bend roundabout evaluation and design guidelines

7/27/2019 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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C it y o f B e n d Ro u n d a b o u t E v a l u a t i o n a n d De s i g n Gu i d e l i n e s April 2010

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




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C it y o f B e n d Ro u n d a b o u t O p e r a t i o n a l A n a l y s i s G u i d e l in e s April 2010


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)


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)


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 By:

Kittelson & Associates, Inc.


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C it y o f B e n d Ro u n d a b o u t D e s i g n C o n s is t e n c y Gu i d e l in e s April 2010


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.



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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.


20/41



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.


21/41



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


22/41



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.


23/41



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.


24/41



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.


25/41



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


26/41


27/41



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.


28/41



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


29/41

City of Bend

Intersection Form EvaluationFramework

April 2010




30/41

Ci t y o f B e n d I n t e r s ec t i o n Fo r m E v al u a t i on F r a m e w o r k April 2010


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.


31/41



Exhibit 1 Intersection Evaluation Process Framework


32/41



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.


33/41



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.


34/41



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


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)


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


35/41



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)


24-Hour TrafficOperations

Average Delay

Anticipated Users

Pedestrians Crossing distances

Bicyclists

Adjacent bike facilities

Intersection specificconsiderations


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


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)


Costs

Design/Engineering Costs

Construction costs includingROW acquisition

Operations/maintenance costs(includes energy costs forsignals)


36/41



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


37/41


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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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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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


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


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


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.

bend roundabout evaluation and design guidelines

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