenextintersectionisrecommendedtoallowU‐turnsforaccessingdriveways.Ifaroundabouthasless
than4legs,accesstonearbyactivitycentersshouldbeprovidedbyusingaseparatedriveway,insteadof
linkingtheroundabouttotheactivitycenteritself(asshowninFigure58wherevehiclesstoppedinthe
roundabouttopickupapassenger);ifmorethan4legsareincluded,trafficdesignersshouldavoidadding
onemorelegtotheroundaboutbasedonthefindingsinChapter5.Additionally,drivereducationis
necessarytomaintainroundaboutoperations.
6.3AssessmentofSoftware
Anumberofsoftwarepackagescanbeusedtoanalyzetheoperationaleffectofroundabouts.Basedon
theirmethodology,wecandividethemintotwodifferentgroups:deterministicsoftwaretoolsand
simulationtools.Deterministicmethodsmodelvehicleflowsasflowratesandaresensitivetochangesin
flowrateandthegeometricdesignofroundabouts(Rodegerdtsetal.,2010,p.4‐18).Macroscopicanalysis
toolsalsofallintothiscategory(Trueblood,2013).Examplesofsoftwarepackagesthatimplement
deterministicanalysismethodsareHighwayCapacitySoftware(HCS),ARCADY,RoundaboutDelay
(RODEL),SIDRA,andSynchro.Microscopicsimulationisanotherwaytomodelroundabouts.Suchtools
canmodelanddisplayindividualvehiclesandthusaresensitivetofactorsatthatlevel:car‐following
behavior,lane‐changingbehavior,anddecision‐makingatintersectionssuchasgapacceptance
(Rodegerdtsetal.,2010,p.4‐19).Examplesofsoftwarepackagesthatperformmicroscopicsimulationare
CORSIMandVISSIM.
Roundabouts and Access Management
Page 112
Chapter6OperationalAnalysis
6.3.1HCS
HCSstandsforHighwayCapacitySoftware,whichisasoftwarepackagethatimplementsthedeterministic,
macroscopicanalysismethodsoftheHighwayCapacityManual.TheprocessitemploysistheHighway
CapacityManualprocedure,whichusescriticalgapandfollow‐uptimealongwithturningmovementto
computethecapacityofeachapproach.ThenewestversionofHCS2010,basedontheHCM2010,provided
anewanalyticalmethodinassessingroundaboutoperations.Approachcontroldelay,approachLOS,
intersectiondelayandintersectionLOScanbecalculatedbythesoftware(TRB,2010a).
ThemethodologyinHCM2010focusedontheoperationofroundaboutswithintheboundariesofthe
roundabout.Thismethodologyprovidesacombinationofanempiricalapproachandananalytical
approachforevaluatingroundaboutoperationsbasedonrecentU.S.fielddata(Rodegerdtsetal.2010).
Evaluationforbothsingle‐laneanddouble‐laneroundaboutsareprovidedinHCM2010.ThereforeinHCS,
wecanonlymodelroundaboutswithtwoorlesscirculatinglanes.
Figure60.InterfaceofHCS2010
InTable29.InputandOutputforRoundaboutComponentsinHCS2010,theinputforcalculation
roundaboutsinHCS2010isshown.SinceHCS2010adoptedthemethodologyinHCM2010,morefeatures
havebeenavailableinassessingroundaboutperformance.
Roundabouts and Access Management
Page 113
Chapter6OperationalAnalysis
Table29.InputandOutputforRoundaboutComponentsinHCS2010
Input
Parameters
TurningFlows
PeakHourFactor
CriticalGap
Follow‐upHeadway
HCS2010
Input
Input
Input
Input
Output
PerformanceMeasures
Capacity
ApproachDelay
ApproachLOS
Queue
IntersectionDelay
IntersectionLOS
HCS2010
Yes
Yes
Yes
Yes
Yes
Yes
ThetwomostimportantparametersintheHCM2010roundaboutmodelarecriticalgapandfollow‐up
headways.Thesetwovaluesplayanimportantroleintheoperationalanalysisofbothsingle‐laneand
double‐laneroundabouts(TRB,2010a).OneofthedisadvantagesoftheHCM2010modelforassessing
roundaboutandaccessmanagementisthatitdoesn’taccountforeffectsrelatedtogeometrysuchaslane
width,ortrafficflowfromadjacentintersections(Trueblood,2013).HCS2010hastheabilitytocalculate
roundaboutapproachqueuelengths.Thisfeatureisessentialtounderstandingaccessmanagementissue
relatedtoroundabouts.
6.3.2Synchro
Synchroisananalysistoolforstudyingintersectionsatamacroscopicscale.SimilartoHCS,Synchrocan
alsobeusedtoassessroundaboutperformancebasedontheHCM2010methodology.Codingaroundabout
isverystraightforwardwithinSynchro.Theuseronlyneedstospecifytheintersectioncontroltypeasa
roundaboutaftersettingupanintersectionwiththespecificgeometryandvolumedata.IftheHCM2010
methodwasselectedinSynchro,theoutputresultsshouldbepresentedinthemannershowninFigure61.
Figure61.UserInterfaceofSynchro(Trueblood,2013)
Roundabouts and Access Management
Page 114
Chapter6OperationalAnalysis
Synchroalsocomeswithamicro‐simulationtoolcalledSimTraffic.Thistoolallowstheusertodesignand
evaluateadvancedroundaboutsdesignsthatexceedtheHCM2010methodologylimitations.Forinstance,
HCScannotmodelroundaboutswithmorethantwocirculatinglanes(Trueblood,2013).Synchrocanalso
assesstheperformanceofaseriesofroundaboutsinacorridor.
6.3.3SIDRA
SIDRAwasoriginallydevelopedbyARRBTransportResearchLtd.andlaterbyAkcelik&Associates
(Akcelik&Associates,2014).Itisoneofthemostwidelyusedroundaboutanalysissoftwareprogramsin
theUnitedStates(Jacquemart,1998).Themodelisbasedonananalyticalmethod,whichusesgap‐
acceptancetechniquestodetermineroundaboutcapacity,delay,queuelength,andotherperformance
measures.SimilartotheHCM2010,SIDRAincludestwoimportantgapparameters:criticalgapandfollow‐
upheadway.Thecriticalgapandfollow‐upheadwayvaluescanbeeitherspecifiedbytheuseror
automaticallyestimatedbySIDRAaccordingtothegeometryandflowconditionsateachentry(Yinetal.,
2011).
AlthoughSIDRAwasdevelopedinAustralia,itdoesincludeseveralmodeloptionstoaccountfor
roundaboutcapacitydifferencesinotherpartsoftheworld.Anenvironmentfactorof1.2wasadoptedasa
globalcalibrationfactorfortheSIDRAversionissuedintheUnitedStates(Yinetal.,2011).Thisfactor
adjuststhecriticalgapandfollow‐upheadwayvalues;thereforethecapacityvalueisadjusteddownward
andtheresultingroundaboutperformancemeasureswillbeworsethanthoseforaroundaboutin
Australia,allelsebeingequal.ThenewestversionofSIDRAcanaccommodatebothHCMmodelandSIDRA
mode. 6.3.4RODELandARCADY
ThesoftwareARCADYwasdevelopedbyTransportationResearchLab(TRL)intheUnitedKingdom.Ituses
alinearregressionformulatopredictcapacity,queuelength,delays,andcrashfrequenciesasafunctionof
geometry(Elias,2009).Queuesanddelayswerebasedontime‐dependentqueuingtheory.ARCARDYcan
modelroundaboutwiththeinclusionofcrashprediction,geometricdelay,andpedestriancrossing
(Waddell,1997).
RODELstandsforRoundaboutDelay,whichwasfirstdevelopedin1987.Itisusedtoexperimentwith
differentgeometricdesignsofroundabouts.RODELcanprovidecapacityestimates,averageandmaximum
delay,queuesforeachapproach,andanestimateofoveralldelay(Elias,2009).RODELcanuseobserved
variationincapacitytoallowtheuserstosettheirdesiredconfidencelevel.Theinclusionofstatistical
variabilityinRODELgavedesignersapreciselevelofconfidencethattheirdesignswouldmeetthe
requirementofcapacityanddelaywithsignificantflexibility(Waddell,1997).RODELcanalsoprovidethe
maximumprobablequeueover40daysratherthantheaveragequeueasinotherroundaboutmodels.
(Waddell,1997).
6.3.5VISSIM
VISSIMisamicro‐simulationprogramdevelopedbyPTVinGermany(PTVGroup,2013).Criticalfeatures
inVISSIM,suchaslinkandconnectors,routingdecisions,priorityrules,andreducedspeedzones,provides
arealisticrepresentationofroundabouttrafficoperations(TruebloodandDale,2003).
VISSIMusesalinkandconnectorsystemratherthanthelinkandnodesystemthatCORSIMuses.This
systemallowsVISSIMtoemphasizethelinkbyusingconnectorstojoindifferentlinkswithoutconsidering
Roundabouts and Access Management
Page 115
Chapter6OperationalAnalysis
thenode.Forroundaboutsimulation,thissystemissuitablesincealinkinVISSIMallowsmultipleinternal
inflectionpointswithoutaffectingthesimulationoftrafficflow(TruebloodandDale,2003).
Figure62.ExampleofRoundaboutSimulationinVISSIM(FHWA,2011)
ManyotherfeaturesinVISSUMfacilitateitsusagetosimulatetrafficmovementthrougharoundabout.The
availabilityofsettingroutechoicedecisionsinVISSIMallowstheusertodetermineaspecificpaththrough
aroundaboutandthespecificvolumepercentage.Thereforeitalsoallowsausertospecifywhichlanea
vehicleusestocompleteitsroutingdecisionthroughmulti‐laneroundabouts(TruebloodandDale,
2003).ThepriorityrulesinVISSIMallowsuserstospecifytheyieldprocessattheconflictpoint.
Adjustmentofgap‐acceptancetimes,dependingondifferentvehicletypes,canalsobedeterminedusing
thesettingofpriorityrules(2003).ReducedspeedzonesinVISSIMarealsogreatfeaturestousein
modelingroundabouts,sincevehiclesusuallyslowdownto15‐25mi/htocirculatetheroundabout(2003).
VISSIMprovidesaflexibletoolforuserstoaccuratelysimulatetheoperationofroundabouts.Researchalso
pointedoutthatVISSIMallowsuserstofine‐tunethegapacceptanceparametersrequiredforthe
simulation(StanekandMilam,2005).Withgreatflexibilityandaccuratefeatures,itisbelievedthatVISSIM
isthebestmicro‐simulatorforroundaboutmodeling(Elias,2009).
6.3.6CORRIDORSIMULATION(CORSIM)
CORSIMincludestwomicroscopicsimulationsubprograms,NETSIMandFRESIMthatarespecializedfor
urbanstreetsandfreeways,respectively.AlthoughitiswidelyusedintheUnitedStates,CORSIMhas
limitedcapabilitiesforsimulatingroundabouts(Elias,2009).SinceCORSIMusesalinkandnodestructure
tomodelatransportationnetwork,withnodesbeingintersectionsandlinksrepresentingtheconnecting
roadways,itdoesnotprovideadirectrepresentationofroundabouts.TomodelaroundaboutinCORSIM,
theuserneedstocreateaseparatenodeforeachapproachandconnectthesenodestogetherwithaone‐
waylinksegmentinacounterclockwisedirectionasshowninFigure63.(Elias,2009).
TheinputsforroundaboutsimulationinCORSIMincludethefollowing:approachvolumesforeachleg,
origin‐destinationofalltraffic,geometriccharacteristics,andspeeddistribution.TheoutputsfromCORSIM
includecontroldelay,averagequeue,andmaximumqueue,andotherstandardperformancemeasures. Whenstartingthesimulation,thevehicleentryheadwaydistributionshouldbemodifiedbasedonfield
datatocloselymatchthearrivalsateachapproach.Thentheuserneedstoconnecteachapproachusinga
counterclockwiseone‐waylinkasinFigure63.Itisimportanttoverifythatthelengthoftheone‐waylink
matchesthesizeoftheactualroundabout.Inordertoreplicatethetrafficruleatroundabout,itis
importanttoimplementyieldcontrolateachapproachlane.Thefinalstepistoadjustthegapacceptance Roundabouts and Access Management
Page 116
Chapter6OperationalAnalysis
Figure63.ExampleofModelingRoundaboutinCORSIM(Elias,2009)
modeltomatchthefindingonroundaboutdriverbehavior.Additionally,itispossibletomodelorigin‐
destinationsinCORSIMusingconditionalturnmovements(Elias,2009).
OnemajordifficultyinmodelingroundaboutsinCORSIMistheinputofturnmovements.Since
roundaboutsarereplicatedusingdifferentsegmentsoflinksconnectedwitheachotherbyjoiningnodes
withapproaches,theturnmovementsneedtobesetbasedonconditionallogicasshowninFigure64. Figure64.ConditionalTurnMovementinCORSIM(Elias,2009)
ResearchshowedthattheoutputofCORSIMwhensimulatingroundaboutsisinaccurateandquitedifferent
fromsiteobservation.Averagequeuewasleastwellpredictedforthethreeperformancemeasures(Elias,
2009).ThiscouldpotentiallybringsomedifficultieswhenusingCORSIMformodelingroundabouts,
especiallyforaccessissues.
6.3.6.1ImprovementofCORSIMforRoundaboutModeling.SinceCORSIMdoesnotprovideadirect
methodforroundaboutsimulation,severalrevisionstoCORSIM’sdefaultparametersshouldbeconducted
beforesimulatingroundabouts(Elias,2009).BasedontheresearchofElias,thecurrentversionofCORSIM
doesnotreplicateroundaboutoperationsaccurately(Elias,2009).AlthoughCORSIMhasallthenecessary
featuresforroundaboutsimulation,improvementsshouldbeconsideredinordertogiveCORSIMthe
Roundabouts and Access Management
Page 117
Chapter6OperationalAnalysis
abilitytomodelroundaboutswell.(Elias,2009).Elias(2009)hasmadethefollowingrecommendationsfor
improvementstoCORSIM:  Multiplenodesshouldbeabletobegroupedtogetherasaroundabout.Oncegrouped,thesoftware
seeksinputsforinscribeddiameterandsuper‐elevation.Theprogramthenusesthisinformation
tocalculatethelimitingspeedforcirculatingvehicles.
 Addinputsforturnmovementsandconditionalturnmovementsateachapproachnode.
 Revisecriticalgapandfollow‐uptimeparameterstobeapproachspecific.Defaultvaluesshouldbe
basedonNCHRP3‐65,withtheabilitytooverwritebasedonavailablefielddata.Adjustthelink
lengthsandcurvatureforrealisticanimationinTrafVu.
6.3.7Summary
Thesoftwarepackagesincludedintheassessmentsectionarethosewhichareoftenusedtoanalyze
roundaboutoperation.Insum,deterministicsoftware,suchasHCS,Synchro,SIDRA,RODELandARCADY,
canperformqueuinganalysisandprovideusefulinformationrelatedtoaccessmanagement,especiallyfor
placingdriveways.Simulationsoftware,suchasVISSIM,canbeusedtoevaluatetheoperationof
roundaboutsandtheinteractionbetweentrafficflowsatroundaboutandadjacentdrivewaysby
conductingmicroscopicanalysis.Itisclearfromthisanalysisthatdeterministicsoftwarecanprovide
guidanceonwherethedrivewayshouldbeplacedbeforeconstructionofintersections,whilesimulation
canbeusedtoevaluatetheimpactofdrivewayandotheraccessmanagementissuesonroundabout
operation.HCScandoqueuinganalysis,whichcandeterminetherecommendeddistancebetweenthe
roundaboutandadjacentdriveways.Table30showstherecommendationforselectionofanalysistoolfor
differentdesignandevaluationapplicationsregardingroundaboutsandaccessmanagement.
Table30.RecommendedSelectionofAnalysisToolforDifferentApplicationsRegardingRoundaboutsand
AccessManagement
PotentialAnalysisTool
ExpectedOutcome
RequiredInput
Distanceofdrivewayto Trafficvolume,
HCM,deterministic
roundabout(vehicle
roundaboutgeometric
software
queuing)
characteristics
Pedestrianaccessat
Vehicledelay, vehicle
Trafficvolume(vehicle
HCM,deterministic
roundabout
queuing,pedestrian
andpedestrian),
software,simulation
delay
crosswalkdesign
Accesstoactivitycenter, Vehicledelay,vehicle
Trafficvolume,
Simulation
parking
queuing
Evaluationofinteraction Delayandqueues
Trafficvolume,
Simulation
betweendrivewayand
betweenintersections,
roundaboutgeometric
roundabout
traveltime
characteristics
OthermajorsoftwarepackagethattheFDOTusesforperformingLOSanalysisisLOSPLAN.However,at
thistime,theabilitytoanalyzeroundaboutsisnotincludedinanyoftheLOSPLANcomponentsoftware
programs:ARTPLAN,FREEPLAN,andHIGHPLAN.Thereforediscussionsofthesesoftwarepackagesare
notincludedinthisstudy.Furtherdevelopmentofsuchsoftwarepackagesmaytakeroundaboutsinto
consideration.Somesoftwarepackages,suchasCAP‐X(developedbyFHWA),GIRABASE(French)and
Kreisel(German),canalsoanalyzeroundabouts,butarenotcurrentlyusedbyFDOT. Application
Planningdriveway
location
Roundabouts and Access Management
Page 118
Chapter7Discussion
ChapterSeven:Discussion
7.1Overview
Floridahasrecentlybeguntoencouragetheuseofroundaboutsonthestatehighwaysystemandis
systematicallyupdatingitsguidancedocuments(e.g.,PlansPreparationManual,IntersectionDesign
Manual,andManualonUniformTrafficStudies)butneedsadviceonwhattoincludeintheMedian
Handbook,andDrivewayInformationGuide.Thepolicyjustificationforthischangeinpolicyresultsfrom
increasingevidencethatroundaboutsmaycostlesstoinstall,havegreatersafetypotentialbyreducingthe
numberofconflictpoints,anddependinguponthecontext,haveloweroperationsandmaintenancecosts.
Toaccomplishthisgoalitisimportanttounderstandtheconnectionbetweenroundaboutsandaccess
managementandotherformsoftrafficcontrol.
Roundaboutsarebeingimplementedinavarietyofcontexts,butexistingresearchdoesnotprovide
detailedguidancetoevaluatehowtheroundaboutscanbeimplementedasaformofaccessmanagement.
AccessmanagementisdefinedbytheTRBAccessManagementCommitteeas“thesystematiccontrolofthe
location,spacing,design,andoperationofdriveways,medianopenings,interchanges,andstreet
connectionstoaroadway”(TRB,2003,pp.3).RoundaboutsfacilitateU‐turnsthatcansubstituteformid‐
blockleftturnsand,whenincorporatedintoacorridorofmultipleroundabouts,canaccommodateaseries
ofU‐turnsandleft‐turnlanesthatcanreducedelayinthecorridor(Rodegerdtsetal.,2010).Left‐turn
lanesandmedianopeningscanbereducedoreveneliminatedasvehiclesthatwanttomakealeftturncan
makeaU‐turnandthenarightturntoadriveway.However,becauseoftheiroperationalcharacteristics,
roundabouts“mayalsoreducethenumberofavailablegapsformid‐blocksignalizedintersectionand
driveways”(Rodegerdtsetal.,2010,p.29).Thismayjustreducethecapacityattheseaccesspoints.Atthe
veryleast,thetrafficalongacorridorchangeswiththeintroductionofroundabouts;thetrafficmaybe
moreuniformlydistributedwithalargenumberofsmallergapsratherthanfewerlargerones.
Additionally,asingleroundaboutfunctionsdifferentlythanacorridorofroundabouts;acorridorof
roundaboutscannotbeactivelymanagedtoprovideprioritytoamajorstreetcorridorinthesamewaythat
coordinatedplatoonsoftrafficcanbemanagedtoimprovetheefficiencyoftrafficsignals.Furthermore,
“roundaboutscannotbemanagedwithacentralizedmanagementsystemtofacilitatespecialevents,divert
trafficflows,andsoonunlesssignalsattheroundaboutsorinthevicinityareusedforsuchapurpose”
(TRB,2010a,pp.2‐6).
Developingguidanceforaccessmanagementnearroundaboutsisfurthercomplicatedbytheneedto
understandtheirbenefitsandchallengesforthevarietyofusersoftheroadway.While,inmostcontexts,
roundaboutsaregenerallyfoundtobesaferthantheprevioustreatmentsinbefore‐and‐afterstudies
(Kittelson&Associates,Inc.2013),theactualandperceivedsafetyofroundaboutsvariesamongusers.Yet,
roundaboutsarenotalwayssafeforallusers.Inparticular,insomecontexts,pedestrians,especiallythose
withvisualimpairments,bicyclists,andtruckdriversmayfacespecificchallengesinnavigatingthrough
roundabouts.Theuseofroundaboutsandotheraccessmanagementtechniquesmayestablishpriorityfor
specificmovementsatornearroundaboutsthataffecttheiroperations.
Thepurposeofthisstudyistounderstandpreviousresearchonroundaboutsandaccessmanagement,to
documenthowotherstatesareprovidingguidanceonroundaboutsandaccessmanagement,andto
provideempiricalresearchonthesafetyandoperationsofroundaboutsinFlorida.Thepurposeisto
presentinformationaboutincorporatingguidanceonroundaboutsandaccessmanagementintotheaccess
managementguidelines,ingeneral,and,specifically,intotheMedianHandbook,andDrivewayInformation
Guide.
Thischapterisorganizedasfollows.First,thecontextforunderstandingtheresearchisprovidedby
describinggapsintheliterature,andtheresultsofsafetyandoperationalanalysis.Next,thefindingsfrom
Roundabouts and Access Management
Page 119
Chapter7Discussion
thereviewofnationalandstates’guidanceonroundaboutsandaccessmanagementaresummarized.
Basedupontheseresults,specificrecommendationsaremaderegardingtheneedforadditionalresearch
onroundaboutsandaccessmanagement,specificguidancefortheroundaboutsandaccessmanagement,
andrecommendationsforsoftwaretoanalyzetheoperationsatroundabouts.
7.2RoundaboutsandAccessManagementinFlorida
ThestateofFloridahasarelativelylargenumberofroundaboutsthataresafelyoperatingandproviding
theoperationalefficienciesofroundabouts,butfewofthemarelocatedonthestatehighwaysystem.The
researchteamidentifiedatotalof283roundaboutsthroughoutthestatebutonlyfourofthose
roundaboutsarelocatedonthestatehighwaysystem.Theroundaboutsarelocatedinavarietyofregional
contextswithdiversedesignsandaccessconsiderations.Theregionalcontextvariesfromurbanto
suburbantoruralanddifferentdistancesfromthenearestcommunitycenters,highways,interstates,and
statehighways.Thedesignoftheroundaboutsvariesfromthemorecommonthreeorfourlegroundabout
toroundaboutswithuptosixlegs.Thetypeofroundaboutvariesfromasingle‐lanetomulti‐laneand
turbo,spiralandothercomplexroundaboutdesigns.Someroundaboutshavemediansononeormore
legs,sliplanesandstub‐outs.Accessconsiderationsinvolvedrivewayplacement,thepresenceorabsence
ofmedians,andthetypeofadjacentlanduses,whichincluderesidentialsingle‐family,residentialmulti‐
family,commercialandmixed‐use.Althoughonlyfourarelocatedonthestatehighwaysystem,the
majorityarelocatednearstatehighwaysandinsomecasesprovideaccessthatallowsdriversalternatives
tousingthestatehighwaysystem.
Inthissection,asummaryofthesafetyandoperationalanalysisispresented.Thesafetyanalysis
consideredfourdifferentaspectsofsafetyrelatedtoaccessmanagementnearroundabouts:(1)impactof
drivewaycornerclearancesonroundaboutsafety;(2)safetyimpactofmedianopeningsinthevicinityof
roundabouts;(3)safetyatroundaboutsthatprovidedirectaccesstoactivitycenters;and(4)safetyof
vulnerableroadusers,includingpedestriansandbicyclists.Next,theresultsoftheoperationalanalysis
weresummarizedbyconsideringthreedifferentaspectsoftheoperationsofroundabouts:(1)conflicts
withinthefunctionalareaofroundabouts;(2)conflictsatroundaboutsinvolvingpedestriansand
bicyclists;and(3)violationoftrafficrulesandtheirimpactontheoperationofroundabouts.Then,a
summaryoftheanalysisofthereviewofthenationalandstateguidanceonroundaboutsandaccess
managementispresented.Finally,theFloridaguidelinesforroundaboutsandaccessmanagementare
exploredandtheresultsareplacedwithinthecontextofFloridapractice.
7.2.1SummaryofSafetyAnalysis
Thefindingsofthesafetyanalysisoneachofthefourdifferentaspectsofsafetyareaddressedafterthe
summaryofthecrashdataispresented.
7.2.1.1SummaryofOverallCrashData.Atotalof1,882crasheswithin500ft.ofthe283roundabouts
locatedinFloridathatweredirectlyrelatedtotheroundaboutwerefoundtooccurduring2007‐2011.
Overall,eachroundaboutexperiencedanaverageof6.65crashesperroundaboutduringthefive‐year
analysisperiodwithcommercialroundaboutsexperiencing8.10crashesperroundaboutwhileresidential
roundaboutsexperienced5.4crashesperroundaboutduringthefive‐yearanalysisperiod.Consistentwith
thepreviousfindingsonthesafetyoftheroundabouts,ananalysisofallofthecrashesrelatedto
roundaboutsshowedarelativelyfewercrashes.
Acollisionwithafixedobjectwasthemostfrequentcrashtype,withaboutaquarter(24.7%)ofallcrashes
inthevicinityofroundaboutsresultingfromvehicleshittingafixedobject,mostly,theroundaboutcenter
island.Abouttwo‐thirds(62.9%)ofthesecrashes(i.e.,collisionwithafixedobject)occurredatnight.After
collisionwithafixedobject,angleandrear‐endcrashesweremostcommon,accountingfor21.0%and
18.5%oftotalcrashes,respectively.Thedistributionofcrashtypeswasfoundtobesimilarincommercial
andresidentialareas.
Roundabouts and Access Management
Page 120
Chapter7Discussion
Overall,aboutone‐thirdofthetotalcrashesweresingle‐vehiclecrashes,whiletherestinvolvedmultiple
vehicles;thesecrasheswereequallydistributedacrosscommercialandresidentialareas.Onehalfofone
percent(0.5%)ofallcrasheshadafatality,4.5%involvedanincapacitatinginjury,andaboutathird
(29.7%)involvedapossibleornon‐incapacitatinginjury;theremaining61.1%involvedonlyproperty
damage.Single‐vehiclecrashes(8.9%)hadahigherproportionofsevereinjuriesthanmulti‐vehicle
crashes(2.9%)andagreaterpercentageofsingle‐vehiclecrashesresultedininjuriescomparedtomulti‐
vehiclecrashes.Ahigherpercentageofmulti‐vehiclecrashes,at68.8%,resultedinPDOcrashes,whileonly
45.9%ofsingle‐vehiclecrasheswerePDOs.Ofthesixfatalsingle‐vehiclecrashes,fiveinvolvedvulnerable
roadusers(fourweremotorcyclistswhowerefoundatfaultandoneinvolvedanintoxicatedpedestrian).
Twoofthefourfatalmulti‐vehiclecrashesinvolvedagolfcart.
7.2.1.2ImpactofDrivewayCornerClearanceonRoundaboutSafety.Ofthe1,882crashesthat
occurredatroundaboutlegs,only74crashes,orabout4%,wereidentifiedtobedriveway‐related.Ofthese
74driveway‐relatedcrashes,37crashes(50%ofthedriveway‐relatedcrashes)occurredatthefirst
driveways(i.e.,thedrivewaythatdefinesthecornerclearance),whileanequalnumberoccurredonall
otherdriveways.Ofthe37crashes,18occurredatthefirstupstreamdriveway,andtheremaining19
occurredatthefirstdownstreamdriveway.Sixof18crashes(33.3%)occurredwhentheupstreamcorner
clearancewaslessthan250ft.;thiscanbecomparedto15of19crashes(78.9%)thatoccurredwhenthe
downstreamdrivewaycornerclearancewaslessthan250ft.Intermsofcrashseverity,ofthe37crashes,
nonewerefatal,tworesultedinincapacitatinginjuries,eightwerenon‐incapacitatinginjurycrashes,and
theremaining27werePDOs.Theabovestatisticsindicatethatthedownstreamdrivewaycornerclearance
hasagreatersafetyimpactthantheupstreamdrivewaycornerclearance.Althoughthisresultisbasedona
smallsample,theresultisconsistentwiththefactthatvehiclesexitingadownstreamdrivewayexperience
reducedgapsduetodispersedplatoonsfromtheupstreamroundabout.Thegeometryoftheroundabout
withalargercornerturningradii,allowsvehiclestoturnrightatahigherspeed.Atcornerswithreduced
sightdistance,itfurtherreducesthetimeavailablefordrivewayvehiclestocompletetheirmaneuvers.
7.2.1.3SafetyImpactofMedianOpeningsintheVicinityofRoundabouts.Crashesinvolvingvehicles
turningleftatmedianopenings(i.e.,vehiclesturningleftfromthemainstreetontoadrivewayandvehicles
turningleftfromadrivewayontothemainstreet)wererelativelyrare.Ofthe283roundabouts,131
roundaboutswerefoundtohaveatotalof157medianopeningswithin500ft.During2007‐2011,a
relativelylowtotalof15crashesoccurredatthese157medianopenings.Ofthese15crashes,eight
involvedvehiclesturningleftfromthemainstreetontoadrivewayandseveninvolvedvehiclesturningleft
fromadrivewayontothemainstreet.
7.2.1.4SafetyatRoundaboutsthatProvideDirectAccesstoActivityCenters.Accesstomajoractivity
centers,suchasbigboxretailstores,shoppingcenters,andmalls,isoftenprovidedatmid‐blocklocations
onacorridor;assuch,aquestionremainsaboutthesafetyofdirectaccesstoactivitycentersascompared
toaccessatmid‐blocklocations.Thesafetyanalysisconfirmsthatroundaboutswiththreeorfourlegswith
directaccesstoactivitycentersareassafeasroundaboutswithoutdirectaccesstoactivitycenters.Once
thenumberoflegsincreasestomorethanfourlegs,theroundaboutswithdirectaccesstotheactivity
centerarelesssafe.
7.2.1.5SafetyofVulnerableRoadUsers,IncludingPedestriansandBicyclists.Atotalof20pedestrian
crashesand47bicycle‐vehiclecrashesoccurredatornearthe131roundaboutsincommercialareas,
constitutinglessthan4%ofallcrashes.Ofthepedestriancrashes,18occurrednearmedians,witha
slightlyhigherrate(6.64)per100roundaboutlegsthanTWLTL(5.56)andmuchhigherthanothermedian
treatments.Becauseofthesmallsamplesizeandthelackofgoodexposuredataforpedestriansand
bicyclists,itisdifficulttogeneralizefromtheresultsofthesafetyanalysis.
Roundabouts and Access Management
Page 121
Chapter7Discussion
7.2.2SummaryofOperationalAnalysis
Thethreeoperationalanalysisissuesrelatedtoaccessmanagementarepresentedinthissection.
Roundaboutssharetheseissuesincommonwithothertypesofintersections.Withtheexceptionofthelast
issue“violationoftrafficrulesanditsimpactontheoperationofroundabouts,”theseconcernsoverlap
withtheissuesinthesafetyanalysis.Oneotherissue,thespillbackintoaroundaboutfromadownstream
bottleneck,wasnotfoundattheroundaboutsincludedintheoperationalanalysis.Incaseswherethis
occurs,itwouldresultinalockedroundabout.
7.2.2.1ConflictswithintheFunctionalAreaofaRoundabout.Conflictcanoccurinthefunctionalarea
ofaroundaboutwhendrivewaysorotherintersectionsarelocatedtooclosetoaroundabout.These
conflictscanoccurwithacoupleoftypesofmovements,suchasleft‐turnsintodrivewaysthatare
preventedordelayedbecauseofatrafficqueueontheopposinglegoftheroundabout(seeFigure65).In
addition,left‐turningvehiclesturningfromadrivewayontooneofthelegsofaroundaboutareprevented
fromenteringtheroadway,aqueue,ortrafficbacksintoanotherintersectionbecausetheyaretooclosely
spaced.Ineachcase,thefailuretodesignforthetrafficqueuecaninterferewiththeoperationoftheentire
intersection,anadjacentintersection,orbothintersections,andcanposeapotentialsafetyrisk,while
reducingthecapacityoftheroundabout.Thesafetyandoperationalconcernsassociatedwithconflicts
withinthefunctionalareaofaroundaboutreinforcestheimportanceofensuringthatintersectionsarenot
toocloselyspacedandthatthefunctionalareabeprotectedtoensuretheefficientmovementoftraffic.The
challengeisthatthefunctionalareaofaroundaboutmaybedifferentfromotherintersections,especiallyin
areaswherethespeedissignificantlylowerthanmostun‐signalizedintersectionscurrentlyoperate.
Figure65‐Figure67showexamplesofdealingwithaccesstodrivewaysatroundabouts.Whenleft‐turn
accesstoaroundaboutisdesigned,spillbackandconflictwithvehiclesfromtheoppositedirectionmay
occur,asFigure65shows.Onesolutiontothissituationistoaddadedicatedleftturnlaneinthemiddle
withenoughstoragecapacity(Figure66).Anothersolutionistodesignthedrivewayattheexitinglaneand
allowright‐turnaccesstothedriveway(Figure67).
Figure65.ConflictandSpillbackassociatedwithLeft‐turnAccesstoDriveway
Roundabouts and Access Management
Page 122
Chapter7Discussion
Figure66.Solution1‐DedicatedLeft‐turnLaneforAccesstoDriveway
Figure67.Solution2–Right‐laneAccess
7.2.2.2ConflictsatRoundaboutsInvolvingPedestriansandBicyclists.Fromanoperational
perspective,locatingroundaboutsinanareawithhighpedestriantrafficcanreducethecapacityof
roundabouts.Whenacarstopsforapedestrianatacrosswalk,thequeuebehindthecarspillsbackinto
thecirculatinglane,andaffectstheoperationoftheroundabout.Thisdelayduetopedestrianmovements
Roundabouts and Access Management
Page 123
Chapter7Discussion
arenotunliketheconflictsbetweenmovingvehiclesandpedestriansincrosswalksatothertypesof
intersections.
7.2.2.3ViolationofTrafficRulesanditsImpactontheOperationofRoundabouts.Examplesof
driversviolatingtherulesoftheroadcanbeseenwhentheystopinthemiddleofroundaboutstoeither
pick‐upordrop‐offapassenger.Whenthedriverstopsintheroundabout,theresultcanbeaqueuethat
causesdriverstoqueueinsidetheroundaboutorchangetheirdirectiontogetaroundthestoppedvehicle.
Pick‐upsanddrop‐offsaremorelikelytooccurinareaswithhighpedestriantrafficoratcertainactivity
centers.Thisresultconflictswiththesafetyanalysis,whichreinforcedtheadvantagesofusing
roundaboutsforaccesstoactivitycentersbecausetheyreducethechallengesofaccessthroughopen
mediansortheplacementofanAWSCintersectionincloseproximitytotheroundabout.
7.3RoundaboutsandAccessManagementGuidance
Inthissection,thenationalandstateguidanceonroundaboutandaccessmanagementissummarized.
Thenthenationalandstateguidanceonboth,incombinationwitheachother,areexplored.Finally,
Florida’sguidanceonroundaboutsandaccessmanagementaresummarized.Followingthissection,the
findingsoftheresearcharecomparedtoeachothertoestablishabasisformakingrecommendations.
7.3.1SummaryofNationalandStateGuidanceonRoundabouts
InthisreportfourNCHRPreportsaresummarizedastheyrelatetoaccessmanagement.Theyinclude:
NCHRPReport672:Roundabouts:aninformationalguide.SecondEdition,(Rodegerdtsetal.,2010),
NCHRPReport674:CrossingSolutionsatRoundaboutsandChannelizedTurnLanesforPedestrianswith
VisionDisabilities,(Schroederetal.,2008),NCHRPReport572:RoundaboutsintheUnitedStates.Report
572,(Rodegerdtsetal.,2007),andNCHRPSynthesis264:ModernroundaboutpracticeintheUnitedStates,
(Jacquemart,1998).Twoofthesedocuments–NCHRPReport672,Roundabouts,AnInformationalGuide
(Rodegerdtsetal.,2010)andNCHRPReport572,RoundaboutsintheUnitedStates(Rodegerdtsetal.,2007)
areofgreatestrelevancetothisstudy.NCHRPReport674(Schroederetal.,2008)focuseson
roundaboutsforpedestrianswithvisiondisabilities.NCHRPSynthesisReport264(Jacquemart,1998)isan
earlyreportontheuseofroundaboutsintheUnitedStates;itincludesdiscussionsofsafety,capacityand
delay,issuesofroundaboutsforvarioususers,locationcriteriaforroundabouts,andexamplesoftheuseof
roundaboutsintheUnitedStates.AnadditionalstudythatisbeingcompletedunderNCHRPProject3‐100
–EvaluatingthePerformanceofCorridorswithRoundabouts–willalsobeofrelevancetothisreport.The
contractor’sreportshouldbeavailablewithinthenextmonth.
NCHRPReport672,Roundabouts,AnInformationalGuide,(Rodegerdtsetal.,2010),iscomprehensive,
coveringplanning,operation,safety,geometricdesign,trafficdesignlandscaping,andsystem
considerationsofroundabouts.Inonesectiononplanning,thisdocumentcomparesoperational
performancefromtheroundaboutswithintersectioncontrols,suchasTWSC,AWSC,andsignalcontrol.
Theoperationsectionincludesthecapacityandperformanceanalysisoftrafficoperation,e.g.degreeof
saturation,delay,queuelength,andfieldobservation.Specificallyforgeometricdesignasrelatedtoaccess
management,thisdocumentexplainshowtodesignroundaboutswith:entrycurvesandexitcurves,
splitterislands,SSD,ISD,andparkingandbusstoplocations.Inthesafetysection,thisdocumentreviews
conflictpointsfordifferentusers,andcommoncrashtypesinroundabouts.Signage,pavementmarkings,
illumination,workzonetrafficcontrol,andlandscapingareexploredinthesectionontrafficdesignand
landscaping.Thelastsectionsystemfocusesonthefollowingconsiderationsrelatedtoaccess
management:trafficsignalsatroundabouts,closelyspacedroundabouts,roundaboutinterchanges,and
roundaboutsinanarterialnetwork.ThisreportistheonemostfrequentlyadoptedbystateDOTs,
includingthestateofFlorida,astheirroundaboutsdesignguidancedocuments.Asisdiscussedbelow,it
alsoincludesconsiderationsofbothroundaboutsandaccessmanagement.
Roundabouts and Access Management
Page 124
Chapter7Discussion
NCHRPReport572:RoundaboutsintheUnitedStates(Rodegerdtsetal.,2007)focusesonpriorresearchon
roundaboutsintheUnitedStatesanddescribesthemethodsofpredictingsafetyandoperationalaspectsof
roundabouts.Thisdocumentincludesfourmainsections:safetyperformance,operationalperformance,
geometricdesign,andpedestrianandbicyclistobservation.Thefindingsonoperationalperformance
includedentrycapacityandcontroldelaymodelforone‐laneandmultilaneroundabouts;theproposedLOS
criteriaaresimilartothoseatunsignalizedintersections;andthedraftproceduresthatincorporatethose
modelsintotheHCM2010.Furthermore,aspectsofdesignthatmaybeimportanttoconsiderare:
accelerationanddecelerationeffectsonspeeds,ISD,anddesigndetailonmultilaneroundabouts,suchas
vehiclepathalignment,lanewidth,anddriverinformationregardinghowtouselanemarkings.
Twenty‐sixstateshaveroundaboutwebsiteswithvaryingdegreesofinformation.Mostofthesestates
adoptthenationalguidancefromNCHRPReport672:Roundabouts,AnInformationalGuide,2ndEdition,
(Rodegerdtsetal.,2010).Theguidanceoffourteenofthesestateswasreviewedingreaterdetailto
understandhowroundaboutsguidancerelatestoaccessmanagement.Thethreestatesthataddressthe
coordinationroundaboutsandaccessmanagementincludeWisconsin,Virginia,andKansas;thesestates
areprofiledingreaterdetailinthesectiononroundaboutsandaccessmanagement,below.Thestate
guidanceinseveralofthefourteenstatesprovidesguidanceontheuseoflocallydevelopedparametersfor
variousaspectsofdesignandoperationalanalysis(e.g.,Californiaforaccelerationanddecelerationeffects;
MichiganforSPFsandCMFs;andWashingtonforcornerclearance,parallelroundabouts,U‐Turns,parking
andtransitstops,andWisconsinforlocationofdrivewaysandsitedistancebetweenusers).Somestates
(e.g.,Minnesota,WisconsinandNewHampshire)recommendspecificsoftwarefortheassessmentofthe
useofroundaboutsforanintersectiondesign.
7.3.2SummaryofNationalandStateGuidanceonAccessManagement
Twelvenationalpublicationsthatdescribetheadvantagesanddisadvantagesofaccessmanagementand
documenthowtoimplementitwereidentified.Thesedocumentsinclude:APolicyonGeometricDesignof
HighwaysandStreets(GreenBook),6thEdition,AASHTO,2011,NCHRPSynthesis404:StateofPracticein
HighwayAccessManagement(GluckandLorenz,2010),NCHRPReport548:AGuidebookforIncluding
AccessManagementinTransportationPlanning(Roseetal.,2005),NCHRPSynthesis351:Accessrights:a
synthesisofhighwaypractice(HuntingtonandWen,2005),NCHRPReport524:SafetyofU‐turnsat
UnsignalizedMedianOpenings(Potts,2004),NCHRPSynthesis337:CooperativeAgreementsforCorridor
Management(Williams,2004),TRBAccessManagementManual(TRB,2003),NCHRPSynthesisofHighway
Practice332:AccessManagementonCrossroadsintheVicinityofInterchanges(ButoracandWen,2002),
NCHRPSynthesis304:DrivewayRegulationPractices(Williams,2002),NCHRPReport420:Impactsof
AccessManagementTechniques(Glucketal.,1999), NCHRPReport395:CapacityandOperationalEffects
ofMidblockLeft‐TurnLanes(BonnesonandMcCoy,1997),andNCHRPReport348:AccessManagement
GuidelinesforActivityCenters(KoepkeandLevinson,1992).
Collectively,thesereportsdocumentvariousaspectsofplanningforaccessmanagement,includingsafety,
capacity,economicdevelopment,andbroadconceptsrelatedtotheimplementationofaccessmanagement,
cooperativeagreementsforcorridormanagement,andtheuseofaccessmanagementasapartof
transportationpractice.Landuseandenvironmenteffectsofaccessmanagementincludeaesthetics,
unificationofactivitycenters,maintainingthecapacityofavailableroadways,minimizingthe
environmentalimpactofindividualaccessroads,andmoreefficientfuelconsumption.Someofthese
documentsfocusonaccessmanagementinspecificcontexts,suchasactivitycenters,U‐turnsat
unsignalizedmedianopenings,crossroadsinthevicinityofinterchanges,drivewayregulations,and
capacityandoperationalaspectsofmidblockleftturns.Becausesomeofthesedocumentswereprepared
inthe1990s,theydonotaddressroundaboutsinmuchdetail.Asisdescribedbelow,noneofthese
documents,withtheexceptionoftheAASHTOGreenBook(AASHTO,2011),specificallyexplainthe
considerationsforroundabouts.
Roundabouts and Access Management
Page 125
Chapter7Discussion
Twoofthesedocuments–NCHRPReport420,ImpactsofAccessManagementTechniques(Glucketal.,
1999)andTRBAccessManagementManual(TRB,2003)–areusefulinprovidinggeneralconsiderations
relatedtoroundaboutsandaccessmanagementthatcouldbeappliedtoroundabouts;bothofthese
documentsareovertenyearsold,whichmayexplainthelackofcoverageofroundabouts.NCHRPReport
420:ImpactsofAccessManagementTechniques(Glucketal.,1999)focusesonthemethodsforevaluating
particularaccessmanagementtechniquesintermsofsafetyandtrafficoperations.Thisresearchidentifies
availabletechniques,andcollectsandanalyzesthemethodsanddatafromvarioussources.Thepriorities
foraccessmanagementanalysisare:trafficsignalspacing,unsignalizedaccessspacing,cornerclearance,
medianalternatives,left‐turnlanes,U‐turnsasalternativestodirectleftturns,accessseparationat
interchanges,andfrontageroads.
Thereportreachesseveralconclusions.Crashratesarehigherwheresignaldensityishigher,orwhereun‐
signalizedintersectionsaremorecloselyspaced.Safetyandoperationsaspectsarebetterifthereismore
cornerclearance.Safetyisalsoassociatedwithraisedmedians.Left‐turnstoragelanesupgradesafetyand
capacitybyprovidingspacesforturningvehicles.Indirectleft‐turnsorU‐turnsmayimprovesafety,
capacityandtraveltime.Frontageroadsalongfreewaysmayneedtobeallocatedproperlytodecrease
arterialleftturns,weavingmovements,andenhanceaccess.Theymayalsoneedtobeplacedfarenough
fromtheramptoavoidconflicts.TRB’sAccessManagementManual(TRB,2003)exploresthegeneral
benefitsofmanagingaccesstoroadways,explainshowaccessmanagementcanbeachieved,itsaspectsand
principles,andtherolesofvariousinstitutionsinaccessmanagement.
Threebasicstepsinimplementingaccessmanagementtoaroadwayaredefiningaccesscategories,
establishingaccessmanagementstandards,andassigningcategoriestotheroadwaysorroadway
segments.Initialfactorstobeconsideredarethedegreeofroadwayimportance,roadwaycharacteristics,
landuseandgrowthmanagementobjectives;andthecurrentandpredictedflowsofgeneraltransit,aswell
aspedestrianandbicycletraffic.Fourgeneralaspectsofdevelopingaccessmanagementstandardsinclude
medians,degreeofurbanization,speed,andsafety(TRB,2003,p.71).Finally,theassignmentofcategories
inroadwaysystemsneedstotakeintoaccountthefollowingfactors:theintendedfunctionoftheroadway
asacomponentofacompletetransportationsystemnetwork;theroadwaysegment’senvironment(rural
andundeveloped,urbanfringe,sub‐urban,urban,anddenselydevelopedorurbancore);theavailabilityof
asupportingroadwaysystemtosupplyalternativeaccess;andthedesiredorappropriatebalancebetween
safetyandfrequencyofaccess(TRB,2003,p.77).
Forty‐threestates,includingtheDistrictofColumbia,haveincorporatedaccessand/oraccessmanagement
intotheirplanninganddesignpolicies.Nineteenstateshaveaccessmanagementmanuals,separatefrom
generaldesignmanuals,andelevenstateDOTsmentionaccessmanagementordesignmanuals,while
anothersixteenDOTshaveotherdocumentswithvariousnames.Onlysevenstatesincorporate
roundaboutsintotheiraccessmanagementguidance;thesestatesarediscussedbelow.
7.3.3SummaryofNationalandStateGuidanceonRoundaboutandAccessManagement
Amongallthenationalguidancedocumentsonroundaboutsandthedocumentsonaccessmanagement,
onlyNCHRPReport672:Roundabouts,AnInformationalGuide,SecondEdition(Rodegerdtsetal.,2010)
referstoaccessmanagementinthecontextofroundaboutsunderthegeneralcharacteristicsof
roundaboutsandaspartofthegeometricprocess(Sections2.2.5p.2.9and6.11,pp.6‐95to6‐98).This
documentreinforcestheideathat“[m]ostoftheprinciplesusedforaccessmanagementatconventional
intersectionscanalsobeappliedatroundabouts.”(Rodegerdtsetal.,2010,p.2‐9)and“[a]ccess
managementatroundaboutsfollowsmanyoftheprinciplesusedforaccessmanagementatconventional
intersections”(p.6‐95).However,thedifferenceinoperationalcharacteristicsofroundaboutscomparedto
othertypesofintersectionsmayjustifythedifferenceincertaindetailsofaccessmanagement.
Roundabouts and Access Management
Page 126
Chapter7Discussion
Asapartofanoverallroadwaysystemthatinvolvesaccessmanagement,thetreatmentofdrivewaysand
parkingwithinthefunctionalareaoftheroundaboutsintersectioniscritical.Theabilitytoprovidepublic
andprivateaccesspointsneararoundaboutisinfluencedbyanumberoffactors,suchasthecapacityof
theminormovementsattheaccesspoints,theneedtoprovideleft‐turnstorageonthemajorstreetto
servetheaccesspoint,theavailablespacebetweentheaccesspointandtheroundabout,andsightdistance
needs.Figure29,above,whichwastakenfromNCHRPReport672,Roundabouts,AnInformationalGuide
(Rodegerdtsetal.,2010)showsthetypicaldimensionsforleft‐turnaccessnearroundaboutsshouldbe
about275ft.subjecttolocalconditions.Thefunctionalareaofabout275ft.fromthecenterdiameter
includesthedistancefromthecenterfortheroundabouttotheedgeofthesplitterisland,aminimumof50
ft.toclearthemedianandaminimumof75ft.toallowfortheleftturningmovementinadditiontothe
distanceformaneuvering,decelerating,andqueuingintotheleftturnlane.
Asmallnumberofstatesexplicitlyrefertoaccessmanagementwithinthecontextofroundabouts.Many
statesadopttheguidanceofNCHRP672,Roundabouts,AnInformationalGuideintheirroundaboutplans
and,assuch,adopttheunsignalizedintersectionspacingguidance.Someincludesuchinformationintheir
roundaboutsmanualsandsomeintheiraccessmanagementmanuals.Fromthesevenstatesthat
specificallyrefertoaccessmanagementinthecontextofroundabouts,twoofthem–KansasandVirginia–
providesignificantsupplementalinformationwhileadoptingthenationalguidance.CaliforniaandIowa
endorsetheuseofroundaboutsasapartofaccessmanagementbutdonotprovidespecificguidanceon
drivewaydistancesandintersectionspacingguidance.Michigan’sAccessManagementGuidebookstates
(MDOT,2008)that“Drivewaysneedtobelocatedasafedistancefromaroundaboutwithadequate
signage.Drivewaysshouldnotbelocatedwithinaroundabout”(MDOT,2008,p.3‐29)buttheydonot
providespecificguidanceonhowtoaccomplishthisgoal.Similarly,Wisconsindescribestheadvantageof
roundaboutsintheretrofitofasuburbancommercialstripdevelopmentinanattempttominimize
conflicts.TheWisconsinreportthendescribessomeofthefactorstobeconsideredinsuchretrofits(e.g.,
drivewayconsolidation,reversefrontage,coordinatedU‐turnsandleftturns,andinterconnectedparking
lots);however,theydonotprovidespecificguidanceonthelengthofthefunctionalareaaround
roundabouts.
BothKansasandVirginiaadopttheunsignalizedintersectionspacingbutprovideadditionalguidance.The
KansasRoundaboutGuide:ASupplementtoFHWA’sRoundabouts(Kittelson&AssociatesandTransystem
Corporation,2003)andKsDOTAccessManagementPolicy(KsDOT,2013)hasinformedandhavebeen
informedbytheNCHRP672,Roundabouts,AnInformationalGuidereport.Virginia’sAccessManagement
DesignStandardsforEntrancesandIntersectionprovidesatable,shownaboveinFigure29,demonstrating
thespacingfromotherintersectionsandthespacingfromotherdrivewaysorroundabouts.Onesignificant
differencebetweenthesesetsofguidancethatmayaffecttheirinterpretationofthelengthofthefunctional
areaisthattheNCHRP672,Roundabouts,AnInformationalGuidemeasuresthefunctionalareafromthe
centerlineoftheroundaboutwhileKansasmeasuresitfromtheendofthesplitterislandandVirginia
measuresfromtheouteredgeofthenearestinscribeddiameter,notthecenterline.
7.3.4SummaryofFlorida’sGuidanceonRoundaboutsandAccessManagement
Floridahastwomajordocumentsrelatedtoaccessmanagement:FDOTMedianHandbook(2006);and
FDOTDrivewayInformationGuide(2008);andfourmajordocumentsthatincludeinformationon
roundabouts:FloridaRoundaboutGuide(FDOT,1996);RoundaboutJustificationStudy(Chapter16in
ManualonUniformTrafficStudies,FDOT,2000);FloridaIntersectionDesignGuide2013;andBicycleand
PedestrianConsiderationsatRoundabouts(FDOT,2000).
FDOTMedianHandbook(2006)doesnotexplicitlymentionroundaboutdesignoraccessmanagement
whiletheFDOTDrivewayInformationGuide(2008)andtheStateHighwaySystemAccessManagement
SystemandStandardsdonotmakeanyreferencetoroundabouts.
Roundabouts and Access Management
Page 127
Chapter7Discussion
TheFloridaRoundaboutGuide(FDOT,1996)waspublishedearlierthanFHWA'sRoundabouts:An
InformationalGuide,1stEdition(Robinsonetal.,2000)andisintheprocessofbeingreplacedwithmore
recentdocuments.TheManualonUniformTrafficStudies,Chapter16–RoundaboutJustificationStudy
(2000)justifiestheuseofroundaboutsintheStateofFlorida,andcomparesthemtothreeother
alternativestointersectioncontrols–trafficsignals,TWSC,andAWSC.TheFloridaIntersectionDesign
Guide,2013,ForNewConstructionandMajorReconstructionofAt‐GradeIntersectionsontheStateHighway
Systememphasizestheneedofconsideringmodernroundaboutsforanynewroadorreconstruction
projectastheymayprovidesafetyandoperationaladvantages.ThisguidealsostatesthatFloridahas
officiallyadaptedNCHRP672,Roundabouts,AnInformationalGuide(Robinsonetal.,2010)asthemain
guidefordesigningroundaboutsinFlorida.Itdescribesmanyadvantagesofbuildingroundabouts.
Regardingroundaboutsandaccessmanagement,thisdocumentacceptsthatroundaboutscanbeusedas
partofanaccessmanagementplanastheycontributeinreducingdownstreamleftturns,becausevehicles
canperformU‐Turnswithintheroundaboutsandthenaccessanareabyturningright.Also,driveways
shouldnotbeallowedinthecirculatoryroadwayunlessthereisenoughdemandtosupporttheir
constructionasadditionallegsoftheroundabout.
Bicyclescanaccessaroundaboutasvehiclesusingthecirculatoryroadwayoraspedestriansusing
sidewalks,sobicyclelanesshouldendatbypassrampstoallowbicyclestousethesidewalkiftheyprefer,
alwaysyieldingtopedestrians.Pedestriantreatmentsatroundaboutsareconsideredthesameasinother
intersectiontypes.Incaseofbusroutesinroadswithroundabouts,busbaysshouldbeplacedcarefullyto
avoidvehiclequeuesthatspillbackintothecirculatoryroadway;Busstopslocatedonthefarsideofthe
roundaboutshouldhavepulloutsorbemovedfurtherdownstreamtothesplitterislandtoavoid
interruptingregulartraffic.Furthermore,theFloridaIntersectionDesignGuideadaptstheSSDformulaand
theISDrequirementsfromNCHRP672,Roundabouts,AnInformationalGuide(Equations6‐5‐6‐7,pp.6‐61‐
6‐63inRodegerdtsetal.,2010).TheBicycleandPedestrianConsiderationsatRoundabouts(Shenetal.,
2000)recommendsthatroundaboutsbeproperlydesignedtoaccommodatethesafetyofbicyclists,
pedestriansanddrivers.Themulti‐laneroundaboutscreatemoretensionandarelesssafeforbicyclists
andpedestriansthanone‐laneroundabouts.Inadditiontotheaforementioneddocuments,FDOT
presentedaPowerPointpresentation—Roundabouts,Florida’sImplementationStrategy(Prytykaand
Sullivan,2012),atthe2012DesignTrainingExpowherethesupplementalaspectsfromFHWA's
Roundabouts,AnInformationalGuide(Robinsonetal.,2000)arecaptured.
7.4SynthesisofFindingsoftheResearch
TheStateofFloridaisintheprocessofchangingitsguidanceontheuseofroundaboutsonthestate
highwaysystem.ThechangeintheState’spolicyguidanceasdescribedintheFloridaIntersectionDesign
Guide2013,FDOTMedianHandbook(2006),FDOTDrivewayInformationGuide(2008),StateHighway
SystemAccessManagementClassificationSystemandStandards(FDOT,2010),andotherguidance
documentswilldefinehowroundaboutsareimplementedintocities,townsandcrossroadsinthestateof
Florida.WhilecommunitiesthroughoutFloridahavesignificantexperiencewithroundabouts,thelevelof
expertiseisunevenandthecontextsinwhichtheroundaboutswillbeimplementedarediverse.The
advantagesofroundaboutsandaccessmanagementareclearlydocumentedintheliterature.Access
managementaffectssafety,operations,economicfactorsrelatedtoretailorcommercialmarketand
propertyvalues,landuse,andtheenvironment.Roundaboutsareseenasaformofaccessmanagement
thathassimilarcharacteristicsandoperational,safety,andcostadvantagescomparedtoothertypesof
intersections.Whenproperlydesignedroundaboutsandaccessmanagementcanenhancetheaesthetic
andenvironmentalaspectsofacorridor.Nonethelessthesameareacanexperienceeconomicdeclineanda
lossofcommunitylivabilitywhenaccessmanagement,includingroundabouts,ispoorlydesignedand
implemented.
Theanalysiscompletedasapartofthisresearchidentifiedseveralareasdirectlyrelatedtoaccess
managementandotherissuesthatmaybecomeapartofthestate’sstrategytoimplementchangein
Roundabouts and Access Management
Page 128
Chapter7Discussion
roundaboutpolicy.Thesafetyanalysiswascompletedonallroundaboutsinthestateand,ingeneral,it
showsarelativelylowrateofcrashesnearroundaboutsbutaslightlyhigherratenearcommercialand
mixedlanduses.Theoperationalanalysiswascompletedonasmallsampleof13roundaboutsidentified
fortheirtrafficvolume,proximitytodriveways,adjacentintersections,andadjacentlanduses.Collectively,
theseanalysesidentifiedacoupleofareasofconcern.Some,suchascollisionswithfixedobjectsatnight,
mayrequiredesign,lighting,orsignagechanges.Others,suchasdriversstoppinginthemiddleof
roundabouts,mayrequiredesignchangesordrivereducation.Stillothers,suchascrashesatmedian
opening,operationalconcernsaboutleft‐turningvehicles,accesstoactivitycenters,andsafetyand
operationalconcernsaboutvulnerableroadusers,willrequiregreaterattentiontoaccessmanagement
issues.
RoundaboutsaredifferentfromothertypesofintersectionsbecausetheycanprovideU‐turn
opportunities,allowingforareductionoffullaccesspointsalongaroadwaysegment,whileatthesame
timeenhancingaccess.Theyhavedifferentoperationalcharacteristics–slowerspeedsatintersections,
continuousmovementoftraffic,fewerconflictpointsbetweenvehiclesandfewersafetyissuesassociated
withleftturningvehiclesinsidetheroundabout.Inturn,theseoperationalcharacteristicscreate
challengesforvulnerableroadwayusersandfortrucksandotherlargevehicles.Additionally,specific
operationalcharacteristicsandcontextualaspectsofroundabouts–newvs.retrofit,urbanvs.suburbanvs.
rural,singlevs.multi‐lanevs.complexintersections(turbo,spiralorinvolvingoneormoresliplanes)
affectthedesigncharacteristicsofroundabouts.
ThisresearchinformsusaboutthesafetyandoperationsofexistingroundaboutsinthestateofFlorida.
However,thetypesofroundaboutscurrentlyinusearenotrepresentativeofthetypesofroundaboutsthat
arelikelytobebuiltunderthenewstateguidelines.Thesampleincludedonlyfourroundaboutsonstate
highways.Theroundaboutcorridorsthatwereevaluatedarelocatedoffthestatehighwaysystem.
Roundaboutsbuiltundertheproposedguidelinesarelikelytoincludehighertrafficvolumes,more
complexlocations,morecomplexagreementsbetweenthestateandlocalgovernment,andinthecaseof
retrofits,havemorecomplexaccessmanagementissues.Assuch,roundaboutcorridors,whichwereonly
examinedinalimitedmanner,willbecomeamoreimportantissueinthefuture.Thisraisesthequestion
ofhowtodesignasetofrecommendationsthataddressthecomplexityofcontextsinwhichroundabouts
arebeingimplementedinthestate.
Recommendationsofthisstudyneedtospecificallyaddressthelocationofdrivewaysandintersectionsin
closeproximitytoroundabouts,roundaboutsnearactivitycenters,theISDandSSDnearintersections,and
theneedsofbothvulnerableroadusersandtrucksinproximitytoroundabouts.Thefirsttwotopicsare
directly related to access management while the third topic is less directly related but is an important
considerationinthedeploymentofroundabouts.
Both the safety and operational analysis identified issues related to the location of driveway and roads
within close proximity to the intersection. The operational analysis identified two situations where
drivewayandroaddistancesaffectedoperations:vehiclesturningleftintoanintersectionthatis located
withinthefunctionalareaofaroundabout,andaroundaboutlocatedtooclosetoanotherintersectionatan
activitycenter.Thesafetyanalysisshowedavarietyofsituationsinwhichleftturningvehicles,eitheron
the leg of a roundabout and/or turning onto a driveway near a roundabout may have caused a crash.
However, the crash data does not indicate serious safety issues with median openings in the vicinity of
roundabouts. While losing median openings located between two adjacent roundabouts could prevent
some of the median opening related crashes, the location of median openings needs to be considered
withinthecontextofoverallaccessmanagementinandaroundtheroundabout.
The review of national and state guidance on roundabouts and access management, and the operational
analysis of this study, suggest that roundabouts are similar to unsignalized intersections in the way that
Roundabouts and Access Management
Page 129
Chapter7Discussion
theyoperate.ThisisconfirmedbyHCM2010,p.4‐14,whereitstatesthat“[t]heoperationofroundabouts
issimilartothatoftwo‐waystop‐controlledintersections.Inroundabouts,however,enteringdriversscan
only one stream of traffic—the circulating stream—for an acceptable gap.” In HCM 2010, the service
measure and thresholds for roundabouts have been made consistent with those for other unsignalized
intersections. This is covered primarily via control delay calculation, as it is for TWSC and AWSC
intersections, by adjusting for the effect of yield control. Also, “roundabouts discharge vehicles more
randomly, creating small (but not necessarily usable) gaps in traffic at downstream locations” (p. 8‐5).
These gaps are different than signalized intersections which create vehicle platoons but similar to gaps
createdbyotherunsignalizedintersections,suchasAWSCintersections.Assuch,roundaboutsmayhave
different requirements with respect to their functional area because of differences in overall speed,
acceleration, deceleration and queuing. While the access management guidance recognizes these
differences, noresearch studyhas explicitlyconsideredhowcontextualfactorsaffectthefunctionalarea.
The guidance on access management, which would include roundabouts, should consider the intended
function of the roadway as a component of a complete transportation system network. This evaluation
would include the roadway segment’s environment, whether rural and undeveloped, urban fringe, sub‐
urban,urban,anddenselydevelopedorurbancore.Itwouldalsoincludetheavailabilityofasupporting
roadwaysystemtosupplyalternativeaccess,andthedesiredorappropriatebalancebetweensafetyand
frequencyofaccess(TRB,2003).
Activity centers represent a specific context for roundabouts that were identified in the research but for
whichnoclearguidanceonintersectionspacingandeventheirusecanbeclarified;assuch,thiscontext
may require additional research. The crash data did not show increased safety hazards at roundabouts
that provided direct access to activity centers. Providing direct access to activity centers through a
dedicatedlegisdesirabletoimprovetrafficoperationsonthecorridor,aslongastheprovisiondoesnot
increasethenumberofroundaboutlegsbeyondthestandardfourlegs.Theoperationalanalysisidentified
twosituationsinwhichroundaboutsmayrequirespecialdesignconsiderationstoensurethecontinuous
and safe flow of traffic. First, if an adjacent intersection for circulating traffic is located too close to the
roundabout,theoperationsoftheroundaboutandtheintersectioncanbeadverselyaffected.Second,ifa
roundaboutislocatednearanurbanactivitycenter,wheretheflowofpedestriansishigh,thedesignofthe
roundaboutshouldincorporateconvenientandaccessibledrop‐offandpick‐uplocationsincloseproximity
totheroundabout.
Anotheraccessmanagementissueassociatedwithroundaboutsforwhichtheresearchcouldnotprovide
clearguidancerelatestotheSSDandtheISD.Driversenteringandexitingaroundaboutneedtoseeand
reacttothedriversinfrontofthemwithchangesintheirspeed;assuchtheSSDandISDareanimportant
partofensuringthatthefunctionalareaofaroundaboutisadequatetoensurethesafetyandefficiencyfor
allusersaroundroundabouts.Bothoftheseissueswereidentifiedinthesafetyanalysis,butthecrashdata
showsthatdownstreamdrivewaycornerclearanceshaveagreatersafetyimpactthanupstreamdriveway
corner clearances. Longer downstream corner clearances are desirable because they provide additional
timefordrivewayvehiclesthatexperiencereducedvehiclegapsandhigherapproachvehiclespeedfrom
upstreamroundabouts.AlthoughtheISDandSSDwereshowntoberelatedtothesafetyoftheoperations
oftheroundabout,thesampleofroundabouts(n=37)isrelativelysmall.Theoperationalanalysisdidnot
provide any additional insights into how the ISD and SSD affect the capacity and operation of the
roundabouts.However,theISDandSSDneedtobeconsideredinthedesignoftheroundaboutbecause
theycandirectlyaffectsafetyandtheoperationsofaroundaboutinitsfunctionalarea.
Drivewayslocatedatorneartheroundaboutcancreateconflictswiththecirculatoryroadway,dueto
accelerationanddecelerationalongthecorridor.Yetbecauseoftheslowerspeeds,drivewaysmaypose
lessofachallengeforaccessmanagementthanforothertypesofintersectionincludingunsignalized
intersections.However,alongmanypartsofthestatehighwaysystem,theexistingdrivewaysmayposea
challengewhenaroundaboutisretrofittedintoanurbanenvironment.Insomesituations–forexample,if
Roundabouts and Access Management
Page 130
Chapter7Discussion
thedrivewayhaslowtrafficvolumes–accesswasprovidedpriortotheinstallationoftheroundabout.In
thiscase,noalternativeaccesspointsareavailable.Thedrivewayisproperlydesignedtoallowvehiclesto
turnaroundandexitfacingforward–thedrivewayscouldbelocatedinthefunctionalareaofaroundabout
ifitincludesadequateISDandSSD.Wheredrivewaysarelocatedinorneararoundabout,thedesign
shouldgiveaclearvisualindicationthatprivatedrivewaysareadjacenttotheroundaboutandarenotfor
publicuse.
Accessmanagementintheproximityofaroundaboutislargelyconnectedtotheoperationinthefunctional
areaaroundtheroundabout,whichisinfluencedbytheISDandtheSSD,thelocationofdriveways,andthe
distancetotheclosestintersectionorroundabout.Thesafetyanalysissuggeststhatthedownstream
functionalareaneedstobelongerforthedownstreamlegthanfortheupstreamlegbecausedriversare
likelytobedeceleratingastheydrivetowardsaroundabout.Figure29,above,showsthatthetypical
dimensionsforleft‐turnaccessnearroundaboutsshouldbeaminimumof275ft.,subjecttolocal
conditions.Inadditiontothedistancefromthecenteroftheroundabouttoitslegs,thisso‐called
functionalareaincludesthedistancefromthecenterfortheroundabouttotheedgeofthesplitterisland,a
minimumof50ft.toclear,75ft.toallowforleftturningmovements,and90ft.fordeceleration.NCHRP
672,Roundabouts,AnInformationalGuideisclearaboutthecomponentsofthefunctionalarea,but
differentstatesmeasurethatdistancedifferentlysoitisimportanttobeclearabouthowtomeasurethe
distance.
7.5Recommendations
Thesynthesisoftheresearchfindingssuggeststhat,whilesignificantresearchhasbeencompletedon
roundaboutsandonaccessmanagement,additionalresearchisneededonthecombinationofroundabouts
andaccessmanagementindifferentcontextsandconditions.Roundaboutshavegenerallybeenconsidered
similartounsignalizedintersections,buttheyhavedifferentoperationalcharacteristicsrelatedtothe
downstreamflowofvehicles,andthespeedwithwhichvehiclesenterthem.Roundaboutscanbeseenasa
partofaccessmanagement,likemedianswhentheyfacilitateU‐turns,or,astheyaregenerallycategorized,
asatypeofintersection.However,theyhavedesignconsiderationsthatdifferfromdrivewaysandleft‐turn
medians.Irrespectiveofhowtheyarecategorized,andthecontextinwhichtheyareimplemented,
roundaboutsneedtobedesignedinamannerthatensurestheoperationalefficiencyoftheintersection
andthesafetyofallusers.Guidancethatresultsinroundaboutswithlengthyqueuinglanescouldunduly
decreasethenumberofroundaboutsthatareimplemented,whilepoorlydesignedguidancecouldcreate
unsafedrivingconditionsforroadwayusersandreducetheaccessandeconomicviabilityofbusinesseson
adjacentland.
Inthissection,threetypesofrecommendationsaremaderegardingaccessmanagementaround
roundabouts.ThefirstsetofrecommendationsprovidesdirectionfortheFDOTonupdatingtheirguidance
onroundaboutsandaccessmanagement,includingaccessmanagementtools,theMedianHandbook,the
DrivewayInformationGuide,andthesoftwareusedtoanalyzeroundabouts.Nextasetofrecommendations
ismadeforfutureresearchregardingroundaboutsandaccessmanagement.Inparticular,
recommendationsaremadetoproposeanNCHRPProjectonroundaboutsandaccessmanagement,a
before‐and‐afterstudyoftheproposedroundaboutretrofitinDowntownSarasota,andastudytoestablish
Florida‐specificparameterstousewiththeHCSandothersoftwareemployedtoanalyzethecapacityof
roadwaysonwhichroundaboutsareproposed.
7.5.1RecommendationsforFlorida’sGuidanceonRoundaboutsandAccessManagement
AsFloridaincorporatesroundaboutsintoitspractices,allpolicyguidanceneedstoprovideaconsistentset
ofguidanceontheuseofroundaboutsthataddressthediversesituationsunderwhichroundaboutsare
implemented.Essentialtothisguidanceisconsiderationofthedifferencesbetweenroundaboutsandother
typesofintersections,aswellasothertypesofaccessmanagement,suchasdriveways,andmedians,which
arediscussedinlatersections.Thedesignspeedsforroundaboutsissignificantlylowerthanthedesign
Roundabouts and Access Management
Page 131
Chapter7Discussion
speedforunsignalizedintersections,withadesignspeedof20to30mphand25to35mphforasingle‐
laneandmulti‐laneroundabout,respectively.Theguidanceshouldaddressthedifferencesinoperational
considerationsbetweenroundaboutsandotherformsofaccessmanagement,anddifferencesinthe
operationofthefunctionalarea,includingqueuing,decelerationandacceleration,accommodationof
pedestrians,andotheraspectsofthemovementofvehicleswithinthefunctionalarea.
Thefindingsalsoidentifiedtwospecificissuesrelatedtoroundaboutsthatshouldbeaddressedinthe
accessmanagementguidance:theuseofroundaboutstoprovideaccesstoactivitycenters,andthe
accommodationofallusersaroundasingleoracorridorofroundabouts.
TheFloridastateguidanceonaccessmanagementneedstoreinforcetheexistingprocessforimplementing
accessmanagementasroundaboutsareincorporatedintotheaccessmanagementguidance.Inparticular,
TRB’sAccessManagementManualrecommendsthreebasicstepstoimplementaccessmanagementona
roadway:definingaccesscategories,establishingaccessmanagementstandards,andassigningcategories
totheroadwaysorroadwaysegments.Initialfactorstobeconsideredarethedegreeofroadway
importance,roadwaycharacteristics,landuseandgrowthmanagementobjectives;andthecurrentand
predictedflowsofgeneraltransit,aswellaspedestrianandbicycletraffic.Fourgeneralaspectsof
developingaccessmanagementstandardsincludemedians,degreeofurbanization,speed,andsafety(TRB,
2003,p.71).
Finally,theassignmentofcategoriesinroadwaysystemsneedstotakeintoaccountthefollowingfactors:
theintendedfunctionoftheroadwayasacomponentofacompletetransportationsystemnetwork;the
roadwaysegment’senvironment(ruralandundeveloped,urbanfringe,sub‐urban,urban,anddensely
developedorurbancore);theavailabilityofasupportingroadwaysystemtosupplyalternativeaccess;and
thedesiredorappropriatebalancebetweensafetyandfrequencyofaccess(TRB,2003,p.71).Tothe
extentpossible,thestateshouldconsidertheuseoflocallydevelopedparametersforvariousaspectsof
designandoperationalanalysisofroundabouts.Otherstateshavedevelopedlocalparametersthatrelate
totheinfluenceofdriverbehaviorasitaffectscapacityandoperationalcharacteristicsofroundabouts.
ThestatehasalreadyadoptedNCHRP672,Roundabouts,AnInformationalGuideforitsguidanceon
roundabouts,andguidanceonthefunctionalareashouldbeincludedinthestateguidance.Differencesin
theoperationswithinthefunctionalareashouldbehighlighted.Theguidanceneedstobeexplicitaboutthe
definitionofthefunctionalareaofaroundabout,especiallyifitdeviatesfromtheguidanceprovidedin
NCHRP672,Roundabouts,AnInformationalGuide.Establishingthelengthsofthefunctionalareabased
uponthefunctionalclassificationoftheroadwayiscomplex.Whilemuchoftheguidanceisbuiltonthe
assumptionthatroundaboutsoperatelikeunsignalizedintersections,thespeedwithwhichvehiclesentera
roundaboutismuchslowerthanunsignalizedintersections.Assuch,thismightsuggestthatthefunctional
areaofaroundaboutisshorter.TheexistingguidanceforunsignalizedintersectionsandVirginia’s
MinimumSpacingStandardsforCommercialEntrances,Intersections,andCrossovers,asshowninFigure29,
shouldbereviewedtoestablishinitialguidanceforlocalgovernmentstouseastheybegintoexploretheir
optionsforroundaboutsandaccessmanagement.Itisnoteworthythattheintersectionspacingstandards
forthestateofVirginia,asshowninthelastcolumninFigure29,arecloserthantheintersectionspacing
forunsignalizedintersections.Additionally,guidanceondrivewayandintersectionspacingneedsto
addressthefactthatthespeedsnearroundaboutsaresignificantlylowerthanthe45mphusedinthe
existingguidance.
7.5.1.1AccommodationofAllUsersAroundRoundabouts.Asthestatebeginstoimplement
roundaboutsinagreatervarietyoflocations,theneedsofallroadwaysusers,includingbicyclists,
pedestrians,andlargevehicles,needtobeaccommodated.Thefindingsofboththesafetyandoperational
analysisidentifytheneedtoaccommodatebicyclistsandpedestriansaroundroundabouts.Becauseofthe
lowerspeedsassociatedwithroundabouts,experiencedbicyclistsmaybeabletomergewithmotoristsas
theynavigatethroughtheroundabouts.Becauseofthesplitterislandandthelocationofthecrossing
Roundabouts and Access Management
Page 132
Chapter7Discussion
behindthevehicleenteringtheroundabout,driversmayencounterlessdelaythanvehiclesatunsignalized
intersections.However,becauseofthecontinuousmovementthroughroundabouts,pedestrians,andin
particularvisuallyimpairedpedestrians,areatgreaterriskatroundaboutsthanatotherunsignalized
intersections.Additionally,asdiscussedbelow,roundaboutspresentaparticularchallengetopedestrians
nearactivitycentersifpick‐upanddrop‐offisnotproperlyhandled.
Whilethesafetyandoperationalanalysisofthisstudydidnotidentifysignificantproblemswithtrucksand
otherlargevehicles,theyarelikelytobecomeanissueasroundaboutsaremorewidelyusedalongstate
roadways,whichcanhavemoretrucktraffic.During2007‐2011,atotalof18crashesinvolvedheavy
vehiclesatthe131commercialroundabouts.Theguidanceneedstobedesignedtoaccommodatetrucksas
apartofaccommodatingallusersinthesystem.Whenroundaboutsareimplementedengineersandlocal
officialsmaybelievethattheycanremoveorrestrictmovementatmediansorotheraccessmanagement
devicesbasedupontheideathatleft‐turningmovementscanbeaccommodatedattheroundabout.TheU‐
turnalternativemayincreasethenumberofsideswipecrashesatroundabouts,especiallyforlarge
vehicles.
Largetrucksandbusesoftenfinditdifficulttonegotiateasmallerroundabout.Inparticular,lackof
adequatelateralclearancecouldresultinheavyvehiclessideswipingothervehiclesorbecominginvolved
inacollisionwithafixedobject,usuallywiththeroundaboutcenterisland.Whileasingleroundaboutmay
notbeabletoaccommodatetrucks,theymaybemoreeasilyaccommodatedalongaroundaboutcorridor
orthroughalternative,parallelaccessthatallowstruckstoreachcommercialdestinations.Furthermore,
forplaceswherethepercentageofheavyvehiclesishigh,thedesignoftheroundaboutsshouldtakethe
radiusintoconsideration.Whenthelackofspacepreventstheinstallationofalargeroundabout,itis
recommendedthatothertypesofintersectionarepreferred.
7.5.1.2UseofRoundaboutsNearMajorActivityCenters. Theresultsofthisresearchshowconflicting
resultswithrespecttotheuseofroundaboutsattheentrancetomajoractivitycenters.Accessaround
activitycenterscanbecomplexduetotheneedtoprovideaccesstoavarietyofdestinationswithinashort
distance.Becauseroundaboutsallowacontinuousflowoftraffic,theymaybeseenasamoreefficient
solutionthanusingcontinuousrightandleftturnlaneswithdirectionmediansandotherformsofaccess
management.Thesafetyanalysisfoundthatroundaboutswiththreeorfourlegsattheentrancetoactivity
centersarejustassafeasroundaboutsinothercommerciallocations.Theoperationalanalysisfoundthat
ifaroundaboutislocatedtooclosetoanadjacentintersectionspilloverandadecreaseincapacitycantake
place.Assuch,thestateshouldconsiderdevelopingguidanceontheuseofroundaboutsatornearmajor
activitycenters.Thisguidanceshouldconsiderwhethertheactivitycenterislocatedinanurban,suburban
orruralcontext;howtheactivitycenterissituatedwithinthestreetnetwork;andhowtrucksare
accommodatedinthevicinityoftheroundabout.Forexample,cantruckshaveaccesstothestoresfor
loadingandunloadingofdeliveriesusingaparallelroadway?Inanurbancontextwhereactivitycenters
arelocatedalongaroad,aroundaboutcouldpotentiallyprovidebetteraccesstotheactivitycenter.With
medianclosingandtheuseofaseriesofroundaboutsinacorridor,safeoperationandaccesstoactivity
centerscanbothbeguaranteed.
Ifroundaboutsarenotproperlydesignedtoaccommodatepick‐upsanddrop‐offsnearmajoractivity
centers,driversmayneedtomaneuveraroundstoppedvehiclesorstopinthemiddleoftheroundabout.
Additionally,largepedestrianvolumesatcrosswalkswithintheroundaboutcanalsocauseaqueuewithin
theroundabout.Theguidanceforroundaboutlocationrecommendsagainsttheuseofroundaboutswhere
therearehighpedestrianvolumes.However,otherpropertiesofroundabouts,suchasaestheticsand
landscaping,mayjustifytheirusageeveninlocationswithhighpedestrianvolumes.Ifaroundaboutis
usedinhighpedestrianareas,pedestrianscouldbeaccommodatedwithunderpassesoroverpasses,or
withsidewalksfurtherfromthecirculatoryroadway.Regardlessofwhethertheroundaboutislocatedin
anurbanorsuburbancontext,nosignificantimpactonoperationisshown.
Roundabouts and Access Management
Page 133
Chapter7Discussion
7.5.1.3RecommendationsontheSoftwareforAnalysisofRoundabouts.Softwareforanalysisof
roundaboutsneedstobeavailableforavarietyofapplicationsincludingplanninglevelsizing,preliminary
design,analysisofpedestriantreatments,systemsanalysis,andpublicinvolvement.Generally,theseneeds
canbeaddressedwithHCS.Otherdeterministicsoftwarecanconducttheplanning‐levelandpreliminary
designreview,whilesimulationsoftwarecanbeusedforthesystemsanalysis,publicinvolvementand
analysisofpedestriantreatments.
Deterministicsoftware,suchasHCS,Synchro,SIDRA,RODELandARCADY,canperformqueuinganalysis
andprovideusefulinformationrelatedtoaccessmanagement,especiallyforplacingdriveways.Simulation
software,suchasVISSIM,canbeusedtoevaluatetheoperationofroundaboutsandtheinteraction
betweentrafficflowsatroundaboutsandadjacentdrivewaysbyconductingmicroscopicanalysis.Itisclear
fromthisanalysisthatdeterministicsoftwarecanprovideguidanceonwherethedrivewayshouldbe
placedbeforetheconstructionofintersections,whilesimulationcanbeusedtoevaluatetheimpactof
drivewayandotheraccessmanagementissuesonroundaboutoperation.
ThenewversionofHCS2010providesaviabletooltoconductqueuinganalysisforroundabout,whichcan
beusedtodeterminethelocationofaccesspointandthelengthoffunctionalarea.CORSIM,whichisused
forotherapplicationsinFlorida,whencomparedtoothersimulationsoftwarepackages,requiressome
modificationinordertoaccuratelyreplicateroundaboutoperations.Roundaboutsshouldbemade
availableinCORSIMbyallowingmultiplenodestobegroupedtogetherasoneroundabout,andfollowup
timeandcriticalgapshouldbemadeapproach‐based.
7.5.2RecommendationsforAdditionalResearch
Whilethenumberofroundaboutshasincreasedsignificantlyoverthepastcoupleofdecades,researchhas
notkeptupwithourunderstandingofthedifferencesbetweenthesafetyandoperationalcharacteristicsof
roundaboutsastheyhavebeenimplementedinadiversityofsituations.NCHRP672,Roundabouts,An
InformationalGuideprovidesguidanceonavarietyofaspectsoftheanalysisanduseofroundaboutsandit
characterizesthesimilaritiesbetweenroundaboutsandothertypesofintersections.However,itdoesnot
providedetailedguidanceonroundaboutsandaccessmanagement.TheFDOTshouldconductitsown
researchandworkwithAASHTOandotherpartnerstoensurethatguidance,includingroundaboutsasa
componentofaccessmanagement,beincorporatedintopractice.Inthissection,threeseparateresearch
initiativesareidentifiedbasedupontheresearchconductedinthisstudyincluding:nationalresearchon
roundaboutsandaccessmanagement,abefore‐and‐afterstudyofproposedroundaboutsintheUS41
corridorinSarasota,andstudiesonthedevelopmentoflocalvariablesforparametersintheanalysistools
forassessmentofroundabouts.Thefirstresearchwouldbeproposedforanationalstudy,whilethelast
twowouldberecommendedforFDOTfunding.
7.5.2.1NationalResearchEffortonRoundaboutsandAccessManagement
Throughoutthisresearchithasbecomeincreasinglyclearthatlittleresearchhasbeenconductedon
roundaboutsincombinationwithotherformsofaccessmanagementandroundaboutsasaformofaccess
management.Roundaboutscanbeseenasaformofaccessmanagementbecausetheycanaccommodate
left‐turnsandallowtheremovalofdirectionalleft‐turnlanes,yettheyfunctionasintersections.The
differencesinsafetyandoperationalcharacteristicsfromothertypesofaccessmanagementandother
intersectionsmeansthatthesitedistances,stoppingdistances,functionalareacharacteristics,and
intersectionanddrivewayspacingmaybedifferentforroundabouts.Furthermore,theuseofroundabouts
inavarietyoftransportationandlandusecontextsmaymeanthatthesefactorsdifferbycontext.While
NCHRP672,Roundabouts,AnInformationalGuideprovidesagreatstartonthisresearch,aprojectis
neededthatspecificallyfocusesonguidanceonaccessmanagementformajorarterialsandothersimilar
roadwaysfoundinthestatehighwaysystem.
7.5.2.2Before‐and‐AfterStudyoftheSarasotaRoundabouts
Roundabouts and Access Management
Page 134
Chapter7Discussion
Anotherareawherefurtherresearchisneededisrelatedtounderstandingthedifferencesinoperational
characteristicsbetweencorridorsusingroundaboutsandotherstandardintersections.Thecontractor’s
reportonNCHRP3‐100,whichevaluatestheuseofroundaboutsalongcorridors,islikelytoincreaseour
understandingofthesedifferences.However,thisstudyisacross‐sectionalstudy,whichmaynothavea
completesetofoperationaldatathatallowsforacomprehensiveunderstandingofthesedifferences.FDOT
hasauniqueopportunitytocompletesuchastudyontheUS41corridorinSarasotawheretwo
roundaboutsareproposedinaportionofthedowntownarea.Thisprojectiscurrentlyscheduledinthe
lateryearsoftheregionalTransportationImprovementProgram(TIP).Assuch,theFDOThasthe
opportunitytocompleteabefore‐and‐afterstudybycollectingthebeforedatawithinthenexttwoyears
andthenattwopointsafterwhentheprojectiscompleted.Asecondsetofdatacouldbecollectedto
understandtheadjustmentofroadwayuserstothenewroundaboutandotheraccessmanagement
features,whilethethirdsetofdatacouldbecollectedafterdrivershaveadjustedtothechangeinthe
corridor.Tocompletesuchanevaluationwouldrequirethecollectionofthefollowingtypesofdata:
 Existinggeometry(numberoflanes,typesoflanes,etc.).FDOTshouldbeabletoprovideas‐built
plans.Thesecanthenbeverifiedthroughfieldobservation.
 Traveltime.Thiscanbeverifiedusinganinstrumentedvehiclemakingnumerousrunsalongthe
corridor.Eachrunwouldbevideo‐recordedsothattheresearcherscanaccuratelyidentifysources
ofvariationinthetraveltimes.
 Trafficvolumes.ThisdatacouldcomefromstationaryvideocamerasorexistingFDOTsensor
infrastructure,ifitexists.
 Turningmovementpercentages(right,through,left,U‐turn).Again,thiscouldcomefrom
stationaryvideocamerasorexistingFDOTsensorinfrastructure,ifitexists.
 Intersectionapproachlegaveragequeuelengths(thiscanbeestimatedfromvideorecordings).
 Signaltimings(assumingtherearecurrentlysignalizedintersectionsalongthiscorridor).These
datashouldbeabletobeprovidedbyFDOT.Theycanbeverifiedthroughfieldobservation.
7.5.2.3FocusedStudiesonState‐specificlocationsguidance
Amajorchallengewiththeuseofnationalguidance,orguidancefromotherstates,isthatdriversinFlorida
mayresponddifferentlytodifferentformsofaccessmanagement,theymayhavedifferentreactiontimes
andtheymaydrivecloserorfurtherfromotherdriversastheyenterintersectionsandroundabouts.The
roundaboutsguidanceinseveralstatesprovidesdocumentationofuseoflocally‐developedparametersfor
variousaspectsofdesignandoperationalanalysis(e.g.,Californiaforaccelerationanddecelerationeffects;
MichiganforSPFsandCMFs;andWashingtonforcornerclearance,parallelroundabouts,U‐Turns,parking
andtransitstops,andWisconsinforlocationofdrivewaysandsitedistancebetweenusers).Thesefactors
mayinfluencethecalculationoftheentryflowrate,conflictingflowrateandexitflowrateofroundabouts.
TotheextentthatFloridadriversbehavedifferentlythandriversinotherstates,theFDOTshouldfund
researchtojustifytheuseofdifferentparametersforsoftwareandotheranalyticaltoolsforplanning‐level
design,preliminarydesign,analysisofpedestriantreatments,andsystemsanalysis.
Roundabouts and Access Management
Page 135
Chapter8Conclusions
ChapterEight:Conclusions
ThisFDOTresearchprojectfocusedonprovidingadviceonhowtoevaluatetheuseofroundaboutsasa
formofaccessmanagementandconsequentlyonwhatshouldbeincludedintheFDOT’sMedianHandbook,
andDrivewayInformationGuide.Inordertoaccomplishthisgoalitisimportanttounderstandthe
connectionbetweenroundaboutsandaccessmanagementandotherformsoftrafficcontrol.Therefore,
thisprojectincludedthreeprimarycomponents:areviewandassessmentofnationalandstateguidance
relatedtoroundaboutsandaccessmanagement;asafetyanalysisofall283roundaboutsinFlorida;andan
operationalanalysisofselectedroundabouts.Thischaptersummarizestheconclusionsofthisresearch
effort.
8.1ConclusionsoftheReviewofNationalandStateGuidance
Thereviewofnationalguidanceonroundaboutandaccessmanagementshowedthatthereareonlyfive
nationalreportsthatrefertoroundabouts:AASHTOGreenBook(2011),NCHRPReport672,NCHRP
Report572,NCHRPReport674,andNCHRPSynthesis264,ofwhichonlytheformerthreearerelevantto
thisstudy.NCHRPReport672,Roundabouts,AnInformationalGuidereferstoaccessmanagementinthe
contextofroundaboutsandreinforcestheideathatmanyoftheaccessmanagementprinciplesappliedto
conventionalintersectionscanbeappliedtoroundaboutsaswell.TheAASHTOGreenBook(2011)explains
accessmanagementconsiderationsforroundabouts.NCHRPProject3‐100,currentlyinprogress,evaluates
thePerformanceofCorridorswithRoundaboutsandwillsoonproduceanothernationalreportwhichwill
berelevanttothisproject.
Regardingstateguidanceonroundabouts,fromthefiftystatesandtheDistrictofColumbia,twenty‐six
stateshavewebsitesonroundaboutswithvaryingdegreesofinformation.Mostofthesestatesadoptthe
nationalguidancefromNCHRPReport672,Roundabouts,AnInformationalGuide(Rodegerdtsetal.,2010).
Infourteenstatesguidanceontheuseoflocallydevelopedparametersforvariousaspectsofdesignand
operationalanalysisisprovided.Minnesota,WisconsinandNewHampshirerecommendspecificsoftware
fortheassessmentoftheuseofroundaboutsforanintersectiondesign.Threeotherstates,Wisconsin,
Virginia,andKansas,addressthecoordinationofroundaboutsandaccessmanagement.Regardingaccess
managementguidance,forty‐threestateshaveincorporatedaccessand/oraccessmanagementintotheir
planninganddesignpolicies.Nineteenstateshaveaccessmanagementmanuals,separatefromgeneral
designmanualsandelevenstateDOTsmentionaccessmanagementordesignmanuals.Anothersixteen
DOTshaveotherdocumentswithvariousnames.However,onlysevenstatesincorporateroundaboutsinto
theiraccessmanagementguidance:Kansas,Virginia,California,Iowa,Michigan,Wisconsin,and
Washington.Generally,whenitcomestoroundaboutsandaccessmanagement,onlyKansasandVirginia
providesignificantsupplementalinformationtoNCHRP672,Roundabouts,AnInformationalGuide,while
mostoftheotherstatessimplyadoptedtheguidancewithoutsupplementation.
Floridahasthreemajordocumentsrelatedtoaccessmanagement.TheFDOTMedianHandbook(2006)
addressessomedesignconsiderationsrelatedtoroundaboutsbutitdoesnotprovideinformationabout
roundaboutdesignoraccessmanagement.Theothertwodocumentsdonotrefertoroundabouts.
8.2ConclusionsAboutSafetyAnalysisofRoundaboutsinFlorida
During2007‐2011,atotalof2,941crasheswerefoundtohaveoccurredwithin500ft.ofthe283
roundabouts.Policereportsofthesecrashesweredownloadedandreviewed.Crashlocationsofthese
2,941crashesweremanuallyverifiedandtheincorrectlocationswerecorrected.Intersection‐related
crashesandthosethatdidnotoccurontheroundaboutsandtheirapproachlegswereexcluded.Finally,a
totalof1,882crashesthatoccurredwithin500ft.ofthe283roundaboutswereincludedintheanalysis.
Roundabouts and Access Management
Page 136
Chapter8Conclusions
Thefollowingpotentialsafetyconcernsassociatedwithroundaboutsincommercialareaswere
investigated:
 Impactofdrivewaycornerclearancesonroundaboutsafety.
 Safetyimpactofmedianopeningsinthevicinityofroundabouts.
 Safetyatroundaboutsthatprovidedirectaccesstoactivitycenters.
 Safetyofvulnerableroadusersincludingpedestriansandbicyclists.
Basedontheresultsfromthesafetyanalysis,thefollowinggeneralrecommendationsrelatedtotheaccess
featuresinthevicinityofroundaboutsaremade:
 Crashdatashowthatdownstreamdrivewaycornerclearanceshaveagreatersafetyimpactthan
upstreamdrivewaycornerclearances.Longerdownstreamcornerclearancesaredesirableto
provideadditionaltimefordrivewayvehiclesthatexperiencereducedvehiclegapsandhigher
approachvehiclespeedfromupstreamroundabouts.
 Crashdatadidnotindicateserioussafetyissueswithmedianopeningsinthevicinityof
roundabouts.However,closingmedianopeningslocatedbetweentwoadjacentroundaboutscould
preventsomeofthemedianopeningrelatedcrashesandisdesirableifthecorridorisdesignedto
servelowheavyvehiclevolumesoriftheroundaboutsaresufficientlylargetosafelyaccommodate
U‐turnsbyheavyvehicles.
 Crashdatadidnotshowanincreasedsafetyhazardatroundaboutsthatprovidedirectaccessto
activitycenters.Providingdirectaccesstoactivitycentersthroughadedicatedlegisdesirableto
improvetrafficoperationsonthecorridoriftheprovisiondoesnotincreasethenumberof
roundaboutlegstobeyondthestandardfour.
8.3ConclusionsAboutOperationalAnalysisofRoundaboutsinFlorida
TheconclusionsfromtheoperationsanalysisofroundaboutsinFloridaaredescribedinthisparagraph.
Theroundabouts’operationalanalysisconductedinFloridashowedthatconflictscanoccurinthe
functionalareaofaroundaboutwhendrivewaysorotherintersectionsarelocatedtooclosetoa
roundabout.Thefunctionalareaofaroundaboutmaybedifferentfromconventionalintersections,
especiallyincaseswherethespeedissignificantlylowerthanmostun‐signalizedintersectionscurrently
operate.Inordertoavoidsuchconflicts,geometricdesignshouldtakeintoconsiderationthetrafficqueue
thatcouldbedevelopduringroundaboutoperationsastheycanaffectprocesseswithintheroundaboutor
withthesurroundingintersections.Duringtheoperationalanalysis,highpedestrianandbicyclesvolumes
canaffectthecapacityandtheeffectiveoperationsofroundabouts.
Theoperationalanalysisalsoindicatederroneousdriverbehaviorsuchasstoppinginthemiddleofthe
intersectiontopickupordropoffpedestrians,causingqueueswhichusuallyhappeninareaswithhigh
pedestrianandbicyclevolumes.Thisconflictswiththesafetyanalysis,whichreinforcedtheadvantagesof
usingroundaboutsforaccesstoactivitycentersbecausetheyreducedthechallengesofaccessthrough
openmediansortheplacementofanAWSCintersectionincloseproximitytotheroundabout.Another
concernisspillbackintotheroundaboutfromadownstreambottleneck,whichwouldresultincompletely
lockingtheroundabout.
8.4FinalRemarks
AsFloridastartsincorporatingroundaboutsintoitspracticesmoreoften,consistentguidanceontheuseof
roundaboutsthataddressthediversesituationsunderwhichtheyareimplementedshouldbeprovided.
Essentialtothisguidanceisconsiderationofthedifferencesbetweenroundaboutsandothertypesof
intersectionsandothertypesofaccessmanagement,suchasdriveways,andmedians.Roundaboutshave
Roundabouts and Access Management
Page 137
Chapter8Conclusions
generallybeenconsideredsimilartounsignalizedintersectionsbuttheyhavedifferentoperational
characteristicsrelatedtothedownstreamflowofvehicles,andthespeedwithwhichvehiclesenterthem.
Irrespectiveofhowtheyareconsidered,andthecontextinwhichtheyareimplemented,roundaboutsneed
tobedesignedinamannerthatensurestheiroperationalefficiencyandthesafetyofallusers.Thefindings
ofboththesafetyandoperationalanalysisidentifytheneedtoaccommodatebicyclistsandpedestrians
aroundroundaboutsbecausepedestrians,andinparticular,visuallyimpairedpedestrians,areatgreater
riskatroundaboutthanatotherunsignalizedintersectionsduetothecontinuousmovementthroughthem.
Additionally,roundaboutspresentaparticularchallengetopedestriansnearactivitycentersifpick‐ups
anddrop‐offsarenotproperlyhandled.
Theresultsofthisresearchshowconflictingresultswithrespecttotheuseofroundaboutsattheentrance
tomajoractivitycenters.Roundaboutsallowacontinuoustrafficflowsotheymaybeseenasamore
efficientsolutionthanusingcontinuousrightandleftturnlaneswithdirectionmediansandotherformsof
accessmanagement.Thesafetyanalysisfoundthatroundaboutswiththreeorfourlegsattheentranceto
activitycentersarejustassafeasroundaboutsinothercommerciallocations.However,theoperational
analysisfoundthatifaroundaboutislocatedtooclosetoanadjacentintersection,spilloverandadecrease
incapacitymayhappen.Assuch,thestateshouldconsiderdevelopingguidanceontheuseofroundabouts
atornearmajoractivitycentersandconsiderthecontextwheretheactivitycenterislocated,howthe
activitycenterissituatedwithinthestreetnetwork,andiftrucksanddeliveryvehiclesareproperly
accommodatedinthevicinityoftheroundabout.Ifaroundaboutisconstructedinhighpedestrianareas,
pedestrianscouldbeaccommodatedwithunderpassesoroverpassesorwithsidewalksfurtherfromthe
circulatoryroadway.Whilethesafetyandoperationalanalysisofthisstudydidnotidentifysignificant
problemswithtrucksandotherlargevehicles,theyarelikelytobecomeanissueasroundaboutsaremore
widelyusedalongstateroadways.Thesecanhavemoretrucktrafficandlargetrucksandbusesmayfindit
difficulttonegotiateasmallroundabout.Therefore,theroundaboutdesignshouldaccountforadequate
lateralclearanceandalargerradius.
FloridahasalreadyadoptedNCHRP672,Roundabouts,AnInformationalGuidebuttotheextentpossible,
thestateshouldconsidertheuseoflocallydevelopedparametersforvariousaspectsofdesignand
operationalanalysisofroundabouts.Otherstateshavedevelopedlocalparametersthatrelatetothe
influenceofdriverbehaviorasitaffectscapacityandoperationalcharacteristicsofroundabouts.
Differencesintheoperationswithinthefunctionalareashouldbehighlighted.Theguidanceneedstobe
explicitaboutthedefinitionofthefunctionalareaofaroundaboutespeciallyifitisdifferentfromtheone
specifiedinNCHRP672,Roundabouts,AnInformationalGuide.
Inordertoestimateandexaminetheeffectsandoperationsofaroundabout,simulationandanalysis
softwareshouldbeavailable.Sofar,HCSandotherdeterministicsoftwaresuchasHCS,Synchro,SIDRA,
RODELandARCADYcanconducttheplanning‐level,preliminarydesignanalysis,queuinganalysisand
provideinformationrelatedtoaccessmanagementandlocationofdriveways.Simulationsoftwaresuchas
VISSIMcanbeusedforthetrafficnetworkanalysis,publicinvolvementandpedestriantreatmentsanalysis.
Notallthesimulationprogramscanadequatelysimulaterealworldapplicationssotheplannersand
engineeringshouldpayattentiontowhichsoftwaretheyuseandwhichparameterstheyconsiderinthe
analysisofroundaboutsordrivewayplacementinthevicinityofroundabouts.
Finally,thisresearchdidnotshowsignificantimpactsoftheroundaboutlocation,whetherinanurbanor
suburbancontext,ontrafficoperations.
8.5AdditionalResearchNeeds
Theresearchfindingsofthisprojectsuggestthatwhilesomeresearchhasbeencompletedonroundabouts,
additionalresearchisneededonthecombinationofroundaboutsandaccessmanagementindifferent
Roundabouts and Access Management
Page 138
Chapter8Conclusions
contextsandconditions.NCHRP672,Roundabouts,AnInformationalGuide,themainnationalguidebook
onroundabouts,doesnotprovidedetailedguidanceonroundaboutsandaccessmanagement.TheFDOT
shouldconductitsownresearchandworkwithAASHTOandotherpartnerstoensurethatguidance,
includingroundaboutsasacomponentofaccessmanagement,isincorporatedintopractice.The
developmentoflocalvariablesforparametersintheanalysistoolsforassessmentofroundaboutsis
necessarybecauseusingnationalguidanceorguidancefromotherstatesmaynotcapturethewayinwhich
driversinFloridarespondtodifferentformsofaccessmanagement.Theymayhavedifferentreaction
timesordrivecloserorfurtherfromotherdriversastheyenterintersectionsandroundabouts.The
roundaboutsguidanceinseveralstatesprovidesdocumentationofuseforlocally‐developedparameters
forvariousaspectsofdesignandoperationalanalysis(e.g.,Californiaforaccelerationanddeceleration
effects;MichiganforSPFsandCMFs;Washingtonforcornerclearance,parallelroundabouts,U‐Turns,
parkingandtransitstops;andWisconsinforlocationofdrivewaysandsitedistancebetweenusers).These
factorsmayinfluencethecalculationoftheentryflowrate,conflictingflowrateandexitflowrateof
roundabouts.TotheextentthatFloridadriversbehavedifferentlythandriversinotherstates,FDOT
shouldfundresearchtojustifytheuseofdifferentparametersforthesoftwareandotheranalyticaltools
forplanning‐leveldesign,preliminarydesign,analysisofpedestriantreatments,andsystemsanalysis.
Also,inordertoenhanceunderstandingoftheeffectsofroundaboutsontrafficconditions,safety,and
trafficnetworkoperations,thereisaneedtoconductnationalresearchonroundaboutsandaccess
managementthatspecificallyfocusesonaccessmanagementformajorarterialsandothersimilar
roadwaysfoundonthestatehighwaysystem.
Throughoutthisresearchithasbecomeincreasinglyclearthat,whilemuchresearchhasbeenconducted
aboutroundaboutsandaboutaccessmanagement,littleresearchhasbeenconductedonroundaboutsin
combinationwithotherformsofaccessmanagementandroundaboutsasaformofaccessmanagement.
Roundaboutscanbeseenasaformofaccessmanagementbecausetheycanaccommodateleft‐turnsand
allowtheremovalofdirectionalleft‐turnlanes,yettheyfunctionasintersections.Thedifferencesintheir
safetyandoperationalcharacteristicsfromothertypesofaccessmanagementandotherintersections
meansthatsitedistances,stoppingdistances,functionalareacharacteristics,andintersectionand
drivewayspacingmaybedifferentforroundabouts.Furthermore,theuseofroundaboutsinavarietyof
transportationandlandusecontextsmaymeanthatthesefactorsdifferbycontext.Additionally,thereisa
lackofresearchonaccessmanagementandroundaboutsoraseriesofroundaboutsincorridors.NCHRP3‐
100,whichevaluatestheuseofroundaboutsalongcorridors,isonprogressanditislikelytogivesome
insightofthedifferencesbetweenroundaboutsandconventionalintersections.However,thisstudymay
nothaveacompletesetofoperationaldatathatcanallowforamorecomprehensiveunderstandingof
thesedifferences.Recently,thecityofSarasotaproposedaseriesofroundaboutsonUS41.Conductinga
before‐and‐afterstudytherewouldgiveabetterunderstandingoftheoperationalandsafety
characteristicsofcorridorswithroundaboutsinsteadofconventionalintersections.Therefore,FDOThasa
uniqueopportunitytocompletearealdatastudyontheUS41corridorinSarasotawheretworoundabouts
areproposedinaportionofthedowntown.
Roundabouts and Access Management
Page 139
ReferencesCited
ReferencesCited
Akcelik,R.,Chung,E.,&Besley,M.(1997,July).Analysisofroundaboutperformancebymodellingapproach
flowinteractions.InProceedingsoftheThirdInternationalSymposiumonIntersectionsWithoutTraffic
Signals(pp.15‐25).
Akçelik,R.(2004).Aroundaboutcasestudycomparingcapacityestimatesfromalternativeanalytical
models.PresentedattheSecondUrbanStreetSymposium,Anaheim,CA,2004.
Akçelik,R.(2011).AnassessmentoftheHighwayCapacityManual2010roundaboutcapacitymodel.In
TRBInternationalRoundaboutConference,Carmel,Indiana,USA.
Akcelik&Associates.(2014).AboutAkcelik&Associates.Retrievedfrom:
http://www.sidrasolutions.com/Company/About
Al‐Ghandour,M.,Schroeder,B.,Rasdorf,W.,&Williams,B.(2012).Delayanalysisofsingle‐laneroundabout
withasliplaneundervaryingexittypes,experimentalbalancedtrafficvolumes,andpedestrians,using
microsimulation.TransportationResearchRecord:JournaloftheTransportationResearchBoard,
2312(1),76‐85.
Al‐Masaeid,H.R.,&Faddah,M.Z.(1997).CapacityofroundaboutsinJordan.TransportationResearch
Record:JournaloftheTransportationResearchBoard,1572(1),76‐85.
Al‐Masaeid,H.R.(1999).Capacityandperformanceofroundabouts.CanadianJournalofCivilEngineering,
26(5),597‐605.
AmericanAssociationofStateHighwayTransportationOfficials(AASHTO)(2011).APolicyonthe
GeometricDesignofHighwaysandStreets.AmericanAssociationofStateHighwayandTransportation
Officials,Washington,DC,1,990.
Angelastro,M.,McFadden,J.,&Mehta,Y.(2012).Evaluationofsightdistanceandcrashfrequencyat
roundaboutsintheUnitedStates.InTransportationResearchBoard91stAnnualMeeting(No.12‐2412).
Ariniello,A.J.(2004,December).AreRoundaboutsGoodforBusiness?InTransportationResearchBoard
NationalRoundaboutConference,VailColorado.
ArizonaDepartmentofTransportation(AzDOT).(2012).RoadwayDesignGuidelines,Section403.2.
Arndt,O.K.,&Troutbeck,R.J.(1998).Relationshipbetweenroundaboutgeometryandaccidentrates(No.E‐
C003).
Ashmead,D.H.,Guth,D.,Wall,R.S.,Long,R.G.,&Ponchillia,P.E.(2005).Streetcrossingbysightedand
blindpedestriansatamodernroundabout.JournalofTransportationEngineering,131(11),812‐821.
Aty,M.A.,&Hosni,Y.(2001).State‐of‐the‐ArtReportOn:RoundaboutsDesign,ModelingandSimulation(No.
FinalReport).Retrievedfromhttp://ntl.bts.gov/lib/17000/17700/17782/PB2001104294.pdf
Bagdade,J.,Persaud,B.N.,Mcintosh,K.,Yassin,J.,Lyon,C.A.,Redinger,C.,Whitten,J.,&Butch,W.A.(2011).
EvaluatingthePerformanceandSafetyEffectivenessofRoundabouts(No.RC‐1566).
Bansen,J.,&Sullivan,F.(2013,June)ModernRoundaboutsinFlorida.PresentationmadeatFloridaDesign
TrainingExpo,Orlando,FL.
Bared,J.,&Edara,P.K.(2005,May).Simulatedcapacityofroundaboutsandimpactofroundaboutwithina
progressedsignalizedroad.InNationalRoundaboutConference:2005Proceedings.Transportation
ResearchBoardoftheNationalAcademies,Vail,USA.
Berthaume,A.,&KnodlerJr,M.A.(2013).HazardousBicycleManeuversatSingle‐LaneRoundaboutsin
Massachusetts:AConflictandEventsStudy.InTransportationResearchBoard92ndAnnualMeeting
(No.13‐4216).
Bie,J.,Lo,H.K.,Wong,S.,&Hung,W.(2005).Safetyanalysisoftrafficroundabout:conventionalversus
Alberta‐typemarkings.JournaloftheEasternAsiaSocietyforTransportationStudies,6,3309‐3324.
Roundabouts and Access Management
Page 140
ReferencesCited
Bonneson,J.A.andMcCoy,P.T.(1997)CapacityandOperationalEffectsofMidblockLeft‐TurnLanes
NCHRPReport395.Washington,DC:TransportationResearchBoard.Retrievedfrom
http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_395.pdf
Brown,M.(1995).Thedesignofroundabouts.TransportResearchLaboratory,London.
Butorac,M.A.andWen.J.C.(2002).AccessManagementonCrossroadsintheVicinityofInterchanges,A
SynthesisofHighwayPractice.NCHRPSynthesisReport332,Washington,DC:Transportation
ResearchBoard.Retrievedfromhttp://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_syn_332.pdf
CaliforniaDepartmentofTransportation(Caltrans).(2007).HighwayDesignManual.
Chen,Y.,Persaud,B.,&Lyon,C.(2011).EffectofSpeedonRoundaboutSafetyPerformance:Implicationsfor
UseofSpeedasSurrogateMeasure.InTransportationResearchBoard90thAnnualMeeting(No.11‐
2846).
Churchill,T.,Stipdonk,H.,&Bijleveld,F.D.(2010).Effectsofroundaboutsonroadcasualtiesinthe
Netherlands(Vol.2010,No.21).InstituteforRoadSafetyResearch.
Dabbour,E.,&Easa,S.M.(2008).Evaluationofsafetyandoperationalimpactsofbicyclebypasslanesat
modernroundabouts.CanadianJournalofCivilEngineering,35(10),1025‐1032.
Daniels,S.,&Wets,G.(2005).Trafficsafetyeffectsonroundabouts:areviewwithemphasisonbicyclist's
safety.
Daniels,S.,Nuyts,E.,&Wets,G.(2008).Theeffectsofroundaboutsontrafficsafetyforbicyclists:an
observationalstudy.AccidentAnalysis&Prevention,40(2),518‐526.
Daniels,S.,Brijs,T.,Nuyts,E.,&Wets,G.(2010).Externalityofriskandcrashseverityatroundabouts.
AccidentAnalysis&Prevention,42(6),1966‐1973.
Daniels,S.,Brijs,T.,Nuyts,E.,&Wets,G.(2011).Extendedpredictionmodelsforcrashesatroundabouts.
Safetyscience,49(2),198‐207.
DeBrabander,B.,Nuyts,E.,&Vereeck,L.(2005).RoadsafetyeffectsofroundaboutsinFlanders.Journalof
SafetyResearch,36(3),289‐296.
DeBrabander,B.,&Vereeck,L.(2007).SafetyEffectsofRoundaboutsinFlanders:Signaltype,speedlimits
andvulnerableroadusers.AccidentAnalysis&Prevention,39(3),591‐599.
Demosthenes,P.(2007,Oct18).WhyManageAccesstotheStateHighwaySystem.PowerPoint
PresentationattheDivisionofResearchandInnovationCaltrans.Retrievedfrom
http://www.dot.ca.gov/researchconn/past_speakers/MrDemosthenes_1/demosthenes_accessmgt_calt
rans_oct18_07wtext.pdf
Dixon,K.,&Zheng,J.(2013).DevelopingSafetyPerformanceMeasuresforRoundaboutApplicationsinthe
StateofOregon(No.FHWA‐OR‐RD‐13‐08).
Elias,A.(2009).RoundaboutmodelinginCORSIM.MSThesis.UniversityofFlorida.
Elvik,R.(2003).Effectsonroadsafetyofconvertingintersectionstoroundabouts:reviewofevidencefrom
non‐USstudies.TransportationResearchRecord:JournaloftheTransportationResearchBoard,
1847(1),1‐10.
FederalHighwayAdministration(FHWA).(2006).Roundabouts:TechnicalSummary.Retrievedfrom
http://safety.fhwa.dot.gov/intersection/roundabouts/fhwasa10006/fhwasa10006.pdf
FederalHighwayAdministration(FHWA).(2011).PublicRoads.Retrievedfrom
http://www.fhwa.dot.gov/publications/publicroads/11septoct/02.cfm
FederalHighwayAdministration(FHWA)(2013).FHWAOfficeofSafetyRoundaboutOutreachand
EducationToolbox,Retrievedfrom
http://safety.fhwa.dot.gov/intersection/roundabouts/roundabouttoolbox/
Roundabouts and Access Management
Page 141
ReferencesCited
Fisk,C.S.(1991).Trafficperformanceanalysisatroundabouts.TransportationResearchPartB:
Methodological,25(2),89‐102.
Flannery,A.,&Datta,T.K.(1996).ModernroundaboutsandtrafficcrashexperienceinUnitedStates.
TransportationResearchRecord:JournaloftheTransportationResearchBoard,1553(1),103‐109.
Flannery,A.,&Datta,T.(1997).OperationalperformancemeasuresofAmericanroundabouts.
TransportationResearchRecord:JournaloftheTransportationResearchBoard,1572(1),68‐75.
Flannery,A.,Elefteriadou,L.,Koza,P.,&McFadden,J.(1998).Safety,delay,andcapacityofsingle‐lane
roundaboutsintheUnitedStates.TransportationResearchRecord:JournaloftheTransportation
ResearchBoard,1646(1),63‐70.
Flannery,A.(2001).Geometricdesignandsafetyaspectsofroundabouts.TransportationResearchRecord:
JournaloftheTransportationResearchBoard,1751(1),76‐81.
FloridaDepartmentofTransportation(FDOT).(1996).FloridaRoundaboutGuide.Tallahassee,Florida.
Retrievedfrom
http://www.dot.state.fl.us/trafficoperations/doc_library/pdf/roundabout_guide8_07.pdf
FloridaDepartmentofTransportation(FDOT).(2000).ManualonUniformTrafficStudies,TopicNo.750‐
020‐007,RoundaboutJustificationStudy.Tallahassee,Florida:FDOT.Retrievedfrom
http://www.dot.state.fl.us/trafficoperations/Operations/Studies/MUTS/Chapter16.pdf
FloridaDepartmentofTransportation(FDOT).(2006).MedianHandbook.Tallahassee,Florida.Retrieved
fromhttp://www.dot.state.fl.us/planning/systems/programs/sm/accman/pdfs/mhb06b.
FloridaDepartmentofTransportation(FDOT)(2007).FloridaIntersectionDesignGuide.ForNew
ConstructionandMajorReconstructionofAt‐GradeIntersectionsontheStateHighwaySystem.
Retrievedfromhttp://www.dot.state.fl.us/rddesign/FIDG‐Manual/FIDG2007.pdf
FloridaDepartmentofTransportation(FDOT).(2008).DrivewayInformationGuide.Tallahassee,Florida.
Retrievedfrom
http://www.dot.state.fl.us/planning/systems/programs/sm/accman/pdfs/driveway2008.pdf
FloridaDepartmentofTransportation(FDOT)RoadwayDesignOffice(2010)DesignStandardsforDesign,
Construction,MaintenanceandUtilityOperationsontheStateHighwaySystem,StandardIndex515
(Driveways).Retrievedfromhttp://www.dot.state.fl.us/rddesign/DS/14/IDx/00515.pdf
FloridaDepartmentofTransportation(FDOT).(2014).SystemsPlanningDocuments:AccessManagement.
Retrievedhttp://www.dot.state.fl.us/planning/systems/documents/sm/default.shtm#access
Fortuijn,L.G.H.(2009).TurboRoundabouts:DesignPrinciplesandSafetyPerformance.Transportation
ResearchRecord:JournaloftheTransportationResearchBoard,2096,16‐24.
Frawley,W.E.,&Eisele,W.L.(2005).AccessManagementGuidebookTexas.TexasTransportationInstitute,
TexasA&MUniversitySystem.
Furtado,G.(2004).Accommodatingvulnerableroadusersinroundaboutdesign.In2004TACAnnual
Conference.
Gluck,J.S.,Levinson,H.S.,&Stover,V.G.(1999).Impactsofaccessmanagementtechniques(No.420).
TransportationResearchBoard.WashingtonD.C.
Gluck,J.S.&Lorenz,M.R.(2010).NCHRPSynthesis404:StateofthePracticeinHighwayAccess
Management.Washington,DC:TransportationResearchBoard.Retrievedfrom
http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_syn_404.pdf
Gross,F.,Lyon,C.,Persaud,B.,&Srinivasan,R.(2013).Safetyeffectivenessofconvertingsignalized
intersectionstoroundabouts.AccidentAnalysis&Prevention,50,234‐241.
Roundabouts and Access Management
Page 142
ReferencesCited
Hagring,O.,Rouphail,N.M.,&Sørensen,H.A.(2003).Comparisonofcapacitymodelsfortwo‐lane
roundabouts.TransportationResearchRecord:JournaloftheTransportationResearchBoard,1852(1),
114‐123.
Hallmark,S.L.,Fitzsimmons,E.J.,Isebrands,H.N.,&Giese,K.L.(2010).RoundaboutsinSignalized
Corridors.TransportationResearchRecord:JournaloftheTransportationResearchBoard,2182(1),139‐
147.
Hallmark,S.L.,&Isebrands,H.(2008).Planning‐levelGuidelinesforModernRoundabouts.Centerfor
TransportationResearchandEducation,IowaStateUniversity.
Harkey,D.L.,&Carter,D.L.(2006).Observationalanalysisofpedestrian,bicyclist,andmotoristbehaviors
atroundaboutsintheUnitedStates.TransportationResearchRecord:JournaloftheTransportation
ResearchBoard,1982(1),155‐165.
Hels,T.,&Orozova‐Bekkevold,I.(2007).Theeffectofroundaboutdesignfeaturesoncyclistaccidentrate.
AccidentAnalysis&Prevention,39(2),300‐307.
Huntington,D.&Wen,J.(2005).Synthesis351:AccessRights.TransportationResearchBoard;National
CooperativeHighwayResearchProgramSynthesisProgram(NCHRPSYN);NCHRPTheNational
AcademiesPress:Washington,DC.Retrievedfromhttp://www.nap.edu/catalog.php?record_id=13557
Hyden,C.,&Varhelyi,A.(2000).Theeffectsonsafety,timeconsumptionandenvironmentoflargescaleuse
ofroundaboutsinanurbanarea:acasestudy.AccidentAnalysis&Prevention,32(1),11‐23.
IndianaDepartmentofTransportation(InDOT).(2006,revised2009).AccessManagementGuide.Retrieved
fromwww.in.gov/indot/files/guide_total.pdf
Inman,V.W.,Davis,G.W.,&Sauerburger,D.(2005,May).Roundaboutaccessforvisuallyimpaired
pedestrians:Evaluationofayieldingvehiclealertingsystemfordouble‐laneroundabouts.In
Proceedings,NationalRoundaboutConference,Vail,CO.
Inman,V.W.,Davis,G.W.,&Sauerburger,D.(2006a).Pedestrianaccesstoroundabouts:Assessmentof
motorists'yieldingtovisuallyimpairedpedestriansandpotentialtreatmentstoimproveaccess(No.
FHWA‐HRT‐05‐080).Retrievedfrom
http://www.fhwa.dot.gov/publications/research/safety/pedbike/05080/
Inman,V.W.,Katz,B.J.,&Hanscom,F.R.(2006b).Navigationsigningforroundabouts.Transportation
ResearchRecord:JournaloftheTransportationResearchBoard,1973(1),18‐26.
IowaDepartmentofTransportation(IowaDOT).(2009,revisedin2010).DesignManualChapter6
GeometricDesign,6A‐3ModernRoundabout‐GeneralGuidance.Retrievedfrom
www.iowadot.gov/design/dmanual/06a‐03.pdf
IowaDepartmentofTransportation(IowaDOT).(2012).IowaPrimaryHighwayAccessManagementPolicy.
Retrievedfromhttp://www.iowadot.gov/traffic/pdfs/AccessPolicy.pdf
Isaacs,B.,&Barrett,J.P.(2003).UseofRoundaboutsinanUrbanSetting.In2ndUrbanStreetSymposium:
Uptown,Downtown,orSmallTown:DesigningUrbanStreetsThatWork.
Isebrands,H.N.(2009a).Roundaboutsandsignals:harmonyevenwithincreasingtrafficvolumes.ITE
Journal,79(2).
Isebrands,H.(2009b).Crashanalysisofroundaboutsathigh‐speedruralintersections.Transportation
ResearchRecord:JournaloftheTransportationResearchBoard,2096(1),1‐7.
Isebrands,H.,&Hallmark,S.(2012).Statisticalanalysisanddevelopmentofcrashpredictionmodelfor
roundaboutsonhigh‐speedruralroadways.TransportationResearchRecord:Journalofthe
TransportationResearchBoard,2312(1),3‐13.
Jacquemart,G.(1998).ModernroundaboutpracticeintheUnitedStates(No.Project20‐5FY1996).
Retrievedfromhttp://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_syn_264.pdf
Roundabouts and Access Management
Page 143
ReferencesCited
Jasper,J.D.(2010).DirectorDivisionofHighwayDesign.DesignMemorandumNo.01‐10,DesignGuidance
forRoundaboutIntersections.Retrievedfromhttp://transportation.ky.gov/Congestion‐
Toolbox/Documents/KYTC%20Roundabout%20Policy.pdf
Jensen,S.U.,&Apes,T.(2013).SafetyEffectsofConvertingIntersectionstoRoundabouts.InTransportation
ResearchBoard92ndAnnualMeeting(No.13‐1319).
Johnson,M.T.,&Isebrands,H.N.(2008).Accessmanagementconsiderationsforhighcapacitymulti‐lane
roundaboutdesignandimplementation.In8thNationalConferenceonAccessManagement.
KansasDepartmentofTransportation(2013,January).KDOTAccessManagementPolicy.Retrievedfrom:
http://www.ksdot.org/accessmanagement/Access_Management_Policy_Jan2013.pdf
KentuckyTransportationCabinet(KYTC).(2008).AccessManagementImplementationReport.Retrieved
fromtransportation.ky.gov/Congestion‐
Toolbox/Documents/Access%20Management%20Implementation%20Report%202008.pdf
KentuckyTransportationCabinet(KYTC).(2010).DesignGuidanceforRoundaboutIntersections.
Retrievedfromtransportation.ky.gov/Congestion‐
Toolbox/Documents/KYTC%20Roundabout%20Policy.pdf
Kittelson&Associates,Inc.(2013).EvaluatingthePerformanceofCorridorswithRoundabouts.ProjectNo.
03‐100.
Kittelson&Associates,Inc.&TranSystemCorporation.(2003).KansasRoundaboutGuide,ASupplementto
FHWA'sRoundabouts:AnInformationalGuide.KansasDepartmentofTransportation.Retrievedfrom
www.ksdot.org/burtrafficeng/Roundabouts/Roundabout_Guide/RoundaboutGuide.asp
Koepke,F.J.,&Levinson,H.S.(1992).Accessmanagementguidelinesforactivitycenters.Washington,D.C.
TransportationResearchBoard.
Layton,R.(2012).InterchangeAccessManagement.OregonStateUniversity;Corvallis,OR.
Lee,J.C.,Robinson,B.,Kidd,B.D.,&Scarborough,W.(2003).Roundabouts:AnArizonacasestudyanddesign
guidelines(No.FHWA‐AZ‐03‐545,).
Lenters,M.(n.d.).“RoundaboutsandBusiness.”PowerPointslidepresentation,undated.Senttoauthorsin
emaildatedFebruary20,2013.
Lindenmann,H.P.(2006).Capacityofsmallroundaboutswithtwo‐laneentries.TransportationResearch
Record:JournaloftheTransportationResearchBoard,1988(1),119‐126.
Lord,D.,vanSchalkwyk,I.,Chrysler,S.,&Staplin,L.(2007).Astrategytoreduceolderdriverinjuriesat
intersectionsusingmoreaccommodatingroundaboutdesignpractices.AccidentAnalysis&Prevention,
39(3),427‐432.
Mahdalová,I.,Seidler,T.,&Cihlářová,D.(2010).InfluenceoftheRoundaboutGeometryonItsSafety.
TransactionsoftheVŠB‐TechnicalUniversityofOstrava.ConstructionSeries,10(1),1‐9.
MarylandStateHighwayAdministration.(2004).StateHighwayAccessManual.Retrievedfrom
roads.maryland.gov/ohd/accesspermits.pdf
MarylandStateHighwayAdministration.(2012).RoundaboutDesignGuidelines.Retrievedfrom
http://roads.maryland.gov/ohd2/mdsha_roundabout_guidelines.pdf
Bagdade, J., Persaud, B. N., McIntosh, K., Yassin, J., Lyon, C. A., Redinger, C., ... & Butch, W. A.
(2011). Evaluating the Performance and Safety Effectiveness of Roundabouts (No. RC-1566).
MinnesotaDepartmentofTransportation(MnDOT).(2008).MnDOTAccessManagementManual.Retrieved
fromwww.dot.state.mn.us/accessmanagement/pdf/manualchapters/chapter2.pdf
Roundabouts and Access Management
Page 144
ReferencesCited
MinnesotaDepartmentofTransportation(MnDOT).(2009).MnDOTRoadDesignManual:Chapter12,
DesignGuidelinesforModernRoundabouts.Retrievedfrom
http://dotapp7.dot.state.mn.us/edms/download?docId=1062365
Møller,M.,&Hels,T.(2008).Cyclists’perceptionofriskinroundabouts.AccidentAnalysis&Prevention,
40(3),1055‐1062.
Montella,A.,Turner,S.,Chiaradonna,S.,&Aldridge,D.(2013).RoundaboutDesignPractices:International
OverviewandInsightstoUpdatetheItalianStandard.InTransportationResearchBoard92ndAnnual
Meeting(No.13‐2129).
NewHampshireDepartmentofTransportation(NHDOT).2007(Revisedin2009).NHDOTSupplemental
DesignCriteria.Retrievedfrom
www.nh.gov/dot/org/projectdevelopment/highwaydesign/roundabouts/documents/RoundaboutFun
damentals.pdf
Park,L.&Pierce,D.(2013).RoundaboutsandtheAccommodationofLargeTrucks:AMotorCarrier
Perspective,Proceedingsofthe92ndAnnualMeetingoftheTransportationResearchBoard,
Washington,D.C.
PennsylvaniaDepartmentofTransportation(PennDOT).(2007).GuidetoRoundabout(PublicationNo.
414).
Persaud,B.N.,Retting,R.A.,Garder,P.E.,&Lord,D.(2001).Safetyeffectofroundaboutconversionsinthe
UnitedStates:EmpiricalBayesobservationalbefore‐afterstudy.TransportationResearchRecord:
JournaloftheTransportationResearchBoard,1751(1),1‐8.
Polus,A.,&Shmueli,S.(1997).AnalysisandEvaluationoftheCapacityofRoundabouts.Transportation
ResearchRecord:JournaloftheTransportationResearchBoard,1572(1),99‐104.
Polus,A.,Lazar,S.S.,&Livneh,M.(2003).Criticalgapasafunctionofwaitingtimeindetermining
roundaboutcapacity.JournalofTransportationEngineering,129(5),504‐509.
Potts,I.B.,Harwood,D.W.,Torbic,D.J.,Richard,K.R.,Gluck,J.S.,Levinson,H.S.,...&Ghebrial,R.S.(2004).
NCHRPReport524:SafetyofU‐TurnsatUnsignalizedMedianOpenings.TransportationResearchBoard
oftheNationalAcademies,Washington,DC.Retrievedfrom
http://www.nap.edu/catalog.php?record_id=13768
Prytyka,G.&Sullivan,F.(2012).Roundabouts,Florida’sImplementationStrategy.Tallahassee,Florida.
Retrievedfromwww.dot.state.fl.us/structures/designExpo2012/Presentations/Roundabouts_Final‐
Expo‐2012.pdf
PTVGroup.(2013)RetrievedDec.29,2013fromhttp://vision‐traffic.ptvgroup.com/en‐us/products/ptv‐
vissim
Retting,R.A.,Persaud,B.N.,Garder,P.E.,&Lord,D.(2001).Crashandinjuryreductionfollowing
installationofroundaboutsintheUnitedStates.AmericanJournalofPublicHealth,91(4),628.
Richfield,V.,&Hourdos,J.(2013).EffectofSignsandStripingonRoundaboutSafety:AnObservational
Before/AfterStudy.InTransportationResearchBoard92ndAnnualMeeting(No.13‐4568).
Robinson,B.W.,Rodegerdts,L.,Scarborough,W.,Kittelson,W.,Troutbeck,R.etal.(2000).Roundabouts:An
InformationalGuide.(FHWA‐RD‐00‐067).Retrievedfrom
http://www.fhwa.dot.gov/publications/research/safety/00067/00067.pdf
Rodegerdts,L.,Blogg,M.,Wemple,E.,Myers,E.,Kyte,M.,Dixon,M.,etal.(2007).RoundaboutsintheUnited
States(Vol.572).TransportationResearchBoardNationalResearch.Retrievedfrom
http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_572.pdf
Roundabouts and Access Management
Page 145
ReferencesCited
Rodegerdts,L.,Bansen,J.,Tiesler,C.,Knudsen,J.,Myers,E.,Johnson,M.,etal.(2010).Roundabouts:An
InformationalGuide.NCHRPReport672.Washington,DC:NationalCooperativeHighwayResearch
Program.Retrievedfromhttp://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_672.pdf
Rodegerdts,L.,Blogg,M.,Wemple,E.,Myers,E.,Kyte,M.,Dixon,M.,List,G.,Flannery,A.,Troutbeck,R.,
Brilon,W.,Wu,N.,Persaud,B.,Lyon,C.,Harkey,D.&Carter,D.(2006).NCHRPWeb‐onlyDocument94:
AppendixestoNCHRPReport572:RoundaboutsintheUnitedStates.Washington,DC:Transportation
ResearchBoard.Retrievedfromhttp://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_w94.pdf
Rose,D.C.,Gluck,J.,Williams,K.,&Kramer,J.(2005).NCHRPReport548:Aguidebookforincludingaccess
managementintransportationplanning.TransportationResearchBoard,Washington,DC.Retrieved
fromhttp://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_548.pdf
Rue,H.,McNally,L.,Rooney,K.,Santalucia,P.,Raulerson,M.,Lim‐Yap,J.,Mann,J.,&Burden,D.(2010).
LivabilityinTransportationGuidebook:PlanningApproachesthatPromoteLivability(No.FHWA‐HEP‐
10‐028).
Russell,E.,Landman,E.D.,&Godavarthy,R.(2012).AStudyoftheImpactofRoundaboutsonTrafficFlows
andBusiness(No.K‐TRAN:KSU‐09‐10).KansasDepartmentofTransportation.
Sacchi,E.,Bassani,M.,&Persaud,B.(2011).Comparisonofsafetyperformancemodelsforurban
roundaboutsinItalyandothercountries.TransportationResearchRecord:JournaloftheTransportation
ResearchBoard,2265(1),253‐259.
Saccomanno,F.F.,Cunto,F.,Guido,G.,&Vitale,A.(2008).Comparingsafetyatsignalizedintersectionsand
roundaboutsusingsimulatedrear‐endconflicts.TransportationResearchRecord:Journalofthe
TransportationResearchBoard,2078(1),90‐95.
Schroeder,B.J.,Rouphail,N.M.,&Hughes,R.G.(2008).TowardRoundaboutAccessibility—Exploringthe
OperationalImpactofPedestrianSignalizationOptionsatModernRoundabouts.Journalof
TransportationEngineering,134(6),262‐271.
Schroeder,B.(2013).BlindPedestriansAccesstoRoundaboutsandOtherComplexIntersections.North
CarolinaStateUniversity.Retrievedfromhttp://www.itre.ncsu.edu/ITRE/research/Pedestrian‐
Accessibility/index.html
Schroeder,B.,Hughes,R.,Rouphail,N.,Cunningham,C.,Salamati,K.,Long,R.,...&Myers,E.(2011).NCHRP
Report674CrossingSolutionsatRoundaboutsandChannelizedTurnLanesforPedestrianswithVision
Disabilities.TransportationResearchBoardoftheNationalAcademies,Washington,DC.Retrievedfrom
http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_674.pdf
Shen,L.D.,Elbadrawi,H.R.,&Ospina,D.I.(2000).BicycleandPedestrianConsiderationsatRoundabouts
(No.FinalReport).
Singer,L.andHicks,T(2000).AnEngineer'sDilemma:AccommodatingtheNeedsofPeoplewithDisabilities
atModernUrbanRoundabouts.InstituteofTransportationEngineers.
Sisiopiku,V.P.,&Oh,H.U.(2001).EvaluationofroundaboutperformanceusingSIDRA.Journalof
TransportationEngineering,127(2),143‐150.
Stamatiadis,N.,House,B.,Brickey,J.,Hartman,D.,Chen,M.,Pigman,J.,Boddu,K.,Patangay,S.&Elwood,E.
(2004).AccessmanagementforKentucky(No.KTC‐04‐05/SPR251‐01‐1F,).
Stanek,D.,&Milam,R.T.(2005).High‐capacityroundaboutintersectionanalysis:goingaroundincircles.
TransportationResearchE‐Circular,(E‐C083).
St‐Aubin,P.,Saunier,N.,Miranda‐Moreno,L.F.,&Ismail,K.(2013).DetailedDriverBehaviourAnalysisand
TrajectoryInterpretationatRoundaboutsUsingComputerVisionData.InTransportationResearch
Board92ndAnnualMeeting(No.13‐5255).
Stone,J.R.,Chae,K.,&Pillalamarri,S.(2002).Theeffectsofroundaboutsonpedestriansafety.Southeastern
TransportationCenter.
Roundabouts and Access Management
Page 146
ReferencesCited
Taekratok,T.(1998).ModernroundaboutsforOregon(No.OR‐RD‐98‐17).OregonDepartmentof
Transportation.
TransportationResearchBoard(TRB).(2000).HighwayCapacityManual.NationalResearchCouncil:
Washington,D.C.
TransportationResearchBoard(TRB).(2003).AccessManagementManual.NationalResearchCouncil:
Washington,D.C.
TransportationResearchBoard(TRB).(2010a).HighwayCapacityManual.NationalResearchCouncil:
Washington,D.C.
TransportationResearchBoard(TRB).TaskForceonDevelopmentoftheHighwaySafetyManual,
AmericanAssociationofStateHighwayTransportationOfficials.JointTaskForceontheHighwaySafety
Manual,&NationalCooperativeHighwayResearchProgram.(2010b).HighwaySafetyManual(Vol.1).
AASHTO.
TransportationResearchBoard(TRB)(2014).NCHRP03‐100EvaluatingthePerformanceofCorridorswith
Roundabouts(ProjectDescription)Retrievedfrom
http://apps.trb.org/cmsfeed/trbnetprojectdisplay.asp?projectid=2950
Trueblood,M.(2013).SynchroandSimTrafficRoundaboutDemo.InITETechnicalConferenceandExhibit.
Trueblood,M.,&Dale,J.(2003).SimulatingroundaboutswithVISSIM.In2ndUrbanStreetSymposium:
Uptown,Downtown,orSmallTown:DesigningUrbanStreetsThatWork.
Turner,S.&Brown,M.(2013).PushingtheBoundariesofRoadSafetyRiskAnalysis,IPENZTransportation
GroupConferenceDunedin,NewZealand
Uddin,W.,Headrick,J.,&Sullivan,J.S.(2012).PerformanceEvaluationofRoundaboutsforTrafficFlow
ImprovementsandCrashReductionsataHighwayInterchangeinOxford,MS.InTransportation
ResearchBoard91stAnnualMeeting(No.12‐3844).
UnitedStatesAccessBoard(USAB).(2006).PedestrianAccesstoModernRoundabouts:Designand
OperationalIssuesforPedestrianswhoareBlindRetrievedfrom
http://safety.fhwa.dot.gov/intersection/resources/fhwasa09027/resources/Pedestrian%20Access%2
0to%20Modern%20Roundabouts.mht
Valdez,M.,Cheu,R.L.,&Duran,C.(2011).OperationsofModernRoundaboutwithUnbalancedApproach
Volumes.TransportationResearchRecord:JournaloftheTransportationResearchBoard,2265(1),234‐
243.
VirginiaDepartmentofTransportation(VDOT).(2007).AccessManagementDesignStandardsforEntrances
andIntersections.
Vlahos,E.,Polus,A.,Lacombe,D.,Ranjitkar,P.,Faghri,A.,&Fortunato,B.R.(2008).Evaluatingthe
ConversionofAll‐WayStop‐ControlledIntersectionsintoRoundabouts.TransportationResearch
Record:JournaloftheTransportationResearchBoard,2078(1),80‐89.
Waddell,E.(1997).EvolutionofRoundaboutTechnology:AHistory‐BasedLiteratureReview.In
CompendiumofTechnicalPapers,67thAnnualMeeting,InstituteofTransportationEngineers,Boston
(August1997).
Wang,Z.,Boon,T.O.,&Rakha,H.(2013).RoundaboutVersusTrafficSignalControl:ComparativeAnalysis.
InTransportationResearchBoard92ndAnnualMeeting(No.13‐4422).
Wei,T.,Grenard,J.L.,&Shah,H.R.(2011).DevelopingCapacityModelsforLocalRoundabouts.
TransportationResearchRecord:JournaloftheTransportationResearchBoard,2257(1),1‐9.
Williams, K. (2002). Driveway Regulation Practices (Vol. 304). Transportation Research Board.
Retrievedfromhttp://www.cutr.usf.edu/research/access_m/pdf/nchrp_syn_304.pdf
Roundabouts and Access Management
Page 147
ReferencesCited
Williams,K.(2004).CooperativeAgreementsforCorridorManagement(Vol.337).TransportationResearch
Board.Retrievedfromhttp://www.cutr.usf.edu/research/access_m/pdf/nchrp_syn_337.pdf
Williams,K.M.,&Levinson,H.S.(2008).AccessManagement:Past,Present,andFuture.In8thNational
ConferenceonAccessManagement.
Williams,K.M.&Marshall,M.A.(1996).ManagingCorridorDevelopment:AMunicipalHandbook.Retrieved
fromhttp://www.cutr.usf.edu/pdf/corridor.pdf
WisconsinDepartmentofTransportation(WisDOT).(2011).TheFacilitiesDevelopmentManual,Chapter11,
Section26Roundabouts.Retrievedfrom
www.dot.wisconsin.gov/safety/motorist/roaddesign/roundabouts/design.htm
WisconsinDepartmentofTransportation(WisDOT).(2013).WSDOTDesignManual22.01.08:Chapter1320
Roundabouts.Retrievedfromhttp://www.wsdot.wa.gov/publications/manuals/fulltext/M22‐
01/1320.pdf
Yin,D.,&Qiu,T.Z.(2011).ComparisonofMacroscopicandMicroscopicSimulationModelsinModern
RoundaboutAnalysis.TransportationResearchRecord:JournaloftheTransportationResearchBoard,
2265(1),244‐252.
Zirkel,B.,Park,S.,McFadden,J.,Angelastro,M.,&McCarthy,L.A.(2013).AnalysisofSightDistance,Crash
Rate,andOperatingSpeedRelationshipsforSingleLaneRoundaboutsintheUnitedStates.In
TransportationResearchBoard92ndAnnualMeeting(No.13‐1847).
Roundabouts and Access Management
Page 148
AppendixAStatePolicies
AppendixA:RoundaboutsFeaturesandDimensions
KeyFeaturesofaModernRoundabout
Source:(Rodegerdtsetal.,2010,6)
TableA.1.KeyFeaturesofaModernRoundabout
Feature
Centralisland
Splitterisland
Circulatoryroadway
Apron
Yieldline
Accessiblepedestrian
crossing
Bicycletreatments
Landscapingbuffer
Description
Thecentralisland istheraisedareainthecenterofaroundaboutaround
whichtrafficcirculates.
Asplitterislandisaraisedorpaintedareaonanapproachusedto
separateenteringfromexistingtraffic,deflectandslowenteringtraffic,
andprovidestoragespaceforpedestrianscrossingtheroadintwostages.
Thecirculatoryroadwayisthecurvedpathusedbyvehiclestotravel
counterclockwisearoundthecentralisland.
Anapronisaraisedsectionofpavementaroundthecentralisland
adjacenttothecirculatoryroadwaythatcanaccommodatethewheel
trackingoflargervehiclesonsmallerroundabouts.
Ayieldlineisapavementmarkingthatdesignatesthepointofentryfrom
anapproachintothecirculatoryroadwayandisgenerallyplacedalongthe
inscribedcircle.Enteringvehiclesmustyieldtoanycirculatingtraffic
comingfromtheleft,beforecrossingthislineintothecirculatoryroadway.
Accessiblepedestriancrossingsshouldbeprovidedatallroundabouts.
Thecrossinglocationissetbackfromtheyieldline,andthesplitterisland
iscuttoallowpedestrians,wheelchairs,strollers,andbicyclestopass
through.
Bicycletreatmentsatroundaboutsprovidebicycliststheoptionof
travellingthroughtheroundabouteitherasavehicleorasapedestrian,
dependingonthebicyclist’slevelofcomfort.
Landscapingbuffersareprovidedatmostroundaboutstoseparate
vehicularandpedestriantrafficandtoencouragepedestrianstocrossonly
atthedesignatedcrossinglocations.Landscapingbufferscanalso
significantlyimprovetheaesthetics.
Dimensions
Sourcefrom:(Rodegerdtsetal.,2010,7)
TableA.2.DimensionsofRoundabouts
Dimension
Inscribedcircle
diameter
Circulatoryroadway
width
Approachwidth
Description
Theinscribedcirclediameteristhebasicparameterusedtodefinethesize
ofaroundabout.Itismeasuredbetweentheouteredgesofthecirculatory
roadway.
Thecirculatoryroadwaywidthdefinestheroadwaywidthforvehicle
circulationaroundthecentralisland.Itismeasuredasthewidthbetween
theouteredgeofthisroadwayandthecentralisland.Itdoesnotinclude
thewidthofanymountableapron,whichisdefinedtobepartofthe
centralisland.
Theapproachwidthisthewidthoftheroadwayusedbyapproaching
trafficupstreamofanychangesinwidthassociatedwiththeroundabout.
Roundabouts and Access Management
Page 149
AppendixAStatePolicies
Dimension
Departurewidth
Entrywidth
Exitwidth
Entryradius
Exitradius
Description
Theapproachwidthistypicallynomorethanhalfofthetotalwidthofthe
roadway.
Thedeparturewidthisthewidthoftheroadwayusedbydepartingtraffic
downstreamofanychangesinwidthassociatedwiththeroundabout.The
departurewidthistypicallylessthanorequaltohalfthetotalwidthofthe
roadway.
Theentrywidthdefinesthewidthoftheentrywhereitmeetsthe
inscribedcircle.Itmeasuresperpendicularlyfromtherightedgeofthe
entrytotheintersectionpointoftheleftedgelineandtheinscribedcircle.
Theexitwidthdefinesthewidthoftheexitwhereitmeetstheinscribed
circle.Itismeasuredperpendicularlyfromtherightedgeoftheexittothe
intersectionpointoftheleftlineandtheinscribedcircle.
Theentryradiusistheminimumradiusofcurvatureoftheoutsidecurbat
theentry.
Theexitradiusistheminimumradiusofcurvatureoftheoutsidecurbat
theexit.
Roundabouts and Access Management
Page 150
AppendixBStatePolicies
AppendixB:StatePolicies
Thissectionsupportsthestates’reviewofroundaboutinformation,accessmanagement,anddriveway
spacingguidancewithadditionaldetailnotincludedinChaptersFourandFive.Thissectionisbroken
downbystate.
TableB.3.StateWebsitesandGuidanceonRoundaboutsandAccessManagement
State
Alabama
Alaska
Arizona
California
Colorado
Connecticut
Delaware
Florida
Georgia
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Roundabout
http://www.alaskaroundabouts.com/index.
html
http://www.azdot.gov/CCPartnerships/Rou
ndabouts/index.asp
http://www.dot.ca.gov/hq/oppd/roundabt/
http://www.coloradodot.info/projects/i70e
dwardsinterchange/area‐roundabout‐
history.html
http://www.ct.gov/dot/cwp/view.asp?a=41
09&q=467780&PM=1
http://deldot.gov/information/community_
programs_and_services/roundabouts/index.
shtml
SearchEngine:roundabout(containsmuch
informationaboutroundabouts)
http://www.dot.ga.gov/travelingingeorgia/t
rafficcontrol/roundabouts/Pages/default.as
px
http://www.iowadot.gov/roundabouts/roun
dabouts.htm
http://www.ksdot.org/burTrafficEng/Round
abouts/roundabout.asp
http://transportation.ky.gov/congestion‐
toolbox/pages/roundabouts.aspx
http://www.dotd.la.gov/administration/pub
lic_info/projects/roundabouts/
Minnesota
http://www.marylandroads.com/Pages/Rou
ndabouts.aspx
http://michigan.gov/mdot/0,1607,7‐151‐
9615_53039‐‐‐,00.html
http://www.dot.state.mn.us/roundabouts/
Mississippi
Missouri
PerLocalDistrict:KansasCity,Northeast,
Michigan
Roundabouts and Access Management
Page 151
AccessManagement
searchengine:accessmanagement
http://www.azaccessmanagement.com/
http://www.dot.state.fl.us/planning/syste
ms/sm/accman/
http://www.dot.ga.gov/doingbusiness/pe
rmits/Pages/AccessManagement.aspx
http://www.in.gov/indot/2512.htm
http://www.iowadot.gov/traffic/access/i
ndex.html
http://www.ksdot.org/accessmanagement
/
SearchEngine:accessmanagement
SearchEngine:accessmanagement
(Brochure)
http://www.maine.gov/mdot/ppp/access
mgmt/index.htm
http://roads.maryland.gov/index.aspx?pa
geid=320&d=95
http://www.michigan.gov/mdot/0,1607,7
‐151‐9621_11041_29705‐‐‐,00.html
http://www.dot.state.mn.us/accessmanag
ement/
SearchEngine:accessmanagement
http://www.modot.org/safety/AccessMan
AppendixBStatePolicies
State
Montana
Nevada
NewJersey
NewYork
Ohio
Oregon
SouthDakota
http://www.oregon.gov/ODOT/hwy/engser
vices/Pages/roundabout_home.aspx
http://www.dot.state.pa.us/Internet/web.ns
f/Secondary?openframeset&frame=main&sr
c=RoundaboutContactInfo?readform
http://www.dot.ri.gov/engineering/trafficde
sign/roundabouts.asp
Vermont
Virginia
http://www.virginiadot.org/info/faq‐
roundabouts.asp
Washington
http://www.wsdot.wa.gov/Safety/roundabo
uts/default.htm
http://www.dot.wisconsin.gov/safety/moto
rist/roaddesign/roundabouts/index.htm
Pennsylvania
RhodeIsland
Wisconsin
Roundabout
Southwest
http://www.mdt.mt.gov/travinfo/roundabo
uts/about.shtml
http://www.nevadadot.com/Traveler_Info/S
afety/Roundabouts.aspx
https://www.dot.ny.gov/main/roundabouts
/background
SearchEngine:roundabout
Roundabouts and Access Management
Page 152
AccessManagement
agement.htm
http://www.mdt.mt.gov/research/toolkit
/m1/pptools/ds/am.shtml
http://www.nevadadot.com/Content.aspx
?id=6274&terms=access%20management
SearchEngine:accessmanagement
http://www.dot.state.oh.us/districts/D01
/PlanningPrograms/trafficstudies/Pages/
Access‐Management.aspx
http://www.oregon.gov/ODOT/HWY/ACC
ESSMGT/Pages/index.aspx
SearchEngine:accessmanagement
http://www.sddot.com/transportation/hi
ghways/management/Default.aspx
http://vtransplanning.vermont.gov/vam
http://www.virginiadot.org/info/access_
management_regulations_and_standards.a
sp
AppendixBStatePolicies
TableB.4.RoundaboutGuidelinesinDrivewayorHighwayManuals
No
Date
State
DocumentTitle
Description
1
2000
Florida
ManualonUniform
TrafficStudies,
Chapter16‐
Roundabouts
2
2007
New
Hampshire
NHDOT
Supplemental
DesignCriteria
3
2009
Iowa
DesignManual
Chapter6
GeometricDesign,
6A‐3Modern
Roundabout
3
2009
Minnesota
MnDOTRoad
DesignManual:
Chapter12,Design
Guidelinesfor
Modern
Roundabouts
4
2011
Maryland
MarylandDesign
Guidelines:Chapter
3C:Roundabout
Markings
5
2011
Washington DesignManual
22.01.08:Chapter
1320‐
Roundabouts
WrittenbyFDOTandpublishedin2000,this16‐pagereportis
thelastchapterintheFDOTManualonUniformTraffic
Studies(MUTS).TheMUTSestablishesminimumstandards
forconductingtraffic‐engineeringstudiesonroadsnearthe
jurisdictionoftheFDOT.Thischapteronroundaboutsjustifies
theiruseintheStateofFlorida,andcomparesthemtothree
otheralternativestointersectioncontrol–trafficsignals,two‐
waystopcontrol(TWSC),andall‐waystopcontrol(AWSC).
Thischaptercitesthe1996FDOTFloridaRoundaboutGuide
forspecificguidelinesonroundaboutlocation,design,and
operation.
WrittenbyNHDOT,the5‐page supplementaldesigncriteria
mentionstheconsiderationsforroundaboutdesign,including
operation(withattachedcapacityworksheet,andRODEL
setting),andgeometricdesign.Designvehiclereceives
additionalattentioninthisdocument.FHWARoundabouts,An
InformationalGuide(Robinsonetal.2000).
WrittenbyIowaDOT,Chapter6oftheGeometricDesign
manualincludesa16‐pagesectiononmodernroundabouts
forIowa.Thechapteroutlineshowroundaboutsareclassified
incomparisonwithothertrafficintersections,keyfeatures
andgeometricelementsofroundabouts,roundabout
operationsanddesign,inadditiontosectionsonroundabout
educationandsafety.Asignificantportionofthechapter
addressesconsiderationsandfeasibilityofroundabout
implementation,takingintoaccountregionalcontext,access
managementissues,andsafetyfactors.
WrittenbyMinnesotaDOT,thisdesignguidelinedocument
showsanenhancementtableoftypicalinscribedcircle
diameterwithdailyservicevolume,intersectioncontrol
evaluationandsiterequirementsections,andspecialdesigns
toaccommodatespecificlanduses.Additionally,this
documentsuggestsRODELandARCADYastoolstoexamine
intersectioncontrolevaluations.
WrittenbytheMarylandStateHighwayAdministration,this
16‐pagechapterincludesdesignguidelinesforpavement
markingsinroundaboutsinMaryland.Itincludesmarkings
forone‐,two‐,andthree‐laneroundabouts,aswellasfor
crosswalk,pedestrian,andbicyclistmarkingsthrough
roundabouts.
WrittenbyWashingtonStateDOT,the50‐pagesectiongives
informationabouttheprocedurestodesignaroundaboutata
specificstatewidelevel.Thisdocumentexplainsmultiple
accesscirculationinsection1320.11includingaccess,parking
andtransitfacilities.Informationaboutaccess:“Noroad
approachconnectionstothecirculatingroadwayareallowed
atroundaboutsunlesstheyaredesignedaslegstothe
roundabout.Itisdesirablethatroadapproachesnotbe
locatedontheapproachordeparturelegswithinthelengthof
thesplitterisland.”(WSDOT,2011,pp.1320‐21).For
driveways,“iftheparceladjoinstwolegsoftheroundabout,it
Roundabouts and Access Management
Page 153
AppendixBStatePolicies
isacceptabletoprovidearight‐in/right‐outdrivewaywithin
thelengthofthesplitterislandsonbothlegs.Thisprovidesfor
allmovements;designbothdrivewaystoaccommodatetheir
designvehicle.”(WSDOT,2011,pp.1320‐21)
6
2010
Kentucky
DesignGuidancefor WrittenbytheDirectoroftheDivisionofHighwayDesignin
Roundabout
theKentuckyDOT,this29‐pagereportgivesspecific
Intersections
explanationsofhowKentuckymayreviewandapprove
roundaboutdesigns.Thedocumentalsoincludesguidesfor
warrantandoperationalanalysis.Thisoperationalanalysis
includestherelationtocapacityaspectintheroundabout.
7
2011
Wisconsin
Facilities
WrittenbytheWisconsinDOTin2011,the79‐pagesection
Development
showsthecompletedesignprocessofaroundaboutandother
Manual,Chapter11, supplementalaspects.Thefirstsupplementisontheguidance
Section26:
ofshared‐usepathsforbicyclists.Inregardtoaccess
Roundabouts
management,thisguidelineconsidersthreeaspectstolocatea
drivewayontheroundaboutentryorexit:volumeof
driveways,operationalimpact,andsightdistancebetween
users.
TableB.5.SpecificManualsonRoundaboutGuidance
No
Date
State
DocumentTitle
1
1996
Florida
Florida
RoundaboutGuide
2
2000
Florida
3
2012
Florida
4
2003
Arizona
5
2003
Kansas
Description
WrittenbytheFDOTandpublishedin1996,the109‐page
reportoutlinesroundaboutdesignandguidanceinFlorida.
ThisdocumentwaspreparedearlierthanFHWA's
Roundabouts,AnInformationalGuide(Robinsonetal.,2000).
ThemainwaythisdiffersfromtheFHWAdocumentisthe
justificationofwhytobuildaroundabout.Another
supplementalaspectistheexplanationaboutSIDRAsoftware
utilization.Inaddition,thisdocumentalsoconsidersother
software,suchasARCADY,andRODEL.Thisdocument
includestheformstodeterminecapacityandotherrequired
documentsforroundaboutjustification.
WrittenbyFDOTandpublishedin2000,thisreportexamines
Bicycleand
Pedestrian
specificconcernsaboutbicyclistsandpedestriansatthe
Considerationsat
roundabouts.Theresultsofthisstudyarethathighbicycle
Roundabouts
crashratesthanthoseoncarandpedestrian,themultilane
roundaboutsprovidealesssafeenvironmentforbicyclists
andpedestriansthanone‐laneroundabouts.
Recommendationsincludebuildinganadditionalbicycle
facilityoutsidetheroundabout(ifspaceisavailable),crossing
provisions,andpropersignage.
Roundabouts,
WrittenbytheDesignTrainingExpoandpublishedin2012,
Florida’s
thisPowerPointpresentationcapturessupplementalaspects
Implementation
fromFHWA'sRoundabouts,AnInformationalGuide(Robinson
Strategy
etal.,2000),especiallyinregardtopedestrians,trucks,and
markinginformation.
WrittenbyLeeEngineeringandKittelson&Associates,the
Roundabouts:An
ArizonaCaseStudy 260‐pagereportisacasestudyofroundaboutsinArizona.
andDesign
(Leeetal.,2003).
Guidelines
Kansas
WrittenbytheKansasDOT,Kittelson&Associates,and
RoundaboutGuide: TransystemCorporationin2003,theKansasRoundabout
ASupplementto
Guideisa176‐pagereportthatshowssupplementalaspects,
FHWA’s
suchasdifferentiatingtrafficcirclesandroundaboutswith
Roundabouts and Access Management
Page 154
AppendixBStatePolicies
Roundabouts
examplesfromKansasroundabouts.Italsospecifiesthe
roundaboutselectionguidance;addingtheroundabout
categoriesonadesigncharacteristictable(whetherurban
andruralroundaboutsaresingleordoublelane),detailingin
designprocess,givingexamplesoffiveprojectsinKansasfor
curbandpavementdesign,detailingthedrawingofsignage
onurban,suburban,multilane,andshowingtheluminance
forintersectionbasedonpavementclassification(the
Portlandcementconcretesurfaceandtypicalasphalt
surface),androadwayclassification.
6
2007
Pennsylvania Guideto
WrittenbyPennsylvaniaDOT,the236‐pagereport
Roundabouts
supplementsthepedestrianaspectofFHWA'sRoundabouts,
AnInformationalGuide(Robinsonetal.,2000),byshowing
detailedrequirementsfordetectablewarningsurfacesand
otherpedestrianfeatures.
WrittenbyHallmarketal.,this32‐pagereportprovidesthe
7
2008
Iowa
Planning‐Level
Guidancefor
IowaDOTwithinformationanddirectiononroundabout
Modern
policies,designguidelines,andpubliceducation.Theproject
Roundabouts
developsaroundabouttaskforce,documentsbestpractices
ofstateswithsuccessfulroundaboutprograms,develops
implementationguidelines,developsdraftroundabout
policies,andassistsinpubliceducationaboutroundabouts.
8
2011
Michigan
Evaluatingthe
WrittenbytheMichiganDOT,thisreportstudiessafety
Performanceand
performanceonroundabouts.Usingthesimplebefore‐after
Safety
andEmpiricalBayesanalysiswithasamplesizeof58
Effectivenessof
roundaboutsinMichigan,thisresearchfindsthat“Singlelane
Roundabouts
has60.55crashesperyearreduction;Doublelane;18.56
crashesperyearreduction;Triplelane;94.76crashesper
yearincrease;andFatal&A‐Level;5.39crashesperyear
reduction”(MDOT,2011,pp.81or7‐1).Thisresearchalso
suggestsadditionalaspectsofroundaboutstobeconsidered
inthenextMichiganStateRoundaboutGuide.Onesuggestion
aboutroundaboutsthathascorrelationtoaccess
managementwouldbeto“considerrestrictingleftturnsinto
andoutofdrivewaysnearroundabouts.Thiswouldreduce
thenumberofconflictpointsandallowvehiclestoutilizethe
roundabouttomakeanindirectleftturn.”(Bagdadeetal.,
2011,pp.86).
WrittenbytheMarylandStateHighwayAdministration,this
9
2012
Maryland
Roundabout
DesignGuidelines 32‐pagereportincludesguidelinesforroundaboutdesignand
operations.
10
2007
California
Roundabout
WrittenbytheCaltrans DivisionofResearchandInnovation,
GeometricDesign
this113‐pagedocumentincludesthreemaintopics:
Guidance
operation,roundaboutfordifferentusers,andgeometric
design.
Roundabouts and Access Management
Page 155
AppendixBStatePolicies
TableB.6.StateGuidanceonAccessManagementManuals
No.
States
NameofDocuments
Year
RetrievedFrom
Pages
1
Alabama
AccessManagementManual
January,
2013
http://www.dot.state.al.us/maweb/d
oc/ALDOT%20Access%20Manageme
nt%20Manual.pdf
65
2
Arizona
RoadwayDesignGuidelines
May,2012
http://www.azdot.gov/highways/Roa
dway_Engineering/Roadway_Design/
Guidelines/Manuals/PDF/RoadwayDe
signGuidelines.pdf
412
3
California
HighwayDesignManual
May7,2012
(web)
4
Colorado
StateHighwayAccessCode
1998
(revised
March
2002)
http://www.dot.ca.gov/hq/oppd/hd
m/hdmtoc.htm
http://www.coloradodot.info/busines
s/permits/accesspermits/references/
601_1_accesscode_march2002_.pdf/vi
ew
5
Connecticut
HighwayDesignManual
http://www.ct.gov/dot/lib/dot/docu
ments/dpublications/highway/cover.
zip
630
6
Delaware
StandardsandRegulations
forSubdivisionStreetsand
StateHighwayAccess
2003
(revised
February.
2013)
2011
http://regulations.delaware.gov/regis
ter/june2011/proposed/14%20DE%
20Reg%201323%2006‐01‐11.pdf
136
7
Districtof
Columbia
(Washington,
DC)
ThePolicyandprocessfor
AccesstotheDistrictof
ColombiaInterstateand
FreewaySystem
2010
http://ddot.dc.gov/DC/DDOT/Project
s+and+Planning/Standards+and+Guid
elines/Interstate+and+Freeway+Acce
ss+Process/Policy+and+Process+for+
Access+to+the+DC+Interstate+and+Fr
eeway+System
(web)
8
Florida
9
Georgia
StateHighwaySystem
2009
AccessManagement
RegulationforDrivewayand 2009
EncroachmentControl
https://www.flrules.org/gateway/Cha
pterHome.asp?Chapter=14‐97
http://www.dot.ga.gov/doingbusiness
/PoliciesManuals/roads/Encroachme
nt/DrivewayFull.pdf
10
Idaho
AccessManagement:
StandardsandProcedures
forHighwayRight‐of‐Way
Encroachments
April,2001
http://itd.idaho.gov/highways/ops/T
raffic/PUBLIC%20FOLDER/Access/Id
aho%20AM%20Standards%20and%2
0Procedures.pdf
11
Illinois
Chapter35‐Access
September
Control/AccessManagement 2010
http://dot.state.il.us/desenv/BDE%20
Manual/BDE/pdf/Chapter%2035%20
Access%20Control‐
Access%20Management.pdf
12
Indiana
AccessManagementGuide
2009
13
Iowa
IowaPrimaryHighway
AccessManagementPolicy
2012
Roundabouts and Access Management
Page 156
http://www.in.gov/indot/files/guide_
total.pdf
http://www.iowadot.gov/traffic/pdfs
/AccessPolicy.pdf
70
(web)
101
93
52
178
47
AppendixBStatePolicies
No.
States
NameofDocuments
Year
RetrievedFrom
Pages
14
Kansas
AccessManagementPolicy
January,
2013
http://www.ksdot.org/accessmanage
ment/Access_Management_Policy_Jan
2013.pdf
300
15
Louisiana
AccessConnectionPolicy
November,
2012
http://www.dotd.la.gov/highways/m
aintenance/maintmgt/documents/AC
_Policy_Manual.pdf
81
16
Maine
AccessManagementRules
Maryland
18
Massachusetts
StateHighwayAccess
Manual
HighwayDesignChapter15
AccessManagement
http://www.maine.gov/mdot/ppp/ac
cessmgmt/amrules.htm
http://roads.maryland.gov/ohd/acces
spermits.pdf
http://www.massdot.state.ma.us/Port
als/8/docs/designGuide/CH_15_a.pdf
(web)
17
March18,
2005
2004
19
Michigan
164
20
http://www.accessmanagement.info/
pdf/GuidebookMI.pdf
http://www.dot.state.mn.us/accessm
anagement/resources.html
http://sp.mdot.ms.gov/RoadwayDesig
n/Documents/MISSISSIPPI%20Access
%20Management%20Guide_v2_Feb20
12.pdf
http://www.modot.org/newsandinfo/
documents/AccessMgmtGuidelines_10
03.pdf
2006
Minnesota
AccessManagement
Guidebook
AccessManagementManual
October1,
2001
2008
21
Mississippi
AccessManagementManual
2012
22
Missouri
AccessManagement
Guidelines
2003
23
Montana
March,2007
24
Nebraska
25
Nevada
Chapter8‐Access
Management
AccessControlPolicytothe
StateHighwaySystem
AccessManagementSystem
andStandards
26
New
Hampshire
DrivewayPermit
27
NewJersey
28
NewMexico
StateHighwayAccess
ManagementCode
StateHighwayAccess
ManagementRequirements
October15,
2001
29
NewYork
HighwayDesignManual
Chapter6‐Interchanges;
July16,
2002
Roundabouts and Access Management
Page 157
232
25
(web)
36
51
http://www.mdt.mt.gov/other/rw/ex
ternal/manual/chapter_8.pdf
http://www.transportation.nebraska.
gov/roway/pdfs/accesscontrol.pdf
http://www.nevadadot.com/uploade
dFiles/TrafEng_AccesMgtSysStandard
s.pdf
21
March10,
2000
http://www.nh.gov/dot/org/operatio
ns/highwaymaintenance/documents/
DrivewayPolicy.pdf
43
2013
http://www.state.nj.us/transportatio
n/business/accessmgt/NJHAMC/
http://dot.state.nm.us/content/dam/
nmdot/Infrastructure/Access_manage
ment_Manual.pdf
89
March1,
2006
1999
https://www.dot.ny.gov/divisions/en
gineering/design/dqab/hdm/hdm‐
repository/chapt_06_new_07162002.
pdf
24
38
197
18
AppendixBStatePolicies
No.
States
NameofDocuments
Year
RetrievedFrom
Pages
30
NorthCarolina
PolicyonStreetand
DrivewayAccesstoNorth
CarolinaHighways
July,2003
https://connect.ncdot.gov/resources/
safety/Congestion%20Mngmt%20and
%20Signing/Congestion%20Manage
ment/Policy%20on%20Street%20an
d%20Driveway%20Access%20to%20
North%20Carolina%20Highways%20
Current%20Edition%20July%202003.
pdf
http://www.dot.nd.gov/manuals/desi
gn/designmanual/DM‐TOC‐
Master_tag.pdf
90
31
NorthDakota
DesignManual‐Driveways
andAccessManagement
July8,2009
32
Ohio
StateHighwayAccess
ManagementManual
2001
http://www.dot.state.oh.us/Divisions
/Engineering/Roadway/AccessManag
ement/Documents/State%20Highway
%20Access%20Management%20Man
ual%20March%202008.pdf
66
33
Oregon
34
SouthCarolina
HighwayApproach
Permitting,AccessControl,
andAccessManagement
Standards
ARMS—Accessand
RoadsideManagement
Standards
June29,
2012
http://www.oregon.gov/ODOT/HWY/
ACCESSMGT/docs/pdf/734‐
051_Perm_Rule.pdf
91
2008(latest
revisionon
Sept26,
2012)
http://www.scdot.org/doing/technica
lpdfs/publicationsmanuals/trafficengi
neering/arms_2008.pdf
130
35
SouthDakota
Chapter17—Access
Management
http://sddot.com/business/design/do
cs/rd/rdmch17.pdf
22
36
Texas
AccessManagementManual
July,2011
http://onlinemanuals.txdot.gov/txdot
manuals/acm/acm.pdf
http://www.udot.utah.gov/main/uco
nowner.gf?n=6599114996078154
46
37
Utah
January,
2006
38
Vermont
AccommodationofUtilities
andtheControland
ProtectionofStateHighway
RightofWay
AccessManagement
ProgramGuidelines
39
Virginia
AccessManagementDesign
StandardsforPrincipal
Arterial/MinorArterials,
Collectors,andlocalstreets/
EntrancesandIntersection
40
Washington
41
WestVirginia
3
100
July1,1999
(Last
Revision:
July22,
2005)
2012/2012
/2007
http://vtransengineering.vermont.gov
/sites/aot_program_development/file
s/documents/rightofway/UandPAccM
anProgGuidelinesRev072205.pdf
33
http://www.virginiadot.org/info/acce
ss_management_regulations_and_stan
dards.asp
18/
19/
116
AccessControl
June,2009
http://www.wsdot.wa.gov/publicatio
ns/manuals/fulltext/m22‐01/520.pdf
8
ManualonRulesand
RegulationsforConstructing
DrivewaysonStateHighway
Rightsofway
May,2004
http://www.transportation.wv.gov/hi
ghways/traffic/Documents/Driveway
Manual.pdf
94
Roundabouts and Access Management
Page 158
AppendixBStatePolicies
No.
States
42
Wisconsin
43
Wyoming
NameofDocuments
AccessControl—Facilities
DevelopmentManual
RulesandRegulations and
policyforAccessesto
WyomingStateHighways
Roundabouts and Access Management
Page 159
Year
RetrievedFrom
Pages
June19,
2013
March,2005
http://roadwaystandards.dot.wi.gov/
standards/fdm/07‐00toc.pdf
http://www.dot.state.wy.us/files/live
/sites/wydot/files/shared/Traffic/W
YDOT%20Access%20Manual.pdf
(web)
48
AppendixBStatePolicies
TableB.7.OtherDocumentsRelatedtoAccessManagement
No.
1
States
Idaho
NameofDocuments
AccessManagementToolkit
2
Oregon
3
RetrievedFrom
http://www.compassidaho.org/docu
ments/planning/studies/AcMgtTlkt_
08Cover_Electronic.pdf
Pages
94
AccessManagementManual Various
(web‐based)
(1996to
2004)
http://www.oregon.gov/ODOT/HW
Y/ACCESSMGT/Pages/accessmanag
ementmanual.aspx
(web)
Michigan
MichiganAccess
ManagementProgram
Evaluation
May,2010
http://www.michigan.gov/documen
ts/mdot/Final_MDOT_Access_Manag
ement_Evaluation_Report_by_TTI_M
ay_2010_324062_7.pdf
112
4
NewYork
January7,
2002
https://www.dot.ny.gov/divisions/e
ngineering/design/dqab/dqab‐
repository/pdmapp8.pdf
19
5
Kentucky
ProjectDevelopment
ManualAppendix8:
InterstateandOther
FreewayAccessControland
Modification
AccessManagementfor
Kentucky(Stamatiadiset
al.,2004)
February,
2004
http://transportation.ky.gov/Conges
tion‐
Toolbox/Documents/KTC%20Acces
s%20Management%20Report.pdf
170
6
AccessManagement
May,2008
Implementationin
KentuckyTechnicalSupport
DocumentandStatus
Report
http://transportation.ky.gov/Conges
tion‐
Toolbox/Documents/Access%20Ma
nagement%20Implementation%20R
eport%202008.pdf
111
7
Utah
May,2006
http://www.udot.utah.gov/main/uc
onowner.gf?n=7861430698992951
150
8
South
Carolina
South
Dakota
AssessingtheSafety
BenefitsofAccess
ManagementTechniques
SouthCarolinaStrategic
CorridorSystemPlan
ReviewofSDDPT's
HighwayAccessControl
Process
http://www.scdot.org/inside/pdfs/
planning/strategiccorridorplan.pdf
http://sddot.com/business/research
/projects/docs/SD1999_01_Final_Re
port.pdf
126
RightofWayManual
March,2013
http://www.wsdot.wa.gov/publicati
ons/manuals/fulltext/M26‐01/M26‐
01.10Revision.pdf
62
9
10
Washington
Year
August18,
2008
February,
2000
214
Date
State
DocumentTitle
Description
2006
Florida
MedianHandbook
2008
Florida
DrivewayInformation
Guide
TheFDOTMedianHandbookisan81‐pagereportthat
borrowed“heavily”fromtheAccessManagementManual,
publishedbytheTransportationResearchBoard;aswellas
TransportationandLandDevelopment(VergilStover)published
byITE.Whilethehandbookaddressesseveraldesign
considerationsrelatedtoroundabouts,itdoesnotexplicitly
detailanythingaboutroundaboutdesignoraccessmanagement.
TheFDOTDrivewayInformationGuideisa94‐pagereportthat
addressesseveraldesignguidelinesfordrivewaydesignin
Florida,suchassightdistanceatdriveways,drivewaylocation,
andpedestrianfactors,butdoesnotmakeanyreferenceto
roundaboutsatall.
Roundabouts and Access Management
Page 160
AppendixCAccessManagementTechniquesinStateGuidelines
AppendixC:AccessManagementTechniquesinStateGuidelines
TableC.8.SpacingRequirements
(Source:GluckandLorenz,2010,pp.47)
Roundabouts and Access Management
Page 161
AppendixCAccessManagementTechniquesinStateGuidelines
TableC.9.AccessManagementElementsontheStates(GluckandLorenz,2010,page48)
Roundabouts and Access Management
Page 162
AppendixCAccessManagementTechniquesinStateGuidelines
TableC.10.AccessManagementTechniquesappliedbytheStateDOTs(GluckandLorenz,2010,pages49‐
50)
Roundabouts and Access Management
Page 163
AppendixCAccessManagementTechniquesinStateGuidelines
Roundabouts and Access Management
Page 164
AppendixDSiteSelection
AppendixD:SiteSelection
ThefigurebelowshowsthedatacollectionoftheClearwaterroundabout,whichhasbeendebatedfor
years.Fourcameraswereplacedonfouroutofthesixlegsofthisroundabouttorecordtrafficinteraction
betweendrivewaysandapproachinglanes.Thisroundaboutislocatedclosetoatouristattractionarea;
thereforetrafficwassignificantatthetimeofdatacollection.
Pinellas
Causeway Blvd and Mandalay Ave
Problems: Huge traffic, lots of spill backs into circulating lanes.
3/22: 3pm-5:30pm
FigureD.1.CameraLocationofRoundaboutatCausewayBlvdandMandalayAve
Roundabouts and Access Management
Page 165
AppendixDSiteSelection
ThefigurebelowshowsaroundaboutinGainesville,Florida.Thissiteisanidealintersectionfor
researcherstoobserveconflictbetweentrafficonapproaching/exitlanesanddrivewayssincethedistance
betweendrivewaysandtheroundaboutisveryclose.
Alachua
SW 2nd Ave and SW 6th St.
Problems: Driveway is too close to the roundabout
4/5: 3pm-5:30pm
FigureD.2.CameraLocationofRoundaboutatSW2ndAveandSW6thSt.
Roundabouts and Access Management
Page 166
AppendixDSiteSelection
ThefigurebelowshowsaroundaboutsiteinOsceolaCounty.Althoughthereisadrivewayclosetothe
roundabout,wedidn’tobservesignificantconflictatthesite.
Osceola
Problems: NA
MLK Blvd. and N. Central Ave.
4/5: 11am-12pm
FigureD.3.CameraLocationofRoundaboutatMLKBlvd.andN.CentralAve.
Roundabouts and Access Management
Page 167
AppendixDSiteSelection
ThefollowingfigureshowsaroundaboutinOrangeCounty,Florida.Thissiteisclosetoashoppingmallso
wepickedaweekendtoconductdatacollection.
Orange
Eagle’s Reserve Blvd and Dyer Blvd
Problems: Design is abnormal
4/14: 12pm-1pm
FigureD.4.CameraLocationofRoundaboutatEagle’sReserveBlvdandDyerBlvd
Roundabouts and Access Management
Page 168
AppendixDSiteSelection
ThefollowingfigureshowsaroundaboutsiteinJacksonville,Florida.ItislocatedintheCBDareaanda
businesscenterwassituatedrightnexttotheroundabout.Trafficattractedandgeneratedbythebusiness
centercausedsignificantimpactonroundaboutoperation.
Duval
Independent Dr. and S. Laura St.
Problems: Huge pedestrian flow, business center right next to roundabout.
4/23: 11am-2pm
FigureD.5.CameraLocationofRoundaboutatIndependentDr.andS.LauraSt.
Roundabouts and Access Management
Page 169
AppendixDSiteSelection
ThefigurebelowshowsaroundaboutinSt.Lucie,Florida.Althoughseveraldrivewaysarelocatednearthe
roundabout,wedidn’tobservedmanyconflictsatthissite.
St. Lucie
CR-707 and Ave A
Problems: Driveway too close to roundabout
5/9: 1pm-3pm
FigureD.6.CameraLocationofRoundaboutatCR‐707andAveA
Roundabouts and Access Management
Page 170
AppendixDSiteSelection
ThefigurebelowshowsaroundaboutinSt.Johns,Florida.Ashoppingcenterwaslocatednearthe
roundabout.
St. Johns
Problems: NA
CR-210 and Mickler Rd.
5/9: 1pm-3pm
FigureD.7.CameraLocationofRoundaboutatCR‐210andMicklerRd.
Roundabouts and Access Management
Page 171
AppendixDSiteSelection
ThenextsiteisaroundaboutinHomestead,Florida.Aswecanseefromthefigurebelow,thereisanAWSC
intersectionnorthoftheroundabout.
Miami-Dade
NE 10th Ct. & SW 152nd Ave.
5/13: 5pm-7:20pm
Problems: You cannot see queue in the driveway from camera 2 due to the high hedges along the roadway.
FigureD.8.CameraLocationofRoundaboutatNE10thCt.andSW152ndAve.
Roundabouts and Access Management
Page 172
AppendixDSiteSelection
ThenexttworoundaboutsformaseriesofroundaboutsinMiami,Florida.Oneofthefeaturesofthesetwo
roundaboutsisonstreetparkingisevidentinthesesites.
Miami-Dade
Greenway Dr. and Sagovia St.
Problems: Long queue build-up on Coral Way westbound on easternmost leg.
5/14: 4:50pm-7:10pm
FigureD.9.CameraLocationofRoundaboutatGreenwayDr.andSegoviaSt.&CoralWay
Roundabouts and Access Management
Page 173
AppendixDSiteSelection
Miami-Dade
Biltmore Way and Sagonia St.
5/15: 4:50pm-7:15pm
Problems: Easternmost leg had heavy traffic traveling east with some spill back into the roundabout.
FigureD.10.CameraLocationofRoundaboutatBiltmoreWayandSagoviaSt.
Roundabouts and Access Management
Page 174
AppendixDSiteSelection
ThemapbelowshowsaroundaboutinBrowardCounty,Florida.Thisareaismostlyresidentialwithsome
factionofmixed‐usedparcel.
Broward
Problems: NA
Holmberg Rd. & Parkside Dr.
5/16: 3:25pm-5:30pm
FigureD.11.CameraLocationofRoundaboutatHolmbergRd.andParksideDr.
Roundabouts and Access Management
Page 175
AppendixDSiteSelection
ThemapbelowshowsaroundaboutsiteinMiami,Florida.Thissiteisinterestingbecausethedesignis
abnormalcomparedtootherroundaboutsinourlist,andyettheaccessissuestillpredominatesatthissite.
Miami-Dade
Ponce De Leon Blvd and Ruiz Ave
5/21: 4:50pm-7:05pm
Problems: For camera 3 we could not place the camera in the median due to the median being covered with bushes
and trees. We had to place it across the street. There is some difficulty seeing the access point because of the cars
crossing through our line of site.
FigureD.12.CameraLocationofRoundaboutatPonceDeLeonBlvd.andRuizAve.
Roundabouts and Access Management
Page 176
AppendixDSiteSelection
ThemapbelowshowsaroundaboutinBrowardCounty,Florida.Thissitewasonthetopofourlistsincea
lotofaccesspointswerefoundateachlane.
Broward
Problems: NA
Margate Blvd and NW 58th Ave
5/23: 7:40am-9:40am
FigureD.13.CameraLocationofRoundaboutatMargateBlvd.andNW58thSt.
Roundabouts and Access Management
Page 177