bend roundabout evaluation and design guidelines
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City of Bend
Roundabout Evaluation andDesign Guidelines
April 2010
Prepared For:The City of Bend
Prepared By:Kittelson & Associates, Inc.
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Kittelson & Associates, Inc. 1
Introduction
Inthe2001TransportationImplementationPlanfortheCityofBendTransportationSystemPlan,theBend City Council adopted a policy stating that roundabouts are the preferred option for
intersection improvements. This policy formalized the Citys practice of giving preference to
roundaboutsas
asafe
and
efficient
form
of
intersection
control.
The
evolution
of
roundabout
implementation inBendhasresulted indesignswithvaryingdetailsandattributes.Furthermore,
theanalysistoolsusedtoevaluatetheoperationalcharacteristicsofroundaboutshaveoftenvaried.
Toachieveconsistency inroundaboutevaluationanddesign, theCityofBendhasdeveloped the
followingguidelines.TheseguidelinesrepresenttheCityspreferredmethodologiesforroundabout
evaluationanddesign.
Roundabout Operational Analysis Guidelines
The
Cityof
Bend
Roundabout
Operational
Analysis
Guidelines
establish
a
consistent
methodology
for
analyzing the operational characteristics of roundabouts within the Citys jurisdiction. These
guidelinesapplytoanalysesconductedfordevelopmentapplicationsortosupporttheevaluation
and selection of Citysponsored capital improvement projects. Methodologies for calculating
capacity,delay,andqueuesarepresentedforsinglelaneandmultilaneroundabouts.
Roundabout Design Consistency Guidelines
Thepurposeof theCityofBendRoundaboutDesignConsistencyGuidelines is topromoteconsistentroundaboutdesignwithintheCityby:
reinforcingthedesignguidelinescontainedintheFHWAsRoundabouts:AnInformationalGuide(FHWAGuide,Reference1)andsubsequentnationalresearch,and
supplementingtheFHWAGuidebydocumentingcriteriaandconsiderationsforcertaindesignelementsaccordingtospecificobjectivesoftheCity.
Inadditiontoprovidinggeneraldesignguidance,theseguidelinespresentspecificdesignelements
theCityofBendexpectsinroundaboutdesignswithintheCitysjurisdiction.
Intersection Form Evaluation Framework
WhiletheCityofBendhasadoptedaroundaboutsfirstpolicy,theCityalsorecognizesthatsite
specificconditionsorotherfactorsmayultimatelynecessitateotherintersectionforms.TheCityofBend Intersection Form Evaluation Framework provides a framework and criteria for comparingroundaboutsandother intersection forms. Acasestudy thatapplies the intersectioncomparison
processisincluded.
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City of Bend
Roundabout OperationalAnalysis Guidelines
April 2010
Prepared For:The City of Bend
Prepared By:Kittelson & Associates, Inc.
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Introduction
The intentof theseoperationalguidelines is to establisha consistentmethodology foranalyzing
roundabouts within the Citys jurisdiction. These guidelines apply to analyses conducted for
development applications or to support the evaluation and selection of Citysponsored capital
improvementprojects.
MuchoftheguidanceprovidedwithinthisdocumentisbasedonNCHRPReport572,Roundabouts
intheUnitedStates(Reference1),whichsummarizesacomprehensivereviewofU.S.roundabouts.
The operational findings and recommendations fromNCHRPReport 572 form thebasis of the
proceduresthatareanticipatedtobeincludedinthe2010updateoftheHighwayCapacityManual
(2010HCM).Oncethe2010HCMisreleased,theseguidelinescouldberevisitedandmodifiedtobe
consistent with national practice. However, the local calibration factors presented in these
guidelineswillstillbeapplicabletotheanticipatedHCMcapacitymodels.
Measures of Effectiveness
VOLUME-TO-CAPACITY RATIO
Volumetocapacity(V/C)ratiosaretheprimarymeasureofeffectivenessforevaluationagainstthe
Citysperformancestandards.V/Cratiosforroundaboutsshouldbecalculatedbasedontheentry
demandandcapacity for themostcriticalapproach (i.e.approachwith thehighestv/c ratio) for
singlelaneroundaboutsandthemostcriticallane(i.e.individuallanewiththehighestv/cratio)for
multilaneroundabouts.
QUEUING
Queuing estimates shouldbe includedwith allnearterm roundaboutoperational analyses (e.g.,
development applications, capital improvement projects).Depending on sitespecific conditions
andat theCitysdiscretion,queuinganalysesmayberequired for longtermoperationalanalysis
(e.g., transportation system plan, transportation planning rule (TPR)). Queues between
roundaboutsandadjacent intersectionsand/ordrivewayshave thepotential to impact the safety
andefficiencyoftheroadwayandintersectionelementsawayfromtheintersectionbeinganalyzed.
DELAY
While the Citys operational performance standard for roundabouts ismeasured against aV/C
ratio,toensureabalancedcomparisonofalternativeintersectionforms,delayestimatesshouldbe
developedwhen comparing alternative intersection forms to the roundabout.As a general rule,
under the same traffic conditions, roundabouts typicallywill result in lower overalldelay than
trafficsignalsandallwaystopcontrolbutmayresult inhigheroveralldelaysthantwowaystop
control.Delay estimates can alsobeused to estimatevehicle emissions that result fromvarious
formsofintersectioncontrol.
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LEVEL OF SERVICE
Level of service shouldbe definedby the delay values presented in Table 1. These values are
consistentwithNCHRPReport572andarethesameasthedelaythresholdsforotherunsignalized
intersections,asdefinedinthe2000HCM(Reference2)andtheproposed2010HCM.
Table 1 Level of Service Thresholds for RoundaboutsLevel of Service Average Control Delay (s/veh)
A 0 - 10
B > 10 - 15
C > 15 - 25
D > 25 - 35
E > 35 - 50
F > 50
Roundabout Operational Analysis Models
ThefollowingmodelsaretheCityspreferredroundaboutoperationalanalysismodelsforcapacity,
delay,andqueuing.Themodelsdescribed in this sectionarecalibrated toeither localconditions
(singlelaneroundabouts)orgeneralU.S.conditions(multilaneroundabouts).
CAPACITY
TheCitys
base
entry
capacity
model
is
consistent
with
NCHRP
Report
572
and
is
shown
in
Equation1.
cpce=Aexp(Bvc,pce) (Equation1)
where,
cpce=lanecapacity,adjustedforheavyvehicles,(pc/h)
A=3600/tf
B=(tc tf/2)/3600
tc=criticalheadway(s)
tf=followupheadway(s)
vc,pce=conflictingcirculatingflowrate,adjustedforheavyvehicles,(pc/h)
If project specific values for critical headway (tc) and followup headway (tf) are identified, this
generalizedmodel shouldbe used to develop a project specific capacitymodel. If the analyst
intendstocollectprojectspecificvaluesfortcandtf,Citystaffshouldfirstbeconsultedtoensurean
appropriatedatacollectionmethodology.
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B e n d Ca l ib r a t e d Ca p a c i t y M o d e l f o r S i n g l e - La n e R o u n d a b o u t s
ThecapacitymodelshowninEquation2shouldbeusedforallsinglelaneentrycapacityanalyses,
unless project specific values for critical headway (tc) and followup headway (tf) have been
developed.
cpce=1333exp(0.0008 vc,pce) (Equation2)where,
cpce=capacity,adjustedforheavyvehicles,(pc/h)
vc,pce=conflictingcirculatingflowrate,adjustedforheavyvehicles,(pc/h)
Equation 2 isbased onBendspecific values for critical headway (4.1s) and followup headway
(2.7s)observedatsinglelaneroundaboutsin2009.
Ca p a c i t y M o d e l f o r M u l t i l a n e R o u n d a b o u t s
Recognizingthe
need
to
provide
avariety
of
lane
configurations
to
accommodate
arange
of
traffic
patterns and the lane use imbalances that may result, multilane capacity analysis should be
conductedonalanebylanebasisandreportedforthemostcriticallane(i.e.lanewiththehighest
volume)oneachapproach.
For entry lanes conflictedby one circulating lane, the singlelane capacitymodel presented in
Equation2shouldbeappliedtothemostcriticallaneoneachapproach.
Forentrylanesconflictedbytwocirculatinglanes,Equation3shouldbeappliedtothemostcritical
laneoneachapproach.
cpce=1130exp(0.0007 vc,pce) (Equation3)
where,
cpce=capacity,adjustedforheavyvehicles,(pc/h)
vc,pce=conflictingcirculatingflowrate,adjustedforheavyvehicles,(pc/h)
Equation 3 reflects themultilane capacitymodel identified inNCHRP Report 572. Given that
singlelane roundabouts in Bend perform at a higher capacity than the national average, it is
possiblethatthecapacitiesofmultilaneroundaboutsinBendwillalsoexceedthenationalaverage.
However,atthetimetheseguidelineswereprepared,nomultilaneroundaboutsinBendoperated
nearor
at
capacity
to
enable
accurate
measurements.
Care
is
recommended
when
assessing
multilaneroundaboutsthatareprojectedtooperatenearcapacity.
(Note: If conditionspermitfuturedata collection atmultilane roundabouts inBend,Equation3 could be
calibratedtolocalconditions.)
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DELAY AND LOS
Equation4providesadelayestimationmodeltobeusedindeterminingdelayforeachapproachor
critical lane.Thismodel isbased on theHCMunsignalizeddelaymodel and is consistentwith
recommendationsfromNCHRPReport572.Thedelayestimatesresultingfromthismodelshould
beused
to
determine
LOS
according
to
the
thresholds
identified
in
Table
1.
(Equation4)
where,
D=averagecontroldelay(s/veh)
x=volumetocapacityratioofthesubjectlane
c=capacityofthesubjectlane(veh/h)
T=timeperiod(h)=0.25fora15minuteanalysis
QUEUING
Queue lengths shouldbe estimated usingEquation 5 for each singlelane approach and for the
critical laneon eachmultilaneapproach. As shown,Equation5will result in the 95thpercentile
queue likely to occur during the peak fifteenminutes of the hourbeing analyzed. If anhourly
queueevaluationisdesired,theflowrateshouldnotbeadjustedbythePHFandTwillequal1.0.
(Equation5)
where,
Q95=queuelength(veh)
x=volumetocapacityratioofthesubjectlane
c
=
capacity
of
the
subject
lane
(veh/h)
T=timeperiod(h)=0.25fora15minuteanalysis
( )3600150
3600
119002
95
c
T
xc
xxTQ
++=
( ) ]1,min[5450
3600
119003600 2
xT
xc
xxTc
D +
+++=
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Roundabout Analysis Process
Thefollowingdiagramoutlinestheanalysisprocesstobeusedforanalyzingroundaboutswithin
theCityofBend.
Step1:Convertmovementdemandvolumes(V,veh/h)toflowrates(v,veh/h).Peak15minuteanalysis:v=V/PHF Hourlyanalysis: v=V
Step2:Adjustflowratesforheavyvehicles.vpce=v/fHV, fHV=1/[1+PT(ET 1)], PT=HeavyVehicle%, ET=2.0forHeavyVehicles
Step3:Determinecirculatingandexitingflowrates.
Step4:Determineentryflowratesbylane.
Step5:Determinethecapacityofeachentrylaneinpassengercarequivalents.(seeEquations2and3)
Step6:Determinepedestrianimpedance(fped)tovehicles.(see
FHWA
Roundabout
Guide(Reference
3)
Exhibits
47and
48)
Step7:Convertlaneflowratesandcapacityintovehiclesperhour.v=vpcefHV c=cpcefHVfped
Step8:Computethevolumetocapacityratioforeachlane.x=v/c
Step9:ComputetheaveragecontroldelayandcorrespondingLOSforeachlane.(seeEquation4)
Step10:Compute95thpercentilequeuesforeachlane.(seeEquation5)
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Comparison with Alternative Intersection Forms
Roundabouts are often evaluated against a signalized intersection alternative. Phase splits at a
signalizedintersectioncanbeadjustedtomeetvaryingdemand.Therefore,analysisforsignalized
intersections is conducted and reported for the intersection as a whole. When comparing a
roundaboutalternative
to
asignalized
intersection
or
all
way
stop
control
alternative,
Equation
6
shouldbeusedtodevelopaweightedaverageofthedelayestimatefortheroundaboutintersection
asawhole.Allmovements,includingthoseusingbypasslanes,shouldbeincludedintheweighted
average.
(Equation6)where,
Dintersection=intersectioncontroldelay,s/veh
Di=controldelayonapproachi,s/veh
Vi=volumeonapproachi,veh/hr
Additional guidance on conducting comparative analyses for different intersection forms is
providedintheCityofBendIntersectionFormEvaluationFramework(Reference3).
Alternative Tools for Operational Analysis
At the discretion of City staff, additional analysis tools, such as other deterministic tools or
microsimulation,maybe required to augment themethodologiesdescribed in thisdocument. If
alternativeoperational
analysis
tools
will
be
used
on
aproject,
the
analyst
should
consult
with
City
staffearlyintheprocesstoensureappropriatecalibrationoftheanalysismethod.
=
i
i
i
ii
V
VD
D onintersecti
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References
1. Rodegerdts, L.,M. Blogg, E.Wemple, E.Myers,M.Kyte,M.Dixon,G. List,A. Flannery,R.
Troutbeck,W.Brilon,N.Wu,B.Persaud,C.Lyon,D.Harkey,andD.Carter.NCHRPReport572:
Roundabouts
in
the
United
States,
TRB,
National
Academy
of
Sciences,
Washington,
D.C.,
2007.
[online]http://www.trb.org/Main/Public/Blurbs/158299.aspx
2. Transportation Research Board. Highway Capacity Manual. TRB, National Research Council,
Washington,DC,2000.
3.Kittelson&Associates, Inc.City ofBend Intersection FormEvaluation Framework.City ofBend,
November2009.
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City of Bend
Roundabout DesignConsistency Guidelines
April 2010
Prepared For:The City of Bend
Prepared By:
Kittelson & Associates, Inc.
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Introduction
TheCityofBendplansand implements transportation facilities thatserveallvehicularandnon
vehicular users.As conventional intersections havebeen applied extensively and their design,
operations, and use are generallywell understood and documented, this document focuses on
roundabouts.This
design
guide
has
been
prepared
to
promote
consistent
roundabout
design
within
theCity.Theintentofthisguideisto:
reinforcethedesignguidelinescontainedintheFHWAsRoundabouts:AnInformationalGuide
(FHWAGuide,Reference1)andsubsequentnationalresearch,and
supplementtheFHWAGuidebydocumentingcriteriaandconsiderationsforcertaindesign
elementsaccordingtospecificobjectivesoftheCity.
Roundabout design is an iterative process, and the roundabouts that the City of Bend has
implementedovertheyearsexhibitavarietyoftraitsandfeatures.BasedontheCitysexperience
with
roundabouts,
this
guide
provides
information
and
approaches
to
roundabout
implementation
topromote consistency anduniformity for future roundabouts.TheCity recognizes roundabout
designsmustadapttothecontextoftheirpotentialapplicationand,therefore,iswillingtoexplore
andconsidertradeoffsandpossibledeviationsthatallowtheCitytheabilitytocontinuetopromote
roundabout implementation. There are no explicit limitations or conditions under which
roundaboutsmaybeconsidered.WhiletheCityhasadoptedaroundaboutsfirstpolicy,theCity
also recognizes that sitespecific conditions or other factors may ultimately necessitate other
intersectionforms.Guidanceonconductingcomparativeanalysesfordifferentintersectionformsis
providedintheCityofBendIntersectionFormEvaluationFramework(Reference2).
Intersection ContextIntersectioncontextincludesfundamentalconsiderationssuchaswhethertheintersectionispartof
anew facility or a retrofit situation.New locations provide increased flexibility in locating and
designingtheintersectionforms,whileretrofitconditionsoftenexhibitconstrainedrightofwayor
secondaryconsiderations, suchasaccess toadjacent landuses. Intersection contextalso includes
understandingusertypesandappropriatelyservingnonmotorizedusersanddesignvehicleneeds.
Designvehiclechoiceshaveasignificanteffectonroundaboutdimensionsandconfigurations.
Thecontextofthetransportationsystemandlandusescenarioinwhicharoundaboutissituated
will often dictate fundamental design decisions. For instance, the roundabout at the Simpson
Avenue/CenturyDrive
intersection
was
constructed
in
abuilt
environment
with
limited
available
rightofway.Thesurrounding landuseshavethepotential togeneratesignificantpedestrianand
bicyclistactivitythroughtheintersection.Thesefactorsappeartomaketheselectionofacompact
urbandesign,asshowninExhibit1,appropriate.
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Exhibit 1 Compact urban design Simpson Avenue/Century Drive
However, if the greater context of this intersection is considered, a designwithmore generous
geometryandhighercapacitymayhavebeenselected.Arockextractionpit in thevicinityofthe
intersection results ina relativelyhighnumberofheavyvehicles through the intersection.These
heavyvehicleshaveadifficulttimenegotiatingthecompacturbandesignofthisroundaboutand
havecauseddamage to thesplitter islandsandcurbs.The tightgeometryalso results in reduced
capacity, which can result in queuing during peak periods. While this may be an acceptable
conditioninanurbanenvironment,thequeueshavethepotentialtoblockafirestationdriveway
on the east approach to the intersection. Fortunately, there are opportunities to retrofit this
roundabout
to
better
accommodate
heavy
vehicles
and
increase
capacity
while
maintaining
slow
speedsdesirableinabuilt,urbanenvironment.
Planning and Design Principles
The roundabout design process is more iterative than that of other intersection forms. Small
changes to the roundabout geometry can have significant effects on the operational and safety
characteristics of the roundabout.Designers should refer to Exhibit 62 of the FHWAGuide for
guidanceonthedesignprocess.Attheinitiallayoutofaroundabout,threefundamentalelements
oftheroundaboutdesignshouldbeconsidered:
1. thelocationoftheroundabout;
2. thesizeoftheroundabout;and
3. theapproachalignment.
Changes to any of these three fundamental elements during the design processmay require a
reassessmentoftheothertwoelements.
Photo: Lee Rodegerdts
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LOCATION
Tominimizeimpacttoadjacentpropertiesandrightofwayacquisition,designersshouldbeginby
attemptingtocenteraproposedroundaboutateitherthecenteroftheexistingintersectionorthe
intersectionofthecenterlinesoftheintersectionapproaches.However,thedesignershouldnotbe
constrainedto
centering
the
roundabout
on
the
center
of
the
existing
intersection.
Reasons
for
shiftingtheroundaboutincludeconstraintsinoneormorequadrantsoftheintersection,providing
moredeflectionon aparticularapproach to reduce speeds,and theability to stage construction
whilemaintainingtraffic.Fundamentally,thelocationoftheroundaboutshouldbeprioritizedtoa
positionthatoptimizesthedesiredoperatingandsafetyperformancecharacteristics.
INSCRIBED CIRCLE DIAMETER
The City desires roundabouts that are as small as possible while still addressing fundamental
capacity, emergency response,designvehicles,andotheroperationaland safetyneeds.Diameter
ranges
and
considerations
are
presented
in
the
FHWA
Guide.
These
values
provide
a
starting
point
indeterminingtheappropriatesizethatresultsindesiredoperationsandsafetyperformance.Site
specificconditionsmayrequireinscribedcirclediametersthatareoutsidethepublishedranges.
APPROACH ALIGNMENT AND DESIGN
To the extent possible, the City desires roundabout approaches that are as near 90 degrees as
possible.Thesealignmentsfacilitatedesirableslowandconsistentspeedsandreducethesizeofthe
roundaboutsinscribedcirclediameter.Regardlessoftheapproachalignment,roundaboutdesigns
shouldpromoteslowoperatingspeedsattheentryandrelativelysmallspeeddifferentialsbetween
successivegeometricelementsorconflictingmovements.
AsdescribedintheFHWAGuide,entrycurvesshouldbecurvilinearlytangentialtotheoutsideedge
of thecirculatory roadwayand thecentral island.Theprimaryconsiderationswhen selectingan
entrycurve radiusare entry speedcontrolandaccommodating thedesignvehicle.Typical entry
curveradiiforasinglelaneroundaboutrangefrom50to100feet.Considerationsforentrydesign
at multilane approaches are similar with the additional consideration of path alignment, as
describedinalatersection.
Similar toentrydesign,exitcurvesshouldbedesignedcurvilinearly tangential to thecirculatory
roadwayand thecentral island.Theexit radiusshouldnot result inavehiclepath radius that is
smaller
than
the
circulating
vehicle
path
radius.
Although
tangential
exit
designs
may
be
acceptable,carefulconsiderationshouldbemade to the resultingvehiclespeedat thepedestrian
crossingdownstreamfromtheexit.
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Safety and Performance Evaluations
SPEED CHECKS
Speedchecksareoneofthekeyperformancechecksofaroundaboutconcept.Thepredictedspeed
valuesareused toassessaroundaboutssafetyperformanceandareused in the intersectionandstoppingsightdistancecomputations.
The first priority in investigating a roundabouts predicted speed is attaining low entry speeds.
Targetmaximumdesignentryspeedsare20 to25mph forsinglelaneroundaboutsand25 to30
mph formultilane roundabouts (assumingdriversdonotadhere to lanemarkingson theentry).
Whiledesignersshouldstrivetoachievethesedesignspeeds,sitespecificconstraintsmayrequirea
tradeoffbetweenachieving thesedesignspeedsandaccommodatingotherdesignelements (e.g.,
promotinggoodpathalignment,accommodating thedesignvehicle). In thesecases, thedesigner
should coordinate with City staff early in the planning process to consider the context of the
intersectionand
acceptable
design
tradeoffs.
In addition to appropriate entry speeds, the roundabout configuration shouldbe evaluated for
speedconsistencybetweensuccessgeometricelements,suchas thepathofa throughmovement
througha roundabout. Inaddition, speed checks should consider the speeddifferentialbetween
conflictingtrafficmovements,suchasathroughmovementpotentiallyovertakingarelativelyslow
left turningmovement.As described previously, the exit geometry should not result in exiting
speedsthatareslowerthancirculatingspeeds.Exhibit2showsasmallexitradiusthatcanresultin
vehiclesslowingwithinthecirculatoryroadwaytonavigatetheexit.
Exhibit 2 Example of tight exit geometry
TheCity follows the guidance provided in the FHWA Guide for assessing vehicle fastest paths.
NCHRP Report 572, Roundabouts in the United States (Reference 3) provides information that
augments the FHWA Guide for exiting speed. NCHRP Report 572 indicates exit speeds maybe
controlledbyspeedreductionattheenteringandcirculatingradiiand,therefore,theactualspeeds
forexitingvehicles(V3)maybelowerthanpredictedbytheFHWAmethodology.Consequently,
speedchecksfortheexitshouldbeconductedusingEquation54ainNCHRPReport572.Designers
shouldgenerallytargetdesignspeedsintherangeof25mphattheexit.
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DESIGN VEHICLE
Theappropriatedesignvehicleisafundamentalconsiderationinfluencingthesizeofaroundabout
and,therefore,shouldbeconsideredintheearlieststagesofintersectionplanning.Inaddition,the
volumes, patterns, and frequency of the design vehicle should be considered early and the
roundaboutconfiguration
customized
for
the
anticipated
movements.
This
means
some
roundaboutsmaybeconfiguredtoservedifferentdesignvehiclesondifferentmovements,which
resultsinacustomizeddesignateachlocationbasedonanticipateddesignvehicleneeds.
AWB50designvehicleshouldbeconsideredasthebaseconditionforintersectiondesignonCity
facilities.However,thespecificcontextoftheintersectionandthefrequencyandsensitivityofthe
needforalargerdesignvehiclemustbeconsideredanddocumented.ThismightmeanthataWB
67vehiclebeconsideredonspecificmovementswithintheroundaboutandtheroundaboutsized
accordingly and customized to anticipated uses and patterns. Similarly, aWB40design vehicle
maybeadequate for certain facilities.Thedesigner shouldconsultwithCity transportation staff
early to identify the contextof the intersection anddocument theappropriatedesignvehicleby
approachastheseevaluationscouldgreatlyinfluencetheroundaboutconfiguration.
Design vehicle checks (using a tool such asAutoTurn) shouldbe conducted as part of concept
developmentandincludedinthe30%designsubmittal.Toincreaseoperatorcomfortandminimize
thepossibilityofoverturning,thedesignershouldstrivetoreserveuseofthetruckapronforonly
thetrailerofthedesignvehicle.Generally,thecabofthedesignvehicleshouldbeabletocirculate
withoutusingthetruckapron.Whilethetrackingofthetrailerofthedesignvehiclewilldefinethe
insideedgeofthetruckapron,abus(e.g.,B40)oremergencyvehicleshouldbeusedtodefinethe
outsideedgeofthetruckapron.Busesandemergencyvehiclesgenerallyshouldnotencroachupon
thetruckapronand,therefore,willdefinetheedgebetweenthecirculatoryroadwayandthetruck
apron.Care
should
be
taken
to
avoid
excessively
large
circulatory
roadway
widths.
Widths
should
generally not exceed 20 feet for a singlelane roundabout to avoid the possibility of drivers
mistaking the roundabout for having two circulating lanes.Adjustments to the inscribed circle
diameter maybe required tobalance the truck accommodations with the circulatory roadway
width.
Thechoiceofdesignvehicleaffectsavarietyofdesignelements.Improperlydesignedroundabouts
may result in longterm maintenance issues. These issues can include broken curbs, crushed
landscaping, and impacted traffic furniture and signing. Operational issues include vehicles
encroachinginpedestrianareasorgenerallydecreasingtrafficflowbecauseofextraslowspeedsby
trucksnavigatingtheroundabout.
PATH ALIGNMENT
The alignment ofvehicle paths shouldbe considered for anymultilane entry or exit.Designers
should take care to promote proper path alignment. Without proper path alignment, the
phenomenonofpathoverlap,asdemonstrated inExhibit3,canoccuratmultilane roundabouts.
Pathoverlapresultswhenthenaturalpathofavehicleinonelaneencroachesonanadjacentlanein
responsetothegeometry.Pathoverlapcanoccuroneithertheroundaboutentryorexit.Exhibit4
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demonstratesanoccurrenceofpathoverlapataroundaboutexitandaretrofitofthesameexitto
eliminatepathoverlap.Exhibit5providesguidancetodesignersonhowtoavoidpathoverlap.The
sametreatmentsareapplicabletothedesignoftheroundaboutexits.Exhibit6showsaroundabout
entrydesignincorporatingdesignfeaturestopromoteproperpathalignment.
Exhibit 3 Example of path overlap (KSDOT Exhibit 6-19, Reference 4)
Exhibit 4 Example of exit path overlap and retrofit
Exhibit 5 Multilane design details (KSDOT Exhibit 6-20)
Photo: Lee RodegerdtsPhoto: Barry Crown
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Exhibit 6 Appropriate entry design with proper path alignment (18th Street/Cooley Road)
Roundabout Design Elements
PEDESTRIAN TREATMENTS
Roundabouts simplifypedestrian roadwaycrossingsbyprovidinga twostagecrossing.As such,
roundaboutsplitter islandsshouldserve theanticipatedvolumeofpedestriansandhavearaised
refuge area of adequate width to accommodate the anticipated user types. The City prefers a
pedestrianrefugeareaofatleast8to9feetinwidth.Innocaseshouldtherefugeareabelessthan6
feetinwidth.Thepedestriancrossingandrefugeareashouldbelocatedaminimumof20feetfrom
thecirculatory
roadway
and
preferably
not
more
than
25
feet.
Atroundabouts,cyclistshavetheoptiontoexittheroadway,whichmeanspedestriansandbicycles
mustcoexistatroundabouts.Therefore,intheareasaroundaroundaboutandboundbythebicycle
ramps,thepedestrianfacilitiesshouldbedesignedasamultiusepathwithawidthofpreferablyno
lessthan10feetandaminimumof8feet.Inconstrainedareas,themultiusepathmayneedtobe
designed curbtight to attain theminimumwidth; alternatively, cyclistsmayneed towalk their
bicyclesiftheychoosenottoridethroughasavehicle.
TheCitysupportsmobilityforallusersandtakescaretoprovideappropriatefacilitiesanddesigns
for
special
user
needs.
Roundabouts
must
include
grades,
access
ramps,
and
detectable
warning
surfacesatallpedestrianroadway interfaces tomakethemaccessibleper therequirementsof the
ADAAccessibilityGuidelines (ADAAG,Reference 5) and thebestpracticesdescribed in thedraft
RightsofWayAccessibilityGuidelines (PROWAG,Reference6).Pedestrian facilities should support
wayfinding.For roadwaycrossings, thecrossing shouldbedesignedperpendicular to the center
lineoftheroadwayandprovideaslinearapathaspossiblefromoneaccessramptotheothervia
the refuge area. If a crosswalkmustbe oriented perpendicular to the traveledway due to site
constraints,adefinedanglepointshouldbeprovided in the refuge island tohelporientvisually
impairedpedestrianstothereceivingaccessramp.
Photo: Lee Rodegerdts
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P e d e s t r i a n S i g n a l s
The U.S. Access Board has developed guidance in the draft Public RightsofWay Accessibility
Guidelines (PROWAG,Reference6) thatwould require installingsome formof signalizationor
other equivalently accessible features on the entries and exits of all multilane roundabout
approaches.Thepedestrianhybridbeacon identified in the2009MUTCD,would likelymeet the
intent of this PROWAG requirement. The rectangular rapid flashingbeacon (RRFB),which hasinterim approval from FHWA, may alsobe an acceptable treatment. In anticipation of a final
PROWAG decision that could include signalizing multilane roundabout approaches, designers
should,ataminimum, includeappropriatelyplacedconduitandjunctionboxes toaccommodate
potentialfutureinstallationsofsignals.
BICYCLE TREATMENTS
Designersshouldstrivetoprovidecycliststheopportunitytoeithernavigatetheroundaboutwith
vehicular traffic or to exit the vehicular travel way and use a multiuse path adjacent to the
roundabout.To
accommodate
those
cyclists
that
would
prefer
to
exit
the
vehicular
travel
way,
exit
and entry ramps shouldbeprovided.Historically, roundaboutdesign inBendhas resulted in a
varietyofbicycleramptreatments.Thisvariationindesignreflectsbothanevolutionoftheoryand,
insomecases,theneedtoadaptdesignstoconstrainedsituations(e.g.,adjacentdriveways).
Exhibit7demonstratestheCityspreferreddesignforbicycleramps.Thedesignershouldconsider
sitespecific conditions when locating bike ramps. Generally, the ramps should be located
approximately 50 to 100 feet from the entrance line and, in all cases, prior to the pedestrian
crossing.Asshown, the rampsareangular to theapproachroadwayand themultiusepath.This
facilitates the exitingmovement from the roadwaywithout requiring cyclists to swing into the
vehicular
travel
way
to
access
the
ramp.
On
the
re
entry
to
the
bike
lane
on
the
exit
side
of
the
approach,theangularrampminimizestheprobabilityofavisuallyimpairedpedestrianmistaking
therampforasidewalk.Inaddition,detectablewarningsurfaces(e.g.,truncateddomesorstamped
or textured concrete) shouldbe installed across the fullwidth of each ramp at the edge of the
sidewalk.AsshowninExhibit7,thetransitionbetweenthebikelaneandthebikeramphasbeen
designedtoaccommodatestreetsweeping.
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Exhibit 7 Bike Treatment Concept
Thesebiketreatmentconceptsshouldbeconsideredatallsinglelaneandmultilaneroundabouts,
evenwherebikelanesarenotpresentinadvanceoftheroundabout.Ifpossible,wherebikelanes
donotexistonanapproach,abikelaneshouldbeprovidedthroughtheextentoftheconstruction
limitsonboth theentryandexitofeachapproach.Thiswillaccommodate the futureadditionof
bike lanes.Under no circumstance should a stripedbike lane or shoulderbe provided on the
roundaboutcirculatoryroadway.
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SPLITTER ISLAND
Splitterislandsserveavarietyofpurposesfrompositiveguidancetosignplacementandshouldbe
included on all roundabouts. Splitter islands should be raised to optimize their effectiveness.
Shouldageometricorsitespecificconstraintappeartoprecludearaisedsplitterisland,extracare
andconsultation
with
City
staff
is
required,
and
the
constraints
should
be
thoroughly
documented.
Fundamentally,thereshouldbeveryfewinstanceswherearaisedsplitterislandisnotfeasibleand
alleffortsshouldbeexploredtorefineadesigntoattainaraisedsplitterisland.Ifaraised island
cannotbe attained, the splitter island shouldbe a textured surface to distinguish it from the
adjacent pavement.Only in rare cases, such as lowvolume approaches, should painted splitter
islandsbeconsidered.
While splitter islands shouldbeat least50 feet long,100 feet isdesirableand,generally, splitter
islandsshouldbeaslongaspracticalwithinthedesignenvironment.Longerislandsaredesirable
tohelpmanagespeedonhighspeedapproaches, tomanageaccessandencourage rightin/right
out
movements
from
adjacent
access,
or
to
help
promote
positive
guidance
on
roadway
approaches
thathavesignificantrealignmentfromtheirformerconfigurations.
TheFHWAGuideandNCHRPReport279 (Reference7)provideexamplesandapplicationsof the
principles of channelization. These documents encourage offsetting splitter islands from the
approachtraveledwayandnarrowingtheoffsetonthedeparturestocreateafunnelingeffect.The
FHWAGuideprovidesendtreatmentdimensionsthatreflectaphilosophyoflargerradiionisland
approachesandsmallerradiionthedepartures.TheCityprefersthatallmedianendtreatmentsbe
nearlyflushwiththeroadway,asshowninExhibit8,toaccommodatesnowplows.Theseprinciples
andtheintentoftheseconfigurationsshouldbeincorporatedinallroundaboutdesigns.
Exhibit 8 Median End Treatment Concept
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APPROACH WIDTH
Ingeneral,roundaboutapproachwidthsshouldbeasnarrowaspracticalwhileservingthetraffic
volumeneeds(singlelaneversusmultilane)andtheapplicabledesignvehicle. Thisholdstruefor
exitwidthsalso.Inmostcasesatractortrailerdesignvehiclewillcontroltheimmediateentryand
exitwidths;
however,
emergency
vehicle
needs
may
control
roundabout
approach,
entry,
and
exit
widths.Asaminimum,20 feetofclearwidth (curb face tocurb face) shouldbeprovidedonall
approaches,entries,andexitstoaccommodateemergencyvehicles.Designersanddesignreviewers
should be especially alert that these approach widths could influence vehicular speeds and,
therefore,fastestpathevaluationsmustbeconductedtoensuretargetspeedscanbeattainedwhile
servingemergencyvehicleandotherdesignvehicleneeds.Ifappropriatedesignspeedscannotbe
achieved while still providing 20 feet of clear width, the designer should consider design
modifications,suchasalargerinscribedcirclediameteroranoffsetleftapproachalignment.
TRUCK APRON
Truckapronsaretypicallyprovidedaroundthecentralislandtoaccommodatethedesignvehicle
while stillmaintaining appropriate speed control through the roundabout.Truck aprons on the
outeredgeofentriesandexitstotheroundaboutshouldgenerallybeavoidedastheycouldresult
in a potential conflictbetween nonmotorized users and trucks. Given that the truck apron is
intendedtoserveheavilyloadedvehicles,allelementsofthetruckapron(edgesandapron)should
be constructed ofhighstrength concrete and include appropriate steel reinforcing to promote a
long service life. The truck apron should have colors and textures that distinguish it from the
traveledwayandadjacentsidewalks.
Cr o s s S lo p e
Thecrossslopeofthetruckapronshouldgenerallybe1to2%butcanvarybasedondesignand
drainageneeds.Iflargercrossslopevaluesareapplied,thedesignshouldspecificallybecheckedto
addresslowclearancetractortrailerdesignvehicles.Inallcasesthetruckapronshouldslopeoutto
thecirculatoryroadway.
Re v e a l
Therevealprovidesacleardistinctionbetweenthecirculatoryroadwayandthetruckapron.Too
smallofarevealmayresultindriversridingupontheapronresultinginundesirablespeeds.Too
muchrevealcancreateanoverlyabruptedgethatcoulddisruptfloworbeimpactedbyvehicles.
TheCity
prefers
to
use
its
Type
B
curb
design
for
the
outer
edge
of
the
truck
apron.
I n t e r i o r Cu r b
Aninteriorcurborotherfixedraiseddemarcationshouldbeprovidedbetweenthetruckapronand
thenontraversablecentralisland.Thiscurbdistinguishesbetweentheapronandplantedareaand
discouragesuseofthelandscapedcentralisland.
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Vehicles running into central island
Vehicles hitting splitter island onapproach
Vehicles making first turn butmissing second
ILLUMINATION
Illuminationshouldbeprovidedatallroundaboutswithspecialcaretakentolightconflictareasat
theproperlevel.Theprimaryconflictareasforconsiderationarefixedobjects(e.g.,splitterislands,
centralisland),pedestriancrossings,biketransitions,andtheenteringandexitingconflictareason
eachapproach.
Luminaire
type
may
vary
to
serve
vehicular
and
pedestrian
traffic
together
or
with
separate lighting systems employed to separately serve user needs.Current guidance from the
IlluminatingEngineeringSociety(IES)suggeststhatilluminationatpedestriancrossingsbeplaced
in frontof the crossing (i.e. as thedriver approaches the crossing) toprovide sufficientpositive
contrast.
Exhibit9depictsareasoftheroundaboutwherelightingissusceptibletovehicularimpact.Tothe
extent possible, these conflict areas shouldbe avoided. If the location cannotbe avoided, other
designtreatments,suchaslargeroffsetsfromthetraveledwayandlongerluminairearms,should
beconsidered.
Exhibit 9 Areas to avoid for pole placement
Generally,thedesignershouldstartbyplacingluminariesalongtheperimeteroftheroundaboutin
advanceofeachpedestriancrossing.Dependingonpoleplacementandthesizeoftheroundabout,
itmaybenecessarytolightthecirculatoryroadwayfromboththeexteriorandtheinterior(i.e.the
centralisland)oftheroundabout.Inallcases,aphotometricanalysisshouldbeconductedtoensure
appropriatelightinglevelsanduniformity.
ThecurrentCitystandard forhorizontal illuminationat roundabouts isaminimum illumination
levelof
1.0
foot
candles
and
auniformity
of
3:1
or
better.
Recent
studies
and
guidance
suggest
that
vertical illuminance, which helps drivers identify pedestrians in crosswalks, should also be
considered.Verticalilluminanceprovidesilluminationofobjectsintheverticalplane.IESsuggests
that vertical illuminance shouldbemeasured at a height of 5 feet along the centerline of each
crossing.The averagevertical illuminance along this line shouldmeet the same standard as the
minimumhorizontalilluminanceanduniformityandthemeasurementshouldbemadeatleastone
safestoppingdistance from thecrossing.Additionaldesignguidancecanbe found in theDesign
GuideforRoundaboutLighting(Reference8)preparedbyIES.
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Application of Traffic Control Devices
AnupdatetotheManualonUniformTrafficControlDevices(MUTCD)wasreleasedbyFHWAinlate
2009.TheupdatedMUTCDincludesrevisedinformationforroundaboutsigningandstriping.The
followingisnotintendedtoreplacetheMUTCDbutinsteadprovideguidancetodesignersonthe
optionalpavement
markings
and
signs
in
the
2009
MUTCD
(Reference
9).
PAVEMENT MARKINGS
Y i e ld L i n e a n d Y i e l d L e g e n d
TheMUTCD identifies theuseofyield lines andyield legends asoptional at roundabouts.The
Cityspreferenceistonotusetheyieldlineorlegendtoreduceongoingmaintenance.
L an e - U se A r r o w s
Laneuse
arrows
should
be
used
on
all
multilane
approaches
and
within
the
circulatory
roadway
of
multilane roundabouts.Laneuse arrows should notbe used at singlelane roundabouts.Where
used, the City prefers to use the traditional (i.e. not fishhook) laneuse arrows without a dot
symbolizingthecentralisland,asdescribedinthe2009MUTCD.
SIGNING
L a n e - U s e S ig n s
TheCitypreferstouselaneusesignsdepictingtraditional(i.e.notfishhook)laneusearrowsand
without
a
dot
symbolizing
the
central
island.
Lane
use
signs
should
only
be
used
on
multilane
approaches.
Y i e l d S i g n
Yieldsigns(R12)shallbeplacedontheoutsideedgeofallsinglelaneapproaches.Dependingon
theapproachgeometryandsignvisibility,ayieldsignmayalsobenecessaryinthesplitterisland
onsinglelaneapproaches.Yieldsignsshallbeplacedonboththeoutsideedgeandinthesplitter
islandonallmultilaneapproaches.
Ci r c u l a r I n t e r s e c t i o n Si g n
TheCircular
Intersection
sign
(W2
6)
should
be
used
in
advance
of
all
roundabout
intersections.
A
plaqueshouldbeincludedwitheachW26signindicatingthenameoftheupcomingcrossstreet.
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P e d e s t r i a n Cr o s s i n g S ig n
Thepedestriancrossingsign(W112)andassociateddownwardarrowplaqueshouldnotbeused
at crossingsof singlelane approaches except at singlelane approacheswithin a school zone.At
crossings ofmultilane approaches, aW112 and associated downward arrow plaque shouldbe
placedon
both
the
outside
edge
and
in
the
splitter
island.
D i r e c t i o n a l A r r o w a n d S t r e e t N am e Si g n A s sem b l y
ConsistentwiththeR64signseriesinthe2009MUTCD,ahorizontal,rectangularsignwithblack
chevronsonawhitebackgroundshouldbeplacedinthecentralislandoppositeeachroundabout
entry.Astreetsignindicatingthenameofthecrossstreetshouldbemountedabovethedirectional
arrowsign.
Exhibits10and11demonstrate the typicalpavementmarkingsand signingdescribed above for
singlelaneandmultilaneroundabouts,respectively.
Exhibit 10 Pavement markings and signing for single-lane roundabouts
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Access Management
TheCity recognizesaccessmanagementasavaluable tool to enhance the flowof traffic, reduce
user conflicts, and improve safety.Access management is a means of optimizing the available
capacity
of
a
roadway
and
perhaps
eliminating
or
postponing
expensive
or
impacting
roadway
widening. Roundabouts provide opportunities to support access management objectives and
shouldbeconsideredexplicitlyforthispurpose.Roundaboutsmayalsosupportflexibilityinaccess
management. For instance, rightin/rightoutdriveways thatwould notbe permittedwithin the
influence area of a conventional intersection may be permitted near a roundabout as the
roundabout will facilitate Uturn movements. Furthermore, the low speed environment in the
influence area of a roundabout may permit driveways that would not be permitted at a
conventional intersection tooperatesafely.Thecontextofeach locationshouldbe reviewedona
casebycasebasiswhenconsideringaccessmanagementneararoundabout.
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References
1. Robinson, B. W., L. Rodegerdts, W. Scarbrough, W. Kittelson, R. Troutbeck, W. Brilon, L.
Bondzio,K.Courage,M.Kyte,J.Mason,A.Flannery,E.Myers,J.Bunker,andG.Jacquemart.
Roundabouts:
An Informational Guide.
Report
FHWA
RD
00
067.
FHWA,
U.S.
Department
of
Transportation,June2000.[online]http://www.tfhrc.gov/safety/00068.htm.
2.Kittelson&Associates, Inc.City ofBend Intersection FormEvaluation Framework.City ofBend,
November2009.
3. Rodegerdts, L.,M. Blogg, E.Wemple, E.Myers,M.Kyte,M.Dixon,G. List,A. Flannery,R.
Troutbeck,W.Brilon,N.Wu,B.Persaud,C.Lyon,D.Harkey,andD.Carter.NCHRPReport572:
Roundabouts in theUnitedStates,TRB,NationalAcademyofSciences,Washington,D.C.,2007.
[online]http://www.trb.org/Main/Public/Blurbs/158299.aspx
4.
Kittelson
&
Associates,
Inc.
and
TranSystems
Corporation.
Kansas
Roundabout
Guide:
A
Supplement
toFHWAsRoundabouts:AnInformationalGuide.KansasDepartmentofTransportation, Topeka,
Kansas,October2003. [online]
http://www.ksdot.org/burTrafficEng/Roundabouts/Roundabout_Guide/RoundaboutGuide.asp
5. United States Access Board. Americans with Disabilities Act Accessibility Guidelines. Federal
Register, July 2004 and amended August 2005. [online] http://www.accessboard.gov/ada
aba/adaag.cfm
6.UnitedStatesAccessBoard.DraftGuidelinesforAccessiblePublicRightsofWay.U.S.Architectural
and Transportation Barriers Compliance Board, Washington, D.C., 2005. [online]
http://www.accessboard.gov/prowac/draft.htm#206
7. Neumann,T.R. NCHRP Report 279: Intersection Channelization Design Guide, TRB, National
ResearchCouncil,Washington,D.C.,1985.
8.IlluminatingEngineeringSociety(IES).DesignGuideforRoundaboutLighting.IESDG1908.2008.
Available for purchase at http://www.ies.org/store/product/designguideforroundabout
lighting1037.cfm.
9. FederalHighwayAdministration (FHWA).Manual on Uniform Traffic Control Devices. FHWA,
Washington,D.C.,December2009.[online]http://mutcd.fhwa.dot.gov/pdfs/2009/pdf_index.htm
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City of Bend
Intersection Form EvaluationFramework
April 2010
Prepared For:The City of Bend
Prepared By:Kittelson & Associates, Inc.
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Kittelson & Associates, Inc. 1
Overview
TheCityofBendmaintainsaroundaboutsfirstapproachtointersectiontreatmentsbutisflexiblein
considering other forms should there be the need to evaluate various intersection options. This
document provides a framework and criteria for evaluating roundabouts against other intersection
forms.The
body
of
this
document
outlines
quantitative
and
qualitative
criteria
that
can
be
used
to
compare intersection forms; the appendix contains a case study that applies the intersection
comparisonprocess.
This sectiondescribesbasic steps to evaluate intersection formswith the intent that completing the
basicstepswillprovidetheinputneededtoconducttheevaluation.Theevaluationcriteriainclude:
SafetyAssessment(crashdata,knowndeficiencies,sightdistance,etc.) TrafficOperations(forecastvolumes,performancemeasures) AnticipatedUsers SystemContext ContextatIntersection(e.g.,ROWimpacts,designvehicle) Benefit/CostRatios
Eachintersectionformevaluationwillbeuniqueinthebreadthoftheevaluationandtheavailabilityof
data. To streamline evaluations, criterion hasbeen prioritizedbased on relative importance. These
prioritiesaredesignatedasTier1andTier2.Tier1criteriaare identifiedas themost importantand
shouldbe considered each time an evaluation is conducted. Tier 2 criteriamaybe considered, if
needed,shouldTier1criterianotbesufficienttoprovideacleardifferentiationbetweenalternatives.If
applicabletoaprojectsneeds,somecriteriamaybeshiftedfromonetiertothenext.Evaluationcriteria
shouldbeconsideredanddiscussedwithcitystaffat theearlieststagesofan intersectionevaluation
process.
Thefollowinginformationprovidesaframeworkforconductinganintersectionplanning,operations,
anddesignevaluationwithaspecialemphasisonroundaboutspecificconsiderations.
Intersection Evaluation Process
Exhibit 1 provides a process framework for considering intersection treatment forms. Since
conventional intersection planning, operations, and design evaluations are well established and
documented(andarearelativelycommonpracticeformostprofessionals),thisframeworkemphasizes
thespecial
needs
of
appropriately
considering
and
evaluating
roundabout
forms.
This
emphasis
is
intended to guide transportationprofessionals in conducting appropriate analysesand conceptually
evaluating and developing roundabout concepts that are consistentwithCitydesired qualities and
attributes.
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Exhibit 1 Intersection Evaluation Process Framework
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INTERSECTION CONTEXT
Intersectioncontext includesfundamentalconsiderationssuchaswhetherthe intersectionispartofa
newfacilityoraretrofitsituation.Newlocationsprovideincreasedflexibilityinlocatinganddesigning
the intersection forms,while retrofit conditions often exhibit constrained right ofway or secondary
considerations,such
as
access
to
adjacent
land
uses.
Context
also
includes
understanding
user
types
includingappropriatelyservingnonmotorizedusersanddesignvehicleneeds.Designvehiclechoices
haveasignificanteffectonroundaboutdimensionsandconfigurations.Iftheintersectionisaretrofit,
the projectmaybebased on improving capacity or addressing a documented safety need.Awell
documentedsummaryofexistingconditionsandfuturedemandshelpsformthebasisforevaluating
alternativeintersectiontreatments.
PRELIMINARY OPERATIONAL ANALYSIS
A preliminary operational analysis evaluates future traffic needs compared to applicablemobility
criteria
to
screen
candidate
intersection
forms.
This
preliminary
analysis
also
establishes
the
intersection lane numbers and arrangements, which defines the basic size of the intersection.
OperationalanalysesshouldbeconductedinaccordancewiththemethodologiesoutlinedintheCityofBendRoundaboutOperationalAnalysisGuidelines(Reference1)forroundaboutsandtheHighwayCapacityManual(Reference2)forallotherintersectionforms.TrafficsignalwarrantsasdefinedintheManualonUniformTrafficControlDevices(MUTCD)(Reference3)maybeusedatthisstagetodeterminewhethertrafficsignalsareaviablealternativeforfurtherconsideration.
CONCEPTUAL LAYOUTS
The lane numbers and arrangements, roadway approach geometry, design vehicle, and safety
performancechecks
influence
the
intersection
conceptual
layouts.
Roundabout
intersection
qualities
shouldconform to theguidelinesandprinciplesoutlined in theFHWApublication,Roundabouts:AnInformational Guide (Reference 4), in general and specifically reflect design qualities and attributesoutlinedintheCityofBendRoundaboutDesignConsistencyGuidelines(Reference5).Conceptuallayoutsshouldconsider fundamentalobjectivesforsafetyperformanceanddesignvehicle types.Theseearly
conceptuallayoutscanbeusedtobeginassessingbasicrightofwayandaccessmanagementneeds.
FEASIBLE ALTERNATIVES
As intersection formsand configurationsareevaluatedand theassociated tradeoffs indentified, the
promisingalternatives
can
be
advanced.
As
these
alternatives
are
advanced,
the
roundabout
concept
designsshouldberefinedandevaluatedbasedondesignvehicleneeds,fastestpathsand,formultilane
configurations, natural path evaluations to consider path alignment.Refining conceptdesigns is an
iterative process to optimize the intersection safety performance balanced with the site specific
constraintsofthatlocation.Shouldthedesignbemodifiedinsuchawayastochangethelanenumbers
orarrangements,additionaloperationalanalysesshouldbeperformedtoassessanticipatedoperational
performance.
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SELECTION CRITERIA AND RECOMMENDED ALTERNATIVE
If conventional and roundabout forms arebeing compared, the tiered selection criteria shouldbe
applied. Tier 1 and 2 criteria are outlined in Tables 1 and 2, respectively. To provide a reasonable
comparison,eachintersectionformshouldbedevelopedataconsistentandcomparablelevelofdetail.
Tier 1 criteria represent those considerations that typically are the highest priority.Capacity, safety,
footprint, and secondary impacts (access management or driveway closures), often guide project
decisions.Tier2criteriaarealso important considerationsand representan increased levelofdetail
(suchaspredictingcrashesordefiningcosts)orsubjectareathatmaynotvarygreatlyenoughtobea
Tier1 consideration.For example,pedestrian crossingdistancesmaynotvary significantlybetween
alternative intersection formsona lowerorder roadway.TheCity,withanunderstandingofproject
contexthas the opportunity to adjust the criteria and the tieredpriorities in the earliest stages of a
project.Forexample, ifpedestriansafety isaprojectdriverorifhighvolumesnecessitatearelatively
large intersection (roundaboutor signalized), itmaybeappropriateat theonset tomakepedestrian
crossingdistancesaTier1consideration.
AppendixAprovidesan example applicationof the intersection form selection criteriausing a case
studyfortheMurphyRoad/ParrellRoadintersection.
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Table 1 Tier 1 Intersection Form Selection Criteria
Category Criteria Resources
Safety
Motor Vehicle SafetyConflict points (exposure)
Severity (speed)
Roundabouts: An Informational Guide (Reference 4)o Chapter 2, Exhibit 2-3
Non-MotorizedVehicle Safety
Conflict points (exposure)
Severity (speed)
Roundabouts: An Informational Guide (Reference 4)o Chapter 2, Exhibit 2-2
Traffic Operations
Peak-Hour TrafficOperations
Volume-to-capacity ratio
Average delay
LOS
Queue lengths
Roundabouts: Bend Roundabout OperationalAnalysis Guidelines (Reference 1)
Non-roundabout intersection forms: HighwayCapacity Manual (Reference 2)
Anticipated Users
Design Vehicle
Appropriate heavy vehicle
Buses
Emergency vehicles
Bend Roundabout Design Consistency Guidelines(Reference 5)
Special User Needs
School children
Elderly
Visually impaired
ADA compliance
Bend Roundabout Design Consistency Guidelines(Reference 5)
System Context
System Effects
Adjacent traffic control
Railroad crossing
Highway Capacity Manual (Reference 2)o Upstream impacts and vehicle arrival
patterns
EnvironmentalImpact
Land use context Review of existing adjacent land use and plannedland use
EmergencyResponse
Response time/control delay Roundabouts: Bend Roundabout OperationalAnalysis Guidelines (Reference 1)
Non-roundabout intersection forms: HighwayCapacity Manual (Reference 2)
Context at Intersection
IntersectionFootprint
Intersection proper (physical &operational influence area)
Roadway approach geometry
Roundabouts: An Informational Guide (Reference 4)o Chapter 3, Section 3.6
Review of conceptual geometric designIntersection
Influence AreaDriveway closures or impacts Review of conceptual geometric design
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Table 2 Tier 2 Intersection Form Selection Criteria
Category Criteria Resources
Safety
Motor Vehicle Safety Crash Prediction AASHTO Highway Safety Manual (Reference 6)Traffic Operations
Peak Hour TrafficOperations
Sensitivity to changes involumes/ travel patterns
Roundabouts: Bend Roundabout OperationalAnalysis Guidelines (Reference 1)
Non-roundabout intersection forms: HighwayCapacity Manual (Reference 2)
24-Hour TrafficOperations
Average Delay
Anticipated Users
Pedestrians Crossing distances
Bicyclists
Adjacent bike facilities
Intersection specificconsiderations
Bend Roundabout Design Consistency Guidelines(Reference 5)
System Context
Environmental Impact Estimated emissions output
Access Management
Facilitates access management
Median and U-turnopportunities
Driveway connections
Review of conceptual geometric design
Emergency ResponseEvaluating likely emergencyresponse routes
Context at Intersection
Environmental Impact Impervious surface area
Aesthetics
Review of conceptual geometric design
Intersection InfluenceArea
Approach and segment cross-section
Benefit/Cost Ratio
Benefits
Crash reduction
Reduced fuel consumption
Reduced delay (15-min delay,
24-hour delay)
AASHTO Highway Safety Manual (Reference 6) Roundabouts: Bend Roundabout Operational
Analysis Guidelines (Reference 1)
Non-roundabout intersection forms: HighwayCapacity Manual (Reference 2)
Costs
Design/Engineering Costs
Construction costs includingROW acquisition
Operations/maintenance costs(includes energy costs forsignals)
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References
1.Kittelson&Associates, Inc.City ofBendRoundaboutOperationalAnalysisGuidelines.City ofBend,November2009.
2.Transportation
Research
Board.
Highway Capacity Manual. TRB, National Research Council,
Washington,DC,2000.
3. Federal Highway Administration (FHWA). Manual on Uniform Traffic Control Devices. FHWA,Washington,D.C.,December2009.[online]http://mutcd.fhwa.dot.gov/pdfs/2009/pdf_index.htm
4.Robinson,B.W.,L.Rodegerdts,W.Scarbrough,W.Kittelson,R.Troutbeck,W.Brilon,L.Bondzio,K.
Courage,M.Kyte,J.Mason,A.Flannery,E.Myers,J.Bunker,andG.Jacquemart.Roundabouts:An
Informational Guide. Report FHWARD00067. FHWA,U.S.Department of Transportation,June
2000.[online]http://www.tfhrc.gov/safety/00068.htm.
5.Kittelson
&
Associates,
Inc.
City of Bend Roundabout Design Consistency Guidelines. City of Bend,
November2009.
6.AmericanAssociationofStateHighwayandTransportationOfficials.DraftHighwaySafetyManual.AASHTO,2009
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Kittelson & Associates, Inc. A-1
Appendix A
CASE STUDY: MURPHY ROAD AND PARRELL ROAD
I n t e r s e c t i o n Co n t e x t
MurphyRoadisakeyeastwestcorridorinsoutheastBend.ParrellRoadisanorthsouthroadwaythat
parallels3rdStreetandprovidesasecondarynorthsouthroadwaywhileservinglocalaccess.Muchof
theaccessisresidential,buturbanrenewalplansinthesouthernareasofBendneartheParkwaywill
likely includemixed uses. Presently the intersection is a twoway stop controlled intersectionwith
prioritytoMurphyRoad.ThecurrentpostedspeedonMurphyRoadis35mphand30mphonParrell
Road.The landaround the intersection isdevelopedwithrelativelycloselyspaceddriveways. Inthe
future,MurphyRoadwillbeextendedtotheeastandwest.WestofParrellRoad,MurphyRoadwillbe
realigned and a new crossing over the Parkway willbe constructed. These enhancements further
solidifytheimportanceofMurphyRoadasakeylongrangenetworklinkageand,asaresult,trafficis
anticipatedto
grow
at
this
location.
Therearepresentlya limitednumberofpedestrians,butpedestrianvolumesarelikelyto increaseas
mixeduse land develops. Parrell Roadmay serve as access to properties that front 3rd Street, and
therefore,theintersectionwilllikelyserveWB50trucks.
PM peak future roadway volumes were estimated by the Oregon Department of Transportation
through the Bend MPO EMME/2 travel demand model for the future 2030 scenario assuming
improvements to the intersection. TableA1 provides a summary of 2030 volumes at theMurphy
Road/ParrellRoadintersection.
Table A-1 2030 Weekday PM Peak Forecast Volume at Murphy Road/Parrell Road
Movement North Leg East Leg South Leg West Leg
Left 45 65 50 15
Through 35 340 25 540
Right 35 45 70 60
P r e l i m i n a r y O p e r a t i o n a l A n a l y s is
Trafficoperationalanalyseswereconductedfortheintersectionbasedona2030futuretrafficvolume
forecast
for
a
roundabout
and
a
signalized
intersection
form.
The
results
of
the
roundabout
and
signal
analyses are provided in Tables A2 and A3, respectively. To provide an equivalent comparison
betweenthesignalizedandroundaboutalternatives,theweightedaverageoftheroundaboutapproach
controldelaywascomputed,usingEquation1.
where,
Dintersection=intersectioncontroldelay,s/veh(Equation1) Di=controldelayonapproachi,s/veh
Vi=volumeonapproachi,veh/hr
=
i
i
i
ii
V
VD
Donintersecti
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Kittelson & Associates, Inc. A-2
TableA2 provides a summary of future intersection lane configurations and operational analysis
resultsforthesignalizedalternative.
Table A-2 Signalized Intersection Analysis at Murphy Road/Parrell Road
Geometry Information North Leg East Leg South Leg West Leg
Entry Lanes L/T/R L T/R L/T/R L T/R
Performance Measure 2030 Operations
Volume/Capacity 0.56
Intersection Control Delay(sec)
12.7
Intersection LOS B
95% Queue Length (ft) 50 50 125 50 25 225
TableA3 provides a summary of future intersection lane configurations and operational analysis
resultsfortheroundaboutalternative.
Table A-3 Roundabout Intersection Analysis at Murphy Road/Parrell Road
Geometry InformationNorthLeg
EastLeg
SouthLeg
WestLeg
Number of Entry/Exit Lanes
1/1 1/1 1/1 1/1
Right Turn By-Pass N N N N
Circulating Lanes 1 1 1 1
Performance Measure 2030 Operations
Approach Volume/Capacity 0.14 0.41 0.21 0.58
Critical Lane Average Delay (sec) 5.5 7.0 6.9 10.3
Intersection Control Delay (sec) 8.4
Intersection LOS A
95% Queue Length (ft) 25 50 25 100
Asshown inTablesA2andA3,thesignalizedandroundaboutalternativesareexpected toprovide
acceptableoperationsin2030.Theroundaboutresultsinsimilarcapacityanddelaywhilemaintaining
the east andwest approaches as a single lane.The roundabout is expected to reducequeueson all
approachescomparedtothesignal.
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Co n c e p t u a l L a y o u t s
ExhibitA1showstheconceptuallayoutofthesignalizedintersection.ExhibitA2showsthelayoutof
the roundabout form.Theconceptswereconfigured to serveWB50designvehiclesand theseearly
conceptuallayoutswereusedtoassesstheplanninglevelrightofwayimpactsandthevariousaccess
managementneeds.
Exhibit A-1 Conceptual Layout of Signal Treatment
Exhibit A-2 Conceptual Layout of Roundabout Treatment
ConceptdrawingbyCH2M
HILL
Conce
ptdrawingbyCH2M
HILL
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Kittelson & Associates, Inc. A-4
A p p l i ca t i o n o f S e l e ct i o n Cr i t e r i a
TheTier1Criteriapresented inTable1were applied to evaluate the two intersection treatments.A
summary of the criteria and evaluation comments for theMurphy Road/Parrell Road intersection
evaluationispresentedinTableA4.
Table A-4 Summary of Tier 1 Criteria Evaluation
Category Criteria Evaluation Comments Conclusion
Safety
Motor VehicleSafety
Conflict points (exposure)
Severity (speed)
Signal:32 conflict pointsRoundabout: 8 conflict points
Signal:85th percentile speed on MurphyRoad = 35 mphRoundabout: Geometry on entry reducesspeeds to 15-20 mph
Highest crash rates along Murphy Roadcorridor occurred at Parrell Road
Roundaboutreduces conflicts andseverity
Non-MotorizedVehicle Safety
Conflict points (exposure)
Severity (speed)
Signal:18Roundabout: 8 conflict points
Signal:85th percentile speed on MurphyRoad = 35 mphRoundabout: Geometry on entry reducesspeeds to 15-20 mph
Roundaboutreduces conflicts andseverity
Traffic Operations
Peak-HourTraffic
Operations
Volume-to-capacity ratio
Average delay
LOS
Queue lengths
See discussion above and resultspresented in Tables A-2 and A-3
Roundabout isforecast to operatewith shorter queuelengths
Anticipated Users
Design Vehicle
Appropriate heavy vehicle
Buses
Emergency vehicles
Both were designed appropriately toaccommodate anticipated users
Neutral
Special UserNeeds
School children
Elderly
Visually impaired
ADA compliance
Roundabout simplifies roadway crossingswith two-stage crossing.
Roundaboutreduces number oflanes required tocross and provides
for two-stagecrossing.
System Context
System Effects
Adjacent traffic control
Railroad crossing
Roundabouts are being considered alongentire Murphy Road Corridor. Signal towest at 3rd Street may send platoons ofvehicles to roundabout.
No railroad crossing conflicts in vicinity
Neutral
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Category Criteria Evaluation Comments Conclusion
EnvironmentalImpact
Land use context Land along Murphy Road is primarilyresidential. Roundabouts can serve as agateway feature to neighborhoods.Roundabouts will result in less stop-and-go traffic and, thereby, reduced noise.
Roundabout fitswithin the contextof a residentialenvironment
EmergencyResponse
Response time/control delay Neither are expected to cause significantincrease in response time as delay isrelatively low on all approaches
Neutral
Context at Intersection
IntersectionFootprint
Intersection proper(physical & operationalinfluence area)
Roadway approachgeometry
Roundabout requires larger footprint toaccommodate inscribed circle diameter,which impacts properties in SW and NWquadrant more so than signal alternative.
Roundabout and signal require similar areaon approach due to need for splitter islandwith roundabout and left-turn for signal
Signal has smallerfootprint for thesite-specificconditions given.
IntersectionInfluence Area
Driveway closures or
impacts
Both will require right-in-right-out at
driveways within 200 feet. Roundaboutincludes raised splitter islands to enforcemovement restriction and provides for u-turns for movements that are restricted.
Roundabout
facilitates u-turn forrestricted driveways
R ec o mm e n d e d A l t e r n a t i v e
AroundaboutisrecommendedasthepreferredintersectioncontrolattheMurphyRoad/ParrellRoad
intersection.AsshowninTableA4,aroundaboutwillsatisfyTier1criteriabetterthanatrafficsignal
alternative.Aroundaboutwouldreducetheseverityofcrashesforvehicularandnonvehicularusers.
Operationally,thetwoalternativesareforecasttooperateatsimilarlevelsofcapacityanddelay,butthe
roundaboutalternativeisforecasttoresultinshorterqueues.Duringoffpeakperiods,theroundabout
will reduce sidestreetdelay asdriverswillnotbe required towait to receive agreen indication.Asignalized intersectionhasasmaller footprintandmay reduce impactsonadjacentproperties,buta
roundaboutisexpectedtofitwithintheresidentialsurroundingsmoresothanasignal.
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