Basic Design Controls - Connecticut · Basic Design Controls 1.2‐1 SECTION 1, CHAPTER 2 Basic...

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Basic Design Controls 1.2‐1 SECTION 1, CHAPTER 2 Basic Design Controls 2.1 Introduction Basic design controls serve as the foundation for establishing the physical form, safety, and functionality of the transportation facility. Some design controls are inherent characteristics of the facility (e.g., its physical context and the existing transportation demands placed upon it). Other basic design controls are selected or determined by the designer, working with communities and users to address a project’s purpose and need. Selecting appropriate values or characteristics for these basic design controls is essential to achieve a safe, effective, and context sensitive design. This chapter illustrates these basic design controls and their influence on the physical characteristics of a roadway or other transportation facility: Roadway Context (Section 2.2) Roadway Users (Section 2.3) Transportation Demand (Section 2.4) Measures of Effectiveness (Section 2.5) Speed (Section 2.6) Sight Distance (Section 2.7) 2.2 Roadway Context The context of a roadway is a critical factor to consider in developing a project’s purpose and need, making fundamental design decisions such as cross‐section determination, and selecting detailed design elements such as street light fixtures or other construction materials. Development of a roadway design that is sensitive to, and respectful of, the surrounding context is important for project success. As described in Chapter 1, context‐sensitive design refers to both the process and its results. An open community process that begins early in project development is needed to ensure that there is consensus about a project’s purpose and need. This process needs to continue through the design phase so that the features of the project are assembled to

Transcript of Basic Design Controls - Connecticut · Basic Design Controls 1.2‐1 SECTION 1, CHAPTER 2 Basic...

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SECTION 1, CHAPTER 2

Basic Design Controls 

2.1 Introduction 

Basicdesigncontrolsserveasthefoundationforestablishingthephysicalform,safety,andfunctionalityofthetransportationfacility.Somedesigncontrolsareinherentcharacteristicsofthefacility(e.g.,itsphysicalcontextandtheexistingtransportationdemandsplaceduponit).Otherbasicdesigncontrolsareselectedordeterminedbythedesigner,workingwithcommunitiesanduserstoaddressaproject’spurposeandneed.Selectingappropriatevaluesorcharacteristicsforthesebasicdesigncontrolsisessentialtoachieveasafe,effective,andcontextsensitivedesign.Thischapterillustratesthesebasicdesigncontrolsandtheirinfluenceonthephysicalcharacteristicsofaroadwayorothertransportationfacility: RoadwayContext(Section2.2) RoadwayUsers(Section2.3) TransportationDemand(Section2.4) MeasuresofEffectiveness(Section2.5) Speed(Section2.6) SightDistance(Section2.7)

2.2 Roadway Context 

Thecontextofaroadwayisacriticalfactortoconsiderindevelopingaproject’spurposeandneed,makingfundamentaldesigndecisionssuchascross‐sectiondetermination,andselectingdetaileddesignelementssuchasstreetlightfixturesorotherconstructionmaterials.Developmentofaroadwaydesignthatissensitiveto,andrespectfulof,thesurroundingcontextisimportantforprojectsuccess.AsdescribedinChapter1,context‐sensitivedesignreferstoboththeprocessanditsresults.Anopencommunityprocessthatbeginsearlyinprojectdevelopmentisneededtoensurethatthereisconsensusaboutaproject’spurposeandneed.Thisprocessneedstocontinuethroughthedesignphasesothatthefeaturesoftheprojectareassembledto

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produceanoverallsolutionthatsatisfiestheproject’spurposeandneed,respectssurroundingresources,andisconsistentwithcommunityvalues.Historically,thehighwayorintersectiondesignprocesshasfocusedonaproject’stransportationelements,particularlythoseassociatedwithmotorvehicletravel.Acontext‐sensitivedesignshouldbeginwithanalysisofthecontextualelements,suchasenvironmentalandcommunityresources,oftheareathroughwhicharoadwaypasses.Asdescribedlaterinthischapter,theconceptofareatypeshasbeendevelopedtohelpthedesignerunderstandtheusers,constraints,andopportunitiesthatmaybeencounteredindifferentsettings.Oncethedesignerhasanunderstandingoftheareasurroundingtheroadandtheroad’susers,thedesignershouldconsiderthetransportationelementsoftheroadway,itsfunctionwithintheregionaltransportationsystem,andtheappropriatelevelofaccesscontrol.Thus,threemainelementsofcontextareconsideredindesign: AreaType—thesurroundingbuiltandnaturalenvironment

RoadwayType—theroletheroadwayplaysintermsofprovidingregionalconnectivityandlocalaccess

AccessControl—thedegreeofconnectionorseparationbetweentheroadwayandthesurroundinglanduse

2.2.1 Area Types 

Thecontextofaroadwaybeginswithitsenvironmentalcontext,whichincludesnearbynaturalresources,terrain,andthemanmadeenvironment(developmentpatterns,historic,cultural,andrecreationalassets).Theenvironmentalcontextcanbeadeterminantofthedesiredtypeofaccommodationfordifferentusers.Thiscontextoftenestablishesthephysicalconstraintsoftheroadwayalignmentandcross‐section,andinfluencestheselectionofmotorvehicledesignspeed.ThroughoutthisPlan,thisenvironmentalcontextisgeneralizedasareatype.Aroadwayfrequentlytraversesavarietyofchangingenvirons.Additionally,thevolumeandcharacterofpedestrian,bicycle,publictransit,andmotorvehicleactivitycanchangeconsiderablyalongitsroute.Landuseisthefundamentaldeterminantinthefunctionofaroad;aslandusechangesalongaroad,theroad’sfunctionsalsochange.Roadwaysmustbedesignedinamannerthatservestheexistinglandusewhilesupportingthecommunity’sfuturelandusegoals.Traditionally,roadwayshavebeenclassifiedeitheras“rural”or“urban.”Itisimportanttorecognizethataroadway’sformalclassificationasurbanorrural(whichisdeterminedfromcensusdatausingperiodically‐adjustedcriteriaadoptedbytheUnitedStatesOfficeofManagementandBudget)maydifferfromactualsitecircumstancesorprevailingconditions.Anexampleincludesanurbanarterialroutepassingthrougha

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portionofNorwalk.Theroutemaynotnecessarilybeclassifiedassuburban,buttheremaybeasignificantlengthoverwhichthesurroundinglanduse,prevailingspeeds,andtransportationfunctionsaremoresuburbanthanrural.Forthisreason,itisimportantforthedesigner,workingwiththecommunityandprojectreviewers,todetermineanappropriateareatypeortypesforaprojectearlyintheplanningprocess.AsmentionedinChapter1,theCityofNorwalkisverydiverseinitscharacter.AreatypesareillustrativeofthebroadrangeofenvironmentsthatthedesignermayencounterthroughouttheCity.Thedesignershouldalsoidentifyuniqueorproject‐specificcontextualelementsthatwillinfluencethedesignbeyondthosegeneralizedforthefollowingareatypes.Thesemightinclude,asexamples,schools,churches,historicfeatures,environmentalresources,areabikefacilities,beachandrecreationalareas,sidewalks,andbusstops.

2.2.1.1 Rural Area Types 

Ruralareasaregenerallyundevelopedorsparselysettledwithdevelopmentatlowdensitiesalongasmallnumberofroadwaysorclusteredinsmallvillages,asillustratedinExhibit2‐1.Ruralareasareoftendistantfromlargemetropolitancenters.Therearenotverymany‘rural’areaswithintheCityofNorwalk.

2.2.1.2 Suburban Area Types 

Suburbanareasvarywidelyincharacterandareusuallyfoundoutsidethecoreofametropolitanarea.Somecomponentsofsuburbanzonesmayappearruralincharacter,whileothersaredenselypopulatedandmorecloselyresembleurbanareas,asillustratedinExhibit2‐2.Threedifferentareatypescharacterizethesuburbancontextzone.MuchofNorwalkcanbeconsidered‘suburban’innature.

2.2.1.3 Urban Area Types 

Urbanareasaretypicallyfoundatthecoreofametropolitanarea.Inmanycases,theurbanareaincludesacentralbusinessdistrict(CBD)withhighdensitycommercialandresidentialdevelopmentsurroundingtheCBD.WhileNorwalkdoesn’thavemany,ifany,traditionalurbanlocationstherearesomeareaswhichexhibit‘urban’characteristics.Openspaceisgenerallyfoundinformalparksorurbanpreserves,asillustratedinExhibit2‐3.Althoughindividualareatypesaredescribedbelowtoillustratethelandusevariationsfoundintheurbanarea,theroadwayelementsdescribedinthesubsequentchaptersrecognizethataconsistentdesignapproachistypicallyappliedtourbanareasgiventhesimilaritiesinparcelaccess,pedestrianactivity,bicycleactivity,andtransitavailabilityacrosstheselandusevariations.

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Exhibit 2‐1  Rural Area Types 

Source: MassDOT

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Exhibit 2‐2  Suburban Area Types 

Source: MassDOT

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Exhibit 2‐3  Urban Area Types 

Source: MassDOT

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2.2.2 Roadway Types 

Thetransportationnetworkiscomposedofseveraldifferenttypesofroadwaysthatprovidedifferentfunctions,traditionallyreferredtoasait’sfunctionalclass(orclassification).Theprimarypurposeofsomeroadsistofacilitatemovementofvehicles(bicycles,cars,trucks,busesandlightrail)betweenpartsofthecommunity.Theprimarypurposeofotherroadsistoprovideaccesstotheadjoiningland.Mostroadsprovideacombinationofthesepurposes,asillustratedinExhibit2‐4.Roadwaytype,definedbythefacility’sroleinthestateandregionaltransportationsystem,togetherwithitsareatype,isanimportantcontextualconsiderationfordesign.Theroadwaytypeshouldbeselectedtoreflecttheactualrolethattheroadwayplaysinthetransportationsystem,asdefinedthroughtheprojectdevelopmentprocess.Atypicaltripwilloftenentailtravelingalongavarietyofroadwaytypes,eachofwhichprovidesadifferentdegreeoflocalaccessandadifferentdegreeofregionalconnectivity.Norwalkisnoexceptiontothiswithmultipleregionalandlocalroadwayscriss‐crossingthecommunity.TheroadwaytypereflectsitsdegreeoflocalaccessandregionalconnectivityasillustratedschematicallyinExhibit2‐5anddescribedbelow: Freewaysareprimarilyforinterstateandregionaltravel(highregional

connectivityathighspeedswithlimitedaccesstoadjacentlandandlimitedaccessforpedestriansandbicyclists).Interstate‐95,theMerrittParkway,andpartsoftheRoute7corridorarethreeexamplesoffreeway‐typeroadwayswithinNorwalk.

Majorarterialsservicestatewidetravelaswellasmajortrafficmovementswithinurbanizedareasorbetweensuburbancenters(highregionalconnectivityatawiderangeofspeeds,andalowerleveloflocalaccessthanthefollowingroadwaytypes).Route1andpartsofRoute7bothserveasmajorarterialsthroughNorwalkastheyconnectthecommunitywithbothregionalandnearbycommunities.

Minorarterialslinkcitiesandtownsinruralareasandinterconnectmajorarterialswithinurbanareas(hightomoderateregionalconnectivityatawiderangeofspeeds,andmoderatedegreesoflocalaccess).

Majorcollectorslinkarterialroadwaysandprovideconnectionsbetweencitiesandtowns(moderatetolowregionalconnectivityatawiderangeofspeeds,andhigherdegreeoflocalaccessthanarterialsandfreeways).

Minorcollectorsconnectlocalroadstomajorcollectorsandarterials(lowerregionalconnectivityatlowerspeedsandhigherdegreesoflocalaccessthanthepreviousroadwaytypes).

Localroadsandstreets—Notintendedforregionalconnectivity(lowspeedswithahighdegreeoflocalcirculationandaccess).

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Exhibit 2‐4  Conceptual Framework of Roadway Type 

Source: Adapted from Safety Effectiveness of Highway Design Features, Volume 1, Access Control, FHWA, 1992

Exhibit 2‐5  Schematic Representation of Roadway Type 

Source: MassDOT

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2.2.2.1 Relationship to the Formal Functional Classification System 

ThefunctionalclassificationsystemdevelopedbytheFederalHighwayAdministrationandappliedtoallroadwaysintheUnitedStatesremainsakeyelementofsystemplanningsothatasafeandefficienttransportationnetwork,providingthedesiredlevelofregionalconnectivityandlandaccess,isdevelopedandmaintained.Thisclassificationsystemisalsousedasadeterminateoffederalfundingeligibility.Formalfunctionalclassificationsinclude:Interstate,PrincipalArterial,RuralMinorArterialorUrbanPrincipalArterial,RuralMajorCollectororUrbanMinorArterial,andRuralMinorCollectororUrbanCollector.Thisformalclassificationoftenservesasausefulstartingpoint,butthedesignershouldnotsimplyrelyonthisformaldesignationasadesigncontrol.Theroadwaytypeshouldbeselectedtoreflecttheactualrolethattheroadwayplaysinthetransportationsystem,asdefinedthroughtheprojectdevelopmentprocess.Forexample,aroadwaymayserveahighnumberofregionaltrips,butmaypassthroughaportionoftownwithfrequentdriveways,closeintersectionspacing,andhighlevelsofpedestrianactivity.Inthiscase,theroadwayservesasbothanarterialANDalocalroad.Thedesignershouldworkcloselywiththecommunitytodeterminetheroadwaycharacteristicsandappropriatedesignconsiderationstoserveboththeregionalpurposeoftheroadwayanditsroleinthelocalsetting.

2.2.3 Access Control 

Accesscontrolisatermusedtodefinehowaccesstoadjacentpropertiesisregulatedanddesignedalongaroadway.Accesscontrolisamongthemostusefultoolsavailabletomaintainsafeandefficientroadwayoperationsforallusers.Judicioususeofmediantreatments,drivewaypermits,drivewayconsolidationandsafedrivewaygeometrycanimproveroadwaysafetyandenhancetheoperationoftheroadwithoutundueburdenonaccessingborderingproperty.Thedegreeofaccesscontrolisinfluencedbytheroadwaytypeandareatype.Forexample,accesscontrolsareusuallymorestringentonarterialsthanoncollectorsandlocalroads,reflectingthemobilityandlandaccessfunctionsoftheseroadways.Likewise,accesscontrolsareoftengivenmoreconsiderationindevelopingareaswherethereisflexibilityforfuturelandusetoconformtoanaccessmanagementplanthanindevelopedareaswherethepatternoflandusehasbeenestablished.However,thedesignershouldconsiderexistingaccesspointsalongaroadwayandthepossibilityforchangesthatareconsistentwiththeproject’spurposeandneed.Forexample,itmaybepossibletorelocate,redesign,orconsolidatedrivewaysalonganexistingroadway.Athoroughunderstandingofaccesscontrolwillhelpthedesignerselectanappropriatedesignspeed,planningparameters,anddesiredlevel‐of‐serviceforthefacility’susers.Accesscontrolisexercisedbystatute,zoning,right‐of‐waypurchases,drivewaycontrols,turningandparkingregulations,geometricdesign(e.g.,raisedmedians,gradeseparations,andfrontageroads),andlocalrightofwaypermitting,administeredbythe

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ConnecticutOfficeofStateTrafficAdministration(OSTA),localPublicWorksDepartmentsorthePlanningandZoningCommission.Roadwayscanbedesignedwiththefollowingapproachestoaccesscontrol: FullControl—Fullcontrolgivesprioritytothroughtrafficbyprovidingaccessonlyat

grade‐separatedinterchangeswithselectedpublicroads.Noat‐gradecrossingsorprivatedrivewayconnectionsareallowed.“Freeway”isthecommontermusedforthistypeofhighway.Fullaccesscontrolmaximizesthecapacity,safety,andspeedsonthefreeway.

PartialControl—Partialcontrolofaccessisanintermediatelevelbetweenfullcontrolandregulatoryrestriction.Underpartialcontrolofaccess,priorityisgiventothroughtraffic,butafewatgradeintersectionsandprivatedrivewayconnectionsmaybeallowed.Partialcontrolofaccessmaybeprovidedforcertainarterialandcollectorroadways.Theproperselectionandspacingofat‐gradeintersectionsandserviceconnectionswillprovideasafebalancebetweentheregionalconnectivityandlocalaccessfunctionsofthefacility.

Statute,ZoningandRegulation—Ifaccesspointsareproperlyspacedanddesigned,theadverseeffectsonroadwaycapacityandsafetywillbeminimized.Thedesignshouldenablevehiclestoenterandexitsafelywithaminimumofinterferencetothroughtraffic.Statutorycontrolmaybeused,forexample,onaruralorurbanarterialhighwaytolimitaccessonlytopublicroadcrossings.Drivewayregulationsandpermitsareoftenusedtocontrolthegeometricdesignofanentrance,drivewayspacing,anddrivewayproximitytopublicroadintersections.Zoningmayalsobeusedtoeffectivelycontroltheadjacentpropertydevelopmentsothatmajorgeneratorsoftrafficwillnotdevelop;however,zoningregulationsareatthediscretionofthegovernmentandPlanningandZoningCommissions,nottheDepartmentofPublicWorks.

Whilethedesignermayhavesubstantialflexibilityindefiningtheaccesscontrolduringtheprojectdevelopmentprocessfornewroadways,theoptionsmaybesubstantiallymorecomplexorlimitedonprojectsthataremodifyingexistingroadways.TheAccessManagementManualpublishedbytheTransportationResearchBoardinMayof2003providesguidanceontheapplicationofaccessmanagementtechniquesforbothexistingandnewroadways.

2.2.4 State Highways 

ThroughouttheCityofNorwalkareanumberofstateownedandmaintainedroadways(StateHighways).WhilethisPlanprovidessuggestionsandguidanceforlocalroadwaysunderthejurisdictionofthecommunity,anadditionallevelofdesignreviewandconsiderationneedstobeconsideredwhendealingwithtransportationfacilitiesthatareconsideredstatehighway.Inthesecases,theConnecticutDepartmentofTransportation(ConnDOT)needstobeconsultedifchangesarebeingconsideredorfundingisbeingsought.WhilethesuggestionsinthisPlanarenotnecessarilyapriority

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Pedestrian Body Ellipse

totheConnDOT,theguidanceprovidedshouldbeusedasastartingpointinthosedealingswiththestateauthorities.

2.3 Roadway Users 

Afundamentalexpectationinroadwaydesignisthatalluserswillbeaccommodatedsafely.Virtuallyallroadwaysserveavarietyofusersincludingpedestrians,bicyclists,motorvehicledriversandpassengers.Inafewcases,suchasfreeways,roadwaysservealmostexclusivelymotorvehicletraffic.Earlyintheprocess,thedesignerneedstodeterminethecompositionofusersanticipatedforthefacility.Appropriatelyaccountingforallusercharacteristicsisessentialforobtainingasafeandefficientroadway.Experiencedemonstratesthatwhenhumanandvehicularfactorsareproperlyaccommodated,thesafetyandeffectivenessofthehighwayorroadsystemisgreatlyenhanced.Considerationofroadwayusers’characteristicsandselectionofappropriateaccommodationcanalsoinfluenceontheroadway’seffectivenessforbusinessesandresidentialusers,theeconomichealthoftheregion,thephysicalhealthofthepopulation,andthequalityofthebuiltandnaturalenvironment.Thecharacteristicsofthesevariedroadwayusersareimportantcontrolsthatinfluencethephysicaldesignofaroadway,asdescribedinthefollowingsections.Spatialneedsandlevelofservice(LOS)foreachoftheseusersshouldbeconsidered,dependingonaproject’scontextandgoals,andtheirrelevancetoeachuser.

2.3.1 The Pedestrian 

Alltravelersarepedestriansatsomepointduringtheirtrip,andpedestriansareapartofeveryroadwayenvironment.Insomecasespedestriansareregularusersoftheroadwaywhileinothers,pedestriansmaybeusingtheroadwayinemergencycircumstances,suchasaccessingadisabledautomobile.Pedestrianfacilitiesincludesidewalks,paths,crosswalks,stairways,curbcutsandramps,andtransitstops.Dependingonthespeedandvolumeofmotorvehicletraffic,pedestriansmayalsosharetheroadoruseshoulderstocompleteatrip.Designersshouldunderstandthatthereisnosingle“designpedestrian”andthatthetransportationnetworkshouldaccommodateavarietyofpedestrians,includingpeoplewithdisabilities.Forexample,childrenperceivetheirenvironmentdifferentlyfromadultsandarenotabletojudgehowdriverswillbehave.Childrenusuallywalkmoreslowly,haveashortergait,andhavealowereyeheightthanadults.Ontheoppositeendofthespectrum,olderadultsmayrequiremoretimetocrossstreets,desiremorepredictablesurfaces,benefitfromhandrailsinsteepareas,andmayrequireplacesto

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restalongtheirroute.Peoplewhoareblindorwhohavelimitedsightrequireaudibleandtactilecuestosafelynavigatesidewalksandcrosswalks.Peoplewithlimitedcognitiveabilitiesmayrelyonsymbolsandtakelongertocrossthestreet.Peopleusingwheelchairsorscootersmaytravelacrossanintersectionfasterthansomeonewalking,butitismoredifficulttoseethemfromtheseatofatruck,SUV,orcar.Itisimportanttorecognizethatpedestriansexhibitawiderangeofphysical,cognitive,andsensoryabilities,buttheyallcomprisethepedestriansthatadesignerneedstoaccommodate.Infact,20percentofthepedestrianpopulationhassomedisability,andthatnumberisgrowingasaresultoftheagingofourpopulation.TheCityofNorwalkintendstoaccommodateallpedestriansinthedesignandconstructionofpedestrianfacilities.Whenthinkingaboutlikelypedestriantravelbetweenactivitycenters(i.e.,residencetoschool,parkingtostore,etc.),distanceistheprimaryfactorintheinitialdecisiontowalk.Mostpeoplearewillingtowalk5to10minutesatacomfortablepacetoreachadestination,whichequatestoadistanceofabout0.2to0.4mile.Althoughlongerwalkingtripsarepossible,atripof1.0mileisgenerallythelongestdistancethatmostpeoplearewillingtowalkonaregularbasis.Thedesignershouldensurethatpedestriannetworkconnectivityandsafecrossingsareprovidedbetweenactivitycenters.Inadditiontothecharacteristicsdescribedabove,thespatialdimensionsofpedestriansandtheiroperatingcharacteristicsarekeycriticalaspectsthatinfluencethedetaileddesignelementsofpedestrianfacilities.

2.3.1.1 Spatial Needs of Pedestrians 

Pedestriansrequireacertainamountofphysicalspaceinordertomaneuvercomfortably.Thespacerequirementsofpedestriansinfluencetheabilityforindividualstofreelyselecttheirspeedandthecarryingcapacityofapedestrianfacility.TheHighwayCapacityManualprovidesmethodologiesforevaluatinghowapathwayservesthedemandplaceduponit,orhowwideasidewalkshouldbeforagivendemand.Spacerequirementsarealsoinfluencedbythecharacteristicsofthosewhousewheelchairsorotherassistivedevices.Asimplifiedbodyellipseof2by1.5feetwithatotalareaof3squarefeetisusedasthebasicspaceforasinglepedestrian.Thisrepresentsthepracticalminimumspacerequiredforstandingpedestrians.Theclearspaceforapersonsittingstationaryinawheelchairisgenerallyunderstoodtobe2.5feetby4feet,althoughpeopleusingscootersandpowerchairsmayrequireevenmorespace.Apersonusingcrutches,aserviceanimal,orawalkertypicallyrequires36inchesclearwidth.Inevaluatingapedestrianfacility,anareaof8.0squarefeetistypicallyconsideredtoallowabufferzoneforeachpedestrianandapproximatelytwicethatisneededforapersonusingawheelchairorawhitecane.Thesedimensionsindicatethata3footpathwayisadequateforsinglefilepedestrianflowinonedirection,intheabsenceofverticalobstructions

Spatial Needs for Wheelchairs

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alongtheroute.However,ataminimum,a4footpathwayshouldbeprovidedexceptatpointobstructionswhereaminimumwidthof3feetisacceptable.Toallowfreepassingofpedestrians,awalkwaythatisatleastfive‐feetwide(excludingthecurbwidth)andclearofobstructionsisrequiredatevery200‐feetintervals.Walkingisoftenasocialactivity,andfrequentlypedestrianswalkinpairsorgroups.Toaccountforthiscommonbehavior,itmaybedesirabletodesignfacilitiesthatenabletwopeopletowalkorridetheirchairabreast,requiringapproximately6feetofwidth((excludingthecurbwidth).Inareaswithhighpedestriantraffic,greaterwidthsaredesirableasdescribedinChapter3.

2.3.1.2 Pedestrian Level‐of‐Service Measures 

TheHighwayCapacityManual(HCM)providesdefinitionsoflevelofservicebasedpedestrians’experiencesonspatial,comfort,exposuretotrafficanddelaymeasurements.TheHCMprovideslevel‐of‐serviceanalysisforavarietyofpedestrianfacilities,includingsidewalks,pathsandcrosswalks.PedestrianlevelsofservicearedefinedsimilartotrafficoperationsusingaLOSAtoLOSFratingsystem.Forconditionsalongstreetsegmentssuchassidewalks,thelevelofserviceisbasedonthepedestriandensityaswellaspedestriancomfortandperceivedexposuretotraffic.Forconditionsonasignalizedintersection,thelevelofserviceisbasedonpedestriandelayandperceivedexposuretoorinteractionwithtraffic.Overall,pedestrianlevelofserviceisimprovedbytheprovisionofsidewalks,widersidewalks,agreaterdegreeofseparationfromtraffic,andreduceddelayscrossingthestreetatbothsignalizedandunsignalizedlocations.Highertrafficvolumes,highertrafficspeed,andwiderstreetsalltendtoreducepedestrianlevelofservice.

2.3.2 The Bicyclist 

Safe,convenientandwell‐designedfacilitiesareessentialtoencouragebicycleuse.Roadsdesignedtoaccommodatebicyclistswithmoderateskillswillmeettheneedsofmostriders.Youngchildrenareprimarilythebicyclistswhomayrequirespecialconsideration,particularlyonneighborhoodstreets,inrecreationalareas,andclosetoschools.Moderatelyskilledbicyclistsarebestservedby: Extraoperatingspacewhenridingontheroadwaysuchasbicyclelanes,usable

shoulders,orwidecurblanes;

Lowspeedstreets(wherecarssharetravellanes);and

Anetworkofdesignatedbicyclefacilities(bicyclelanes,side‐streetbicycleroutesandsharedusepaths).

Thedesignofroadsforbicyclingshouldconsiderthesefactors:

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Providingwidthsufficientformotoriststopassbicyclistswithoutchanginglanesonhighspeedorhighvolumeroadways;

Removingroadwayobstaclesthatcouldcausebicycliststofall;

Directingbicycliststoscenicandlowtrafficroutesbyguidesignsand/orpavementmarkings;

Providingsignalizedcrossingsofmajorroadswhenwarrantedforthosewhoarenotcomfortablemakingleft‐turnsinheavytraffic;

Providingbicycleactuateddetectorswherecurrentactuationdetectionisnotcycleready.

Whenbicyclesareusedonpublicstreetsandroads,bicyclistsaresubjecttothesametrafficrulesasmotorvehicleoperatorswithsomeexceptions.Thefollowingsectionsdescribethespatialneedsandlevel‐of‐servicemeasuresforbicyclists.

2.3.2.1 Spatial Needs of Bicyclists 

Thebicyclist’soperatingcharacteristicsincluderequiredwidth,angleofleanwhennegotiatingcurves,sightdistances,andclearzones.Clearwidthrequirementsmaydiffersomewhatdependingonbicycletype.Typically,bicyclistsrequireaclearwidthofatleast40inches.Aclearwidthofatleast48inchesisnecessarytoaccommodatebicycleswithtrailersoradulttri‐cycles.Therequiredheightofoperatingspaceis100inches.Anoperatingspaceof4feetisassumedastheminimumwidthforonewaybicycletravel.Wheremotorvehicletrafficvolumes,truckandbusvolumes,orspeedsarehigh,amorecomfortableoperatingspaceof5to6feetisdesirable.Also,adjacenttoon‐streetparking,5to6feetisdesirabletoprovidespacefortheopeningofcardoorsintothetravellane.Acriticaldesignconsiderationistheminimaltiresurfacecontactwiththegroundandthesusceptibilityofbicycletirestodamage.Theminimaltirecontactmeansthatlongitudinalseamsandcracks,sand,mud,wetleaves,metalutilitycoversanddecking,andskewedrailroadtrackscanprecipitateacrash.Longitudinalcracksasnarrowasone‐quarterinchandsurfaceedgeshigherthanone‐halfinchcancauselossofcontrol.Avoidanceofroaddebrisorobstaclesforcesbicycliststoswerveandthesemaneuversareoftenunexpectedbyadriversharingthesamelane.Placementof

Bicycle Operating Space

Shared-Use Path Users

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obstaclesinthetravelpathofbicyclistsshouldbeavoided.Lastly,bicycledesignisnotaone‐size‐fits‐alleffort.Considerationshouldbegiventotheexpecteduser’sskilllevelanddesignshouldreflecttheappropriatelevelofcomfortforthatuser.

2.3.2.2 Bicycle Level‐of‐Service Measures 

Thelevelofserviceforbicyclistsonsharedusepaths(wherebicyclistssharethepathwithpedestrians,in‐lineskaters,etc.)isalsoevaluatedonaLOSAtoFscalefordifferentsettings.Forsuchpaths,thelevelofserviceisdeterminedbythenatureandnumberofinteractionsbetweenbicyclists,pedestrians,in‐lineskaters,andotherobstaclessuchasdogsandbabystrollers.BicycleLOSatintersectionsissimilartothatexperiencedbymotorvehiclessincebicyclesaresubjecttothesametrafficcontrol.TheseLOSconsiderationsareexploredinmoredetailinChapter4.Thelevelofserviceforon‐roadbicycletravelisbasedonanumberoffactorsincludingseparationfromtraffic,cross‐streetwidthsandfrequency,adjacenttrafficvolumes,speeds,heavyvehiclepercentage,on‐streetparking,andpavementquality.ProceduresfordeterminingbicyclelevelofserviceonuninterruptedbicyclefacilitiesareprovidedintheHighwayCapacityManual.Othertoolshavebeendevelopedtoassesslevelofserviceforon‐roadfacilities.ThetwoprimarysourcesaretheLandis’TowardaBicycleLevelofService(BLOS)methodologyandtheFederalHighwayAdministration’sDevelopmentoftheBicycleCompatibilityIndex(BCI).

2.3.3 The Driver 

Roadwaydesignisbasedontheassumptionthatmotorvehicledriversarecompetentandcapable;however,thedesignofaroadwayalsoneedstoaccountforalargevariationindriverskillandability.TheAASHTOPolicyonGeometricDesignofHighwaysandStreetsdiscusseshumanfactorsindetail.

2.3.3.1 Spatial Needs of Motorists 

Whenaroadwayorintersectionisunderdesign,thelargestdesignvehiclelikelytousethatfacilityonaregularbasisshouldbeusedtodeterminetheselecteddesignvalues.Typically,trucksandbusesrequirelargerdesignvaluesthanpassengercars,whichmakesdeterminingthetypeofspecificdesignvehicleanimportantdesignconsideration.Exhibit2‐6summarizestherangeofvehicledimensions.Actualvehiclewidthsmayvaryfromthedimensionslistedinthetableduetomanufacturerandaftermarketvehiclevariationssuchassideviewmirrorextensions.

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Onlocalstreetsthedesignmightfullyaccommodatesmallertruckswiththeknowledgethat,atintersections,theoccasionallargertruckcanbackupwhileturningandcanencroachuponopposinglanes.Thesetypesofdecisionsaresituation‐specificanddependonthefrequencyoflargervehicles,theamountofothertraffic,thecharacterofthearea,andotherfactors.

Exhibit 2‐6  Design Vehicle Dimensions 

Vehicle Vehicle Length

(Feet) Vehicle Width

(Feet) Operating Width1

(Feet)

Passenger Cars and Light Trucks 19.0 7.0 9.0

School Bus 35.8 8.0 10.0

Transit Bus 40.0 8.5 10.5

Single Unit Truck2 30.0 8.0 10.0

Tractor Trailer (WB 40) 45.5 8.0 10.0

Tractor-Trailer (WB-62) 69.0 8.5 10.5

Tractor-Trailer (WB-67) 73.5 8.5 10.5 Source: A Policy on the Geometric Design of Streets and Highways, AASHTO, 2011. Table 2.1b Chapter 2 Design Controls and

Criteria 1 Assuming one-foot clearance on both sides of vehicle 2 The SU-30 design vehicle is commonly used to model emergency response vehicle operations

Spatialdimensionsandmotorvehiclespeedsarecloselyrelated.Thefollowingisabriefdiscussionofthemotorvehiclecharacteristicsusedinarrivingatdesignvalues. Stoppingsightdistancesdependonthespeedofoperationandvehiclebraking

characteristics.

Horizontalcurvaturedependsonthesidefrictionbetweentireandroadway,amongotherfactors.

Truckaccelerationanddecelerationratesarefactorsinthedesignofhighwayverticalalignment.

Vehiclesarerestrictedinhowsharplytheycannegotiateaturnbytheirphysicaldimensionsandtirefriction,whichinfluencescurbradiiatintersections.

Anotherturningcharacteristicofvehiclesisthetransitionalnatureoftheirturningpath.Vehiclescannotimmediatelyturntotheirdesiredturningradiusbuthaveanenteringandexitingtransitionintothatradius.Thishasledtotheuseofcompoundcurvesonhighways.

Laneandshoulderwidthsarederivedfromthedesignwidthofvehiclesandhorizontalclearancestoallowsafeoperation.

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FurtherdiscussionofdesignvehiclesisprovidedinAASHTO’sAPolicyontheGeometricDesignofHighwaysandStreets.

2.3.3.2 Driver Level‐of‐Service Measures 

Thelevelofservicefordriversonafacilityreflectsthespeedandcapacityprovidedformotorvehicletravel.Additionally,thevehicularlevelofserviceofteninfluencesthequalityofpublictransitserviceprovidedalongaroadwaycorridor.Differentlevel‐of‐servicemeasuresapplytodifferentcomponentsoftheroadway.Ingeneral,therearetwocategoriesofvehicularlevel‐of‐servicemeasures: Uninterruptedflow(two‐lanehighways,multi‐lanehighways,freewaysegments,

andfreewayramps)forwhichlevelofserviceisbasedontheconceptsofaveragetravelspeed,percenttimefollowing,anddensitymeasures.

Interruptedflow(signalizedintersections,unsignalizedintersections,androundabouts)forwhichlevelofserviceisbasedontheamountofdelayexperiencedbyvehiclesusingthefacility.

LevelsofserviceformotorvehiclesrangefromLOSAtoLOSF,withLOSErepresentativeofoperationapproachingoratcapacity.TheHighwayCapacityManual(HCM)providesproceduresfordetermininglevelsofserviceforavarietyoffacilitytypes.

2.3.4 Public Transit  

Publictransitwithinaroadwayisusuallyprovidedwithtransitbuses.Arepresentativebususedbythelocaltransitagencyshouldbeincludedasadesignvehicleonroadwayswheretransitserviceisprovided,orisanticipatedduringtheexpectedlifeoftheproject.Thedesignershouldalsoconsiderthedesigncharacteristicsandpotentiallocationofbusstops,stations,andotherintermodalfacilities.Mostbusesareliftequipped,generatingtheneedforfive‐foot(measuredatthecurbandparalleltothevehicle)byeight‐foot(measuredfromthecurborvehicleedge)levelpadadjacenttotheaccessiblesidewalk.Thisallowsforthedeploymentoftheliftandspacetomaneuveronandoffofit.Thedesignershouldalsoensurethatpedestrianconnectivity,includingcurbcutrampsandaccessibledropoffareastothesefacilitiesareprovided.Inlessfrequentcircumstances,railtransitisprovidedalongaroadwayorwithinacentermedian.Thedetailedclearance,station,andoperationalneedsofrailtransitshouldbeintegratedintotheroadwaydesignintheseconditions.Otherfeaturessuchasexclusivelanesandtrafficsignalpre‐emptioncanimprovetransitoperationswithinaroadway.TransitdesignconsiderationsarediscussedfurtherinChapters3,4,5,and6.

60-foot transit

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Intermsoflevelofservice,therearemanymeasuresoftransitqualityofserviceasoutlinedintheTransitCapacityandQualityofServiceManual.Mostofthese,suchasvehicletype,operatinghoursandfrequencyofservice,areindependentofroadwaydesign.Forthepurposeofroadwaydesign,thekeyconsiderationsarethelocationanddesignofbusstops,thetraveltimethroughacorridor,thepedestrianandbicycleroutesconnectingtothefacility,andwaitingareastoaccesstransit.Forspecificprojects,theremaybetransitdesignelementsthatinfluencetheroadwaydesign.Wheretransitoperationsarepresentorexpected,thedesignershouldcoordinatewiththetransitagencyduringtheprojectdevelopmentprocesstoensurethattransitoperationalrequirementsareincludedinthedesign.

2.4 Transportation Demand 

Transportationdemands—volume,composition,andpatterns—areimportantdesigncontrols.Thegreaterthedemandforafacility,themoreimportantareitsoperationalandsafetycharacteristics.Thedesignermusthaveagoodunderstandingofexistingandanticipateddemandsbypedestrians,bicyclists,anddrivers.Communityplanninggoals,theselecteddesignyear,andperformancemeasuresforaprojectarekeydeterminantsofhowthedesignachievestheproject’spurposeandneed.

2.4.1 Design Year 

Projectsaredesignedtoaccommodatetraveldemandslikelytooccurwithinthelifeofthefacilityunderreasonablemaintenance.Thisinvolvesprojectingfutureconditionsforaselectedplanninghorizonyear.Projectionsoffuturedemandformajortransportationinvestmentsareusuallymadeforthe15‐to25‐yearrange.Forlargeprojects,thedesignershouldusuallyselect20yearsfromtheexpectedfacilitycompletiondateasthedesignyear.Thisisareasonablecompromisebetweenafacility'susefullife,theuncertaintiesoflongrangeprojections,andtheconsequencesofinaccurateprojections.Forsmaller,lesscapitalintensiveprojects,a5‐to10‐yearplanninghorizonisgenerallyused.Forecastsoffutureactivitylevelsshouldreflectcommunityandregionalplans,communitysetting,andtheproject’spurposeandneed.Basedontheseconsiderations,afutureconditionsforecastrepresentsatechnicalanalysisandpolicyconsensusonthetypeanddevelopedintensityoflanduse,futureregionaleconomicactivity,presenceoftransitservice,theneedsofpedestrianandbicyclists,andmanyotherfactors.Forecastsoffutureactivitylevelsshouldincludeestimatesofpedestrianandbicycleactivity.Particularcaremustbeusedwhenforecastingpedestrianandbicyclevolumes.Manytimesthereislatentdemandaboveobservedpedestrianandbicyclevolumes

To evaluate the future conditions, planners and designers first collect and evaluate existing conditions’ data to establish a baseline. 

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becausepedestrianandbicyclefacilitiesdonotyetexistintheprojectarea,aresubstandard,ordonotprovidecompleteconnectivitytoattractions.Itisimportanttoevaluatefuturelanddevelopment,includinganypotentialattractorssuchastransitstops,schools,parksandretailusesthatmaybelocatednearmoderateandhigh‐densityresidentialdevelopment.Plannersanddesignersneedtodeterminetheappropriateestimatesofactivitylevelsfordesign.Forthetypicalprojectundertakenwithinacommunity,suchasanintersectionimprovementoracorridoraccessmanagementproject,theforecastisbasedonexistingconditions.First,trafficcounts(includingpedestrianandbicycletrips)areconductedtodeterminewhenthepeakhour(s)oftrafficoccurs.Second,seasonaladjustmentsaremade,ifnecessary,toensurethecountdataarerepresentativeofatleastaverageannualconditions.Lastly,futureconditionsareestimatedbyaddingtoorsubtractingfromtheexistingtrafficvolumestoaccountforknowndevelopmentandtransportationprojects,andanannualizedfactorisgenerallyappliedtoaccountforpotentialareawidegrowthordecline.Regionaltraveldemandmodelsareoftenusedinplanninglargertransportationprojects.Althoughthetypicalprocessforforecastingtrafficvolumesassumesthattrafficwillincreaseovertime,therearesituationswheretrafficvolumesmaydeclineorremainrelativelyconstantovertime.Itisimportantthattrafficforecastsforaroadwaydesignprojectreflectlikelyconditionsovertheproject’slifeandarenotselectedarbitrarily.Municipalplanningdepartments,regionalplanningagencies,andConnDOT,canprovideassistanceinseasonaladjustmentsandinvalidatingtheassumptionsregardingfuturetrafficestimates.

2.4.2 Volume and Composition of Demand 

Thecompositionoftransportationdemandisanimportantelementinthedesignofroadways.Thedesignershoulddeveloparealisticdesignscenarioincludingthevolumeandmixofactivityforallmodesasdescribedbelow.

2.4.2.1 Pedestrian Demands 

Pedestriancountsshouldbecompletedtodeterminepedestrianflowsandpatterns.Thepedestriancountsshouldincludesidewalkdemands,crossingdemands,andstoragedemandsatcorners,trafficislands,andmedians(totalnumberofpedestrianswaitingtocrossthestreet).Inadditiontorelyingoncountsofpedestrians,thedesignershouldalsoevaluatetheprojectareatodetermineifthereislatentdemandforpedestrianaccommodationduetoanuncomfortableexistingwalkingenvironment,missinglinksinthepedestriannetwork,orexpectedchangesindevelopmentpatterns.Thelikelihoodoflatentdemandcanbeassessedbylookingatsurroundinglandusesandtheirpropensitytogenerate

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pedestrianactivity.Onecanalsolookforconditionslikepathwayswornalongtheroadsidetodetermineifpedestrianconnectivityisunderserved.Itmaybeimportanttocompletepedestriancountsforothertimesoftheday(beyondthetypicalmorningandeveningpeakhours)and/oronweekends,dependingontheprojectarea.Forexample,ifaprojectareaisheavilyinfluencedbyaschool,itisbeimportanttoobservepedestrianflowsduringmorningandmid‐afternoonperiods.Publicassemblyfacilitiesandtransitstopsorstationsalsomeritspecialconsiderationbecausetheycanproducehighvolumesofpedestriansovershortdurations.Todeterminetheappropriatelocationsforpedestriancounts(includingprojectareaintersections),itisimportanttoreviewcurrentpedestrianroutesbetweenactivitycenters.Informalpathsorcrossinglocationsmaywarrantsupplementalpedestrianobservationsduringprojectplanning.

2.4.2.2 Bicycle Demands 

Bicycledemandsshouldbecountedduringpeakhoursconcurrentwithvehicleturningmovementcounts.Aswithpedestrianactivity,thedesignershouldalsoevaluatetheprojectareatodetermineifthereispotentiallatentdemandforbicycleaccommodation.Additionalconsiderationofbicycledemandsduringotherperiodsofthedayand/oronweekendsmaywarrantsupplementalcounts,asdiscussedinthepriorsection.Methodsforforecastingbicycledemandarestillevolvingthroughnationaltransportationresearch.Commonpracticestogagefuturedemandscurrentlyincludesamplingdemandatsimilarsettingsorfacilitiesandevaluatingsurroundinglandusesfortheirpropensitytogeneratebicycleactivity.

2.4.2.3 Motor Vehicle Traffic Volumes 

Daily,peakhour,andpatternsofmotorvehicletrafficareneededasinputtotheplanninganddesignofroadwayfacilities.Somekeydefinitionsoftrafficvolumemeasuresarelistedbelow: AverageAnnualDailyTraffic(AADT)—Thetotalyearlyvolumeofautomobiles

andtrucksdividedbythenumberofdaysintheyear.

AverageDailyTraffic(ADT)—Thecalculationofaveragetrafficvolumesinatimeperiodgreaterthanonedayandlessthanoneyear.(ADTisoftenincorrectlyusedinterchangeablywithAADT.)

Peak‐HourTraffic(PH)—Thehighestnumberofvehiclespassingoverasectionofhighwayduring60consecutiveminutes.T(PH)isthePHfortrucktrafficonly.

Peak‐HourFactor(PHF)—Aratioofthetotalvolumeoccurringduringthepeakhourtothemaximumrateofflowduringagiventimeperiodwithinthepeakhour(typicallyis15minutes).

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DesignHourlyVolume(DHV)—Theone‐hourvolumeinthedesignyearselectedfordeterminingthehighwaydesign.(Inmanycases,designerslookatthetypicalworstcaseweekdaymorningoreveningpeakhourorthe30thhighesthouroftheyeartoassessthegeometricrequirementsoftheirdesign.)

K‐factor(K)—TheK‐factoristhepercentofdailytrafficthatoccursduringthepeakhour.

Manualturningmovementcounts(TMCs),includingheavyvehiclemovements,atintersections,andautomatictrafficrecorder/vehicleclassificationcounts(ATRs)countsalongroadwaysaregenerallyneededforplanninganddesignoftransportationprojectsandcanbeusedtoprovideestimatesofthevalueslistedabove.Thesecountsshouldalsoincludepedestrianandbicycleactivity,wherepresent.Pedestrianandbicyclecountsshouldbeperformedinfairweather.

2.4.2.4 Design Volumes and Traffic Composition 

Thedesignhourlyvolume(DHV),ordailypeakhours,willaffectmanydesignelementsincludingthedesirednumberoftravellanes,laneandshoulderwidth,andintersectionlayout.Thedesignvolumemayalsoinfluencethelevelofserviceprovidedandtheaccommodationappropriateforpedestriansandbicyclists.Dailytrafficestimatesarealsousefulinmakingdesigndecisionsrelatedtothetotaluserbenefitofaproposedimprovement.Forexample,thebenefitofhighwaysafetyroadsideimprovementsisdirectlyrelatedtothecrashexposure(expressedinADT)ontheroad.Sometimesselectionofthedesignhourentailsjudgmentregardingtheconversionofdailytraffictopeakhourtrafficvolumes.Othertimes,whendatafromcontinuoustrafficcountstationsareused,thedesignhourlyvolumeisbasedonthepeakingcharacteristicsofthefacilityoveranentireyear.Forruralareas,theDHVistypicallybasedonthe30thor50thhighesthour.Inurbanareas,theDHVtypicallyrepresentsthe100thhighesthour.Insomecircumstances,alesserdesignhourisappropriate.ThesedesignhourvolumesareusuallyselectedsincetheycaptureoperatingconditionsexpectedtooccuronaregularbasisandhavebeenshowntohavedependablestatisticalrelationshipstomeasuredADTonaroadway.Thechoiceofthedesignhourvolumehasasignificantimpactonthecharacteristicsofaproject.Designersshouldensurethatthedesignhourvolumeisselectedsuchthatthefacilityiswell‐matchedtothetrafficvolumesitwillcarryonaregularbasisandisnot“over‐designed.”Forexample,accommodatingahighvolumeexpectedtooccurinfrequentlywillresultinaprojectthatiscostlyandhassignificantadverseimpacts.Likewise,accommodatingalowerdesignvolumethatisfrequentlyexceededmayresultinsignificantcongestionandnotmeetthelevelofserviceexpectationsforvarioususers.Largeorheavyvehicles,suchastrucksandbuses,havedifferentoperatingcharacteristicsfrompassengercarsandbicyclesandcanaffecttrafficoperations.

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Therefore,thenumberoftrucksandbusesexpectedtouseafacilityneedstobeestimatedforboththedailyandpeakhourconditions,inplanninganddesign.Forhighwaycapacitypurposes,“heavyvehicles”aretypicallydefinedasallbuses,single‐unittrucks,andtruckcombinationsotherthanlightdeliverytrucks.(Lightdeliverytruckshavetwoaxleswithfourtires).Inaddition,theimpactoftransitoperations(suchasbusesmakingstopsalongaroadway)mustbeconsideredinoperationalanalysisoftheroadway.

2.5 Measures of Effectiveness 

Throughtheprojectdevelopmentprocessandwithpublicinput,thedesignershouldevaluatetheproject(anditsalternatives,ifapplicable)usingseveralmeasuresofeffectiveness.Suggestedmeasuresofeffectivenessandanalysistechniquesforconsiderationduringprojectplanninganddesignaredescribedbelow.Thefollowingsectionsdiscusstransportationorcontextualmeasuresofeffectiveness.

2.5.1 Transportation Measures of Effectiveness 

Thefollowingmeasuresofeffectivenessarerelatedspecificallytothetransportationfunctionofafacilityandhowthefacilityaccommodatesitsusers.

2.5.1.1 Condition of Facilities 

Statetransportationpolicyplacesanemphasisonimprovingtheconditionofexistingfacilities.Projectsonexistingfacilitiesshouldreturnafacilitytoastateofgoodrepairbyaddressingexistingstructural,pavementsurface,orotherdeficiencies.Techniquessuchaspavementtestingandbridgeinspectionscanbeusedtoidentifyexistingdeficiencies.

2.5.1.2 Safety 

Thesafetyoftransportationfacilitiesisaprimaryconcerninplanninganddesign.Someprojectsarespecificallyproposedtoaddressknownsafetyproblems;however,allprojectsshouldresultinafacilitythatsafelyaccommodatesitsusers.Corridorsafetyauditsandanalysisofcrashrecordscanbeusefulforidentifyingexistingsafetyhazards.Projectdesignelementsshouldbeselectedbasedontheirhistoricsafetyperformanceandexpectedoperatingcharacteristics.

The overall objective of the design process is to provide the desired level 

of service for each roadway user, therefore achieving a safe and efficient facility for all 

users. 

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2.5.1.3 Mode Choice 

Manyprojectsresultinimprovedaccommodationforparticularmodes.Theeffectivenessoftheseprojectscanbemeasuredbythedegreetowhichtheyallowuserstochoosethemodebest‐suitedtotheirtrippurposeandpersonalvalueswithinthebroaderframeworkofthecommunity,theregion,andtheenvironment.Thetraditionallevelofservicemeasuresdescribedbelowcanalsobeusefultoolsforevaluatingtheimprovementinaccommodationforeachusergroup.

2.5.1.4 Network Connectivity 

Inmanyinstances,projectsareproposedtofillinmissinglinkswithinanetworksothatconnectionsbyaparticularmodearepossible.Theeffectivenessoftheseprojectscanbeevaluatedbasedonthedemandfortheconnectionandhowwellthefacilitysatisfiesthatdemandusingthetraditionallevel‐of‐servicemeasuresdescribedbelow.

2.5.1.5 Level of Service 

Tocharacterizethequalityofmovementthroughatransportationnetwork,level‐of‐service(LOS)objectivesarebroadlyused.Levelsofservicetraditionallyrelatetotheproject’scontextandthedemandcharacteristicsofthefacility.Asinglelevelofserviceforatransportationfacilitythatreflectsthequalityofserviceprovidedtoalluserswouldbeideal;however,amultimodalLOSframeworkisstillatthepreliminarystageofdevelopment.Therefore,thedesignershouldevaluatetheLOSprovidedtoeachusergroupseparatelyandshouldtestdesignalternativesasnecessarytomeettheLOSgoalsforallusersoftheproject.SeveralanalyticalmethodologiesandcomputersoftwarepackagesareavailabletoestimateLOSforfacilityusers.Thedesignershouldalsocarefullyconsiderthelevel‐of‐serviceinteractionsbetweendifferentusergroupswhendesigningaroadway.Agooddesignwillprovideareasonablelevelofservicetoallusers,withinthecontextoftheproject.Asthedesignisrefined,theresultinglevelsofservicemaydifferfromthegoalsselectedatthebeginningoftheprojectdevelopmentprocess.Particularcaremustbetakenwhendeterminingdesiredlevelsofserviceandhowthatlevelofservicemeetstheneedsofroadwayusersandhelpsmeetthepurposeandneedofaproject.Ingeneral,thedesiredlevelofserviceisdeterminedthroughconsensusoftheaffectedcommunityandthefacilityowner.Likemanyelements,thedesignershouldensurethatprojectparticipantshaveathoroughunderstandingoftheresultinglevelofservicefromthedesignsothatexpectationsaremet,ortheproject’spurposeandneedisrefined.

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2.5.2 Contextual Measures of Effectiveness 

Thefollowingmeasuresofeffectivenessareassociatedwithhowthetransportationfacilityrelatestoitscontextincludingitsphysicalsurroundingsandcommunityfunction. EnvironmentalandCommunityResourcePreservation—Projectscanimpact

environmentalandcommunityresourcestodifferentdegrees.Inmanycases,highly‐effectiveprojectsminimizetheirimpactstotheseresources.GISandlandscapeanalysisarehelpfulforconsideringtheenvironmentalandculturalresourceimplicationsofaproject.Traditionalplanninganddesigntoolssuchasplan,andcross‐sectionanalysescanalsobehelpful.

AestheticsandCommunityEnhancement—Asidefromimpactstonearbyresources,transportationprojectsareanimportantaestheticelementwithintheircontext.Well‐designedfacilitiescancomplementtheirsurroundingswhilepoorly‐designedprojectscanbeadetrimenttothevisualexperienceofusersandfacilityneighbors.Somecommunityenhancementprojectsareproposedspecificallytoimprovetheaestheticsofafacilitywithinacommunity.Visualizationtechniquesincludingthree‐dimensionalmodelingandlandscapeanalysisarehelpfulforconsideringtheaestheticimplicationsofaproject.Traditionalplanninganddesigntoolssuchasplan,andcross‐sectionanalysescanalsobehelpful.

EconomicDevelopment—Economicdevelopmentisoftenanimportantconsiderationinprojectplanninganddesign.Someprojectsareproposedspecificallytospureconomicdevelopment.Inothercases,thereisconcernaroundthedevelopmentimplicationsofaproject,suchassprawl.Economicimpactandlanduseanalysescanhelpintheevaluationoftheeconomicdevelopmentpotentialandlanduseimplicationsofprojects.

EnvironmentalJustice—Projectscanserveorimpactindividualcommunitiesanddemographicgroupsdisproportionately.Demographicanalysesbasedonrace,income,andotherfactorscanbehelpfultounderstandandaddressthesedifferentialimpacts.Aprojectshouldprovideachoiceofmodesbasedontheeconomicconditionsandtypicalincomesofspecificcommunities.

ImpactMitigation—Someprojectsareproposedspecificallytoaddressenvironmentalorcommunityimpactsofexistingtransportationfacilities.Forexample,noisewallsareoftenproposedtoshieldsensitivelandusesfromhighwaynoise.Manyoftheplanningandvisualizationtechniquesdescribedaboveareavailabletoassesstheeffectivenessoftheseprojectelements.Additionally,environmentalmonitoringandmodelingtechniquesfornoise,vibration,andairqualitycanbehelpful.

Accessibility—ThefederalAmericanswithDisabilitiesActrequiresthatpublicentitiessuchastheCityprovideaccessiblesidewalksandcurbcutramps.Accessfeaturesareanimportantpartofanyprojectthatincludespedestrianfacilities.

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2.6 Speed 

Speedisanimportantfactorconsideredbytravelersinselectingatransportationmodeorroute.Speedcanalsoinfluencethephysicalcharacteristicsofthetransportationinfrastructure.Manydesignelementssuchashorizontalandverticalcurvatureandsuperelevationaredirectlyrelatedtospeed.Otherfeatures,suchaslaneandshoulderwidth,andthewidthoftheroadsiderecoveryclearzonesforerrantvehicles,canvarywith,butarenotadirectfunctionofthedesignspeed.Theobjectiveintheplanninganddesignofaroadwayistodetermineaspeedthatisappropriateforthecontext(asdescribedinSection2.2),resultsinasafefacilityforallusers,isconsistentwiththecommunity’sgoalsandobjectivesforthefacility,andmeetsuser’sexpectations.Onceanappropriatespeedisselected,thedesignerneedstotailordesignelementstothatspeed.Speedisdefinedasthedistancetraveledbyanobjectinacertainperiodoftime.Speediscommonlyexpressedinmiles‐per‐hourorfeet‐per‐secondinthecontextoftransportationplanninganddesign.Severalmeasuresandcharacteristicsofspeedareimportanttounderstandwhendesigningaroadway,asdescribedinthefollowingsections.Thesemeasuresaremostoftenusedtodescribemotorvehicleoperations,althoughtheyarealsoapplicabletopedestrianandbicyclemovement.

2.6.1 Speed Limits 

SpeedlimitsareestablishedbytheConnecticutDepartmentofTransportation(CTDOT),theDivisionofTrafficEngineering,andspecificallybytheOfficeofStateTrafficAdministration(OSTA)forstateroadwayswhiletheLegalTrafficAuthority(LTA)ofindividualtownstypicallyinconjunctionwiththeTown’sDepartmentofPublicWorksdeterminethespeedlimitsforlocalroadways.Ingeneral,theOSTAandtheTownssetregulatoryspeedlimitsbasedonanengineeringinvestigationbytheDivisionofTrafficEngineering.Principalfactorsconsideredare: Roadtypeandsurface(curve,hill,etc.)

Locationandtypeofaccesspoints(intersections,entrances,etc.)

Existingtrafficcontroldevices(signs,signals,etc.)

Accidenthistory

Trafficvolumes

Sightdistances

Testdriveresults

Radarobservations

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ThepostedspeedlimitisgenerallydeterminedbasedonanevaluationoftheobservedoperatingspeedsaccordingtothecriteriaintheManualonUniformTrafficControlDevices.Thecurrentacceptedpracticeistoestablishthepostedspeedbasedonexistingspeedinformation.Thepostedspeedshouldbethespeedatwhichthemajorityofexistingmotoristsaretravelingatorbelow.

2.6.2 Motor Vehicle Running Speed 

Runningspeedcharacterizesthetimenecessarytotravelapredetermineddistancealongaroadway(incorporatingbothtimewhilemovingandstoppeddelays).Measuresofrunningspeedcanvarysubstantiallybydayofweekandtimeofdaybasedontrafficconditions.Averagerunningspeedisusuallyusedtocharacterizeconditionsonaroadwayforanalytical(planning,routeselection,airqualityanalyses,etc.)purposesratherthanforthedesignofroadwaygeometrics.

2.6.3 Motor Vehicle Operating Speed 

Operatingspeedisthemeasuredspeedatwhichdriversareobservedoperatingtheirvehiclesinfairweatherduringoff‐peakhours.Operatingspeedismeasuredatdiscretepointsalongaroadway.Operatingspeedsareusuallyreportedusingpercentilespeedswiththe50thpercentile(average)and85thpercentile(thespeedatwhich85percentofvehiclesaretravelingatorbelow)speedsareoftenusedtocharacterizetheoperatingspeedonaroadway.Theroadway’sfeaturessuchascurvesandtopography,width,accesstoadjacentproperties,presenceofpedestriansandbicyclists,parking,trafficcontroldevices,lighting,etc.,affecttheoperatingspeed.Duringpeakperiods,whentrafficcongestionorintersectionoperationsarecontrollingmovementalongacorridor,observedoperatingspeedsmaybesubstantiallylowerthantheoperatingspeedmeasuredduringoff‐peakconditionswhentheroadway’sdesignandcontextarecontrollingspeed.Numerousstudieshaveindicatedthatdriverswillnotsignificantlyalterwhattheyconsidertobeasafeoperatingspeed,regardlessofthepostedspeedlimitunlessthereisconstantheavyenforcement.

2.6.4 Selecting Motor Vehicles Design Speed  

Designspeedistheselectedspeedusedtodeterminevariousgeometricfeaturesoftheroadway.Thedesignspeedshouldbealogicalonewithrespecttothetargetspeedandexistingoperatingspeed.Whenselectingadesignspeed,understandingtheexistingoperatingspeedandtargetspeedaddresses:(1)theneedtomeettheexpectationsofdriversbasedontheroadwayenvironment,and(2)thewaysinwhichthesettinginfluencesthedesiredspeed.Itisimportanttounderstandtheinter‐relationshipbetweenspeedandroadwaygeometry.Selectionofadesignspeedinfluencesthephysicalgeometricsoftheroadway.

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Similarly,thephysicalgeometricsoftheroadwayareimportantdeterminantsoftheoperatingspeedsthatwillresultonthefacility.Typically,thehigherthefunctionalclassification,thehigherthedesignspeed.Exhibit2‐7providesrecommendedrangesofvalues;however,wheresignificantconstraintsareencountered,otherappropriatevaluesmaybeemployed.Therelativelywiderangeofdesignspeedsrecognizestherangeofroadwaytypes,context,andtopography.Theprovisionofarangeindesignspeeds,combinedwithgeneralguidanceonselectionofadesignspeedasnotedabove,representsperhapsthegreatestflexibilityaffordedthedesigner.Designersshouldexercisejudgmentintheselectionofanappropriatedesignspeedfortheparticularcircumstancesandconditions.Ingeneral,anappropriatedesignspeedshouldbewithinapproximately5mphoftravelspeeds.Whendeterminingtheappropriatedesignspeedthedesignershouldalsoconsiderthevolumesandcompositionoftheexpectednon‐vehicularandvehiculartraffic,theanticipateddrivercharacteristics,anddriverfamiliaritywiththeroute.Thedesignershouldconsiderexpectedoperationsthroughouttheday,includingbothpeakandnon‐peakhours.Indeed,non‐peaktrafficflowwillgenerallycontroltheselectionofareasonabledesignspeed.Thedesignspeedmayvaryforanygivenrouteasittraversesrural,suburban,andurbanareas.Oncethesefactorshavebeenevaluatedandanappropriatedesignspeeddetermined,thegeometricelementsshouldbedesignedconsistentlytothatlevel.Thedesignershoulddocumentthefactorsleadingtotheselectionofanappropriatedesignspeed.Thisdocumentationisparticularlyimportantforselecteddesignspeedsbelowtheexistingpostedspeedlimit,belowthe“reasonableandproper”speedforthetypeofroadwayandareaasdiscussedinSection2.6.1,orbelowthemeasuredoperatingspeed.Whereitisnotpossibletomeettheselecteddesignspeedforonelocationordesignelementalongacorridor,adesignexceptionandappropriatewarningsignagemaybejustified,asdiscussedlaterinthissection.

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Exhibit 2‐7  Design Speed Ranges (Miles per Hour) 

Roadway Type

Arterials Collectors Local

Area Type Freeway Major* Minor Major Minor Roads

Rural Natural 50 to 75 40 to 60* 35 to 60 30 to 60 30 to 55 20 to 45

Rural Developed 50 to 75 40 to 60* 35 to 60 30 to 60 30 to 55 20 to 45

Rural Village N/A 30 to 45 30 to 40 25 to 40 25 to 35 20 to 35

Suburban Low Intensity Development 50 to 75 30 to 60* 30 to 55 30 to 55 30 to 55 20 to 45

Suburban High Intensity Development 50 to 75 30 to 50* 30 to 50 25 to 50 25 to 40 20 to 40

Suburban Town Center N/A 25 to 40 25 to 40 25 to 40 25 to 35 20 to 35

Urban 50 to 75 25 to 50 25 to 40 25 to 40 25 to 35 20 to 35 N/A Not Applicable * A higher design speed may be appropriate for arterials with full access control Source: Adapted from A Policy on Geometric Design of Highways and Streets, AASHTO, 2011 – Chapter 3, Elements of Design

Higherdesignspeedsimposegreaterchallengesandconstraintsondesigners.Designersfacedwithdifficultorconstrainedconditionsmayconsiderselectingalowerdesignspeedforanelementorportionofthehighway.Thispracticecancauseproblemsinthatalargenumberofdriversmaynot“behave”asthedesignerdesiresorintendsthemto.Designsbasedonartificiallylowspeedscanresultininappropriategeometricfeaturesthatviolatedriverexpectationsanddegradethesafetyofthehighway.Theemphasisshouldbeontheconsistencyofdesignsoasnottosurprisethemotoristwithunexpectedfeatures.Therefore,thedesignspeedshouldonlybebasedonthespeedlimitifthespeedlimitisconsistentwithexistingoperatingspeedsorphysicalconstraintsofthebuiltenvironment.Designersshouldnotproposeanalternativedesignspeedforahighwayorsegmentofaprojectasadesignexception.Aseriousfundamentalproblemwithacceptingorallowingadesignexceptionfordesignspeedisbasedonitsimportancerelativetoallfeaturesofthehighway.Areductioninthedesignspeedmaybeunlikelytoaffectoveralloperatingspeeds.Itwillpotentiallyresultintheunnecessaryreductionofallofthespeed‐relateddesigncriteriaratherthanjusttheoneortwofeaturesthatledtotheneedfortheexception.Theacceptablealternativeapproachtoadesignspeedexceptionistoevaluateeachgeometricfeatureindividually,addressingexceptionsforeachfeaturewithinthecontextoftheappropriatedesignspeed.Occasionally,projectsretaingeometricelements,suchastightcurves,superelevation,orrestrictedsightdistancesthataredesignedforaspeedlowerthanthedesignspeedforthecorridor.Thismaybeduetoadjacentlanduse,ortoenvironmentalorhistoricconstraints.Inthesecases,thedesignershouldrecommendapostedspeedconsistentwiththegeometricfeatures.Whereitisdesirabletomaintainahigherconsistentspeed

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throughoutacorridor,thedesignershouldinstallappropriatecautionarysigningatlocationswithdesignelementsthatdonotmeetthecriteriaforthepostedspeed.

2.6.5 Design Speed and Traffic Calming 

Thetermtraffic‐calmingreferstoavarietyofphysicalmeasurestoreducevehicularspeedsprimarilyinresidentialneighborhoods.Theloweringofoperatingspeedsisoftentheappropriatesolutiontoaddressingsafetyproblems.Suchproblemstypicallyinvolvevehicleconflictswithpedestrians,bicyclists,andschoolchildren.Researchhasshownthatmeasurablereductionsinoperatingspeedsarepossiblethroughtraffic‐calming.Alocalroadorstreet,andinsomeinstancesotherroadwaysthatfunctionasalocalroadorstreet,mayhaveanexistingoperatingspeedfarinexcessofthespeedlimitorthetargetspeed.Inthesecasesitmaybeacceptable,andconsistentwithgoodengineeringpractice,todevelopadesignthatwilllowertheoperatingspeed.Generally,thedesignspeedselectedfortrafficcalmingelementsshouldbeconsistentwiththetargetspeedforthecorridorasawhole.Thetrafficcalmingelementsshouldnotresultinoperatingspeedssubstantiallylowerthanthetargetspeedatcertainpointsalongthecorridorandhigherspeedselsewhere.Selectionofareasonabledesignspeedfortrafficcalmingelements,selectionoftypeofelements,andthespacingoftrafficcalmingelementscanhelpachievethedesireduniformreductioninoperatingspeedalongaroadway.Greatcaremustbeexercisedtoensurethattheproposeddesignwillactuallyreducetheoperatingspeedstolevelsconsistentwiththedesign.Theburdenisontheindividualdesignerofatraffic‐calmingfeaturetodocumentareasonableexpectationthattheproposedmeasureswillreducetheoperatingspeed.Oncetrafficcalminghasbeenimplemented,monitoringoftheperformanceoftheprojectshouldbeundertakentoassurethatspeedshaveindeedbeenreduced,andtoprovidevaluablelessonsforfuturetraffic‐calmingprojects.Chapter1‐6andChapter2‐3providesmoredetailontoolsandtechniquesfortrafficcalming.

2.6.6 High Speeds and Safety for Pedestrian and Bicyclists  

Ineverycase,thedesignershouldseektomaintainorimprovesafetyforallusergroups.Safetyisoftenmeasuredbothintermsofthelikelihoodofacrashandtheexpectedseverityofacrash.Asmotorvehiclespeedsincrease,theseverityofcrashesbetweenmotorvehiclesandbicyclesorpedestriansincreases.Inthehighspeedenvironment,safetyforpedestriansandbicyclistscanbeenhancedbyreducingtheexposureofbicyclistsandpedestrianstomotorvehicletraffic,therebyreducingthelikelihoodofcrashes.Alongroadwaysegments,greaterseparationofmotorvehicleandnon‐motorizeduserscanbeprovidedbyincludingshoulders,bicyclelanes,orbufferedsidewalks.Atcrossings,theexposureofbicyclistsandpedestrianstohighspeedmotorvehicletraffic

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canbemitigatedthroughsignal‐controlledcrossings,gradeseparation,andinstallationofcrossingislandsormedians.

2.6.7 Selecting Bicycle Design Speed 

Bicycledesignspeedisalsoanimportantconsideration.Inmostcases,thedesignspeedforbicyclesisnomorethan20mph;thus,foron‐roadtravel,thedesignspeedchosenformotorvehiclesappropriatelyaccommodatesbicycles.Sharedusepathsshouldbedesignedforaselectedspeedthatisatleastashighasthepreferredspeedofthefasterbicyclists.Currentpracticesuggestsadesignspeedof20mphforbicyclists.(Althoughbicyclistscantravelfasterthanthis,todosowouldbeinappropriateforthistypeofsharedusesetting.)Designandtrafficcontrolscanbeusedtodeterexcessivespeedandencouragefasterbicycliststousetheroadwaysystem;however,lowerdesignspeedsshouldnotbeselectedtoartificiallyloweruserspeeds.Whenadowngradeexceedsfourpercent,orwherestrongprevailingtailwindsexist,adesignspeedof30mphisadvisable.Downgradesinexcessofsixpercentshouldbeavoidedonsharedusepaths.Onunpavedpaths,wherebicycliststendtoridemoreslowly,lowerdesignspeedsof15mphformostconditions,and20mphwheretherearegrades,areappropriate.Itshouldbenotedthatbicycledesignspeedisusuallyonlyafactorwhenconsideringthedesignofoff‐roadfacilities.

2.6.8 Selecting Pedestrian Design Speed 

Muchlikeotherroadwayusers,thespeedatwhichpeoplewalkvariesconsiderably;however,walkingspeedusuallydoesnothaveasubstantialinfluenceonthegeometricdesignofroadways.Acriticalexceptiontothisisthepedestrian’sinfluenceonthedesignofintersectionsandcrosswalks,andthetimingoftrafficsignals.ThechoiceofwalkingspeedforintersectionsandtrafficsignaldesignisdiscussedintheManualonUniformTrafficControlDevices(MUTCD).

2.7 Sight Distance 

Sightdistanceisthelengthofroadwayaheadthatisvisibletotheroadwayuser.Inmostcases,specificsightdistancemeasuresapplytomotorvehiclesandbicyclists.Thefourfollowingaspectsarecommonlydiscussedformotorvehiclesightdistance: Stoppingsightdistance, Passingsightdistance, Decisionsightdistance,and Intersectionsightdistance.

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Allofthesesightdistancesarerelatedtothedesignspeedoftheroadway.ThedesignershouldrefertoAASHTO’sAPolicyonGeometricDesignofHighwaysandStreetsfordetailedinformationfortheuseandcalculationofsightdistances.

2.7.1 Stopping Sight Distance 

Theprovisionofadequatestoppingsightdistance(SSD)isacriticalsightdistanceconsiderationfordesignandisdescribedinmoredetailbelow.

2.7.1.1 Motor Vehicle Stopping Sight Distance 

Stoppingsightdistanceisthedistancenecessaryforavehicletravelingatthedesignspeedtostopbeforereachingastationaryobjectinitspath.Thesightdistanceateverypointalongaroadwayshouldbeatleastthestoppingsightdistance.Exhibit2‐8providesstoppingsightdistancesforarangeofdesignspeedsandgrades.

2.7.1.2 Bicycle Stopping Sight Distance 

Foron‐roadtravel,thestoppingsightdistanceformotorvehiclesappropriatelyaccommodatesbicycles.However,bicyclestoppingsightdistanceisanimportantconsiderationinthedesignofoff‐roadfacilitiessuchassharedusepaths.

2.7.1.3 Sight Distance for Pedestrians 

Thereisnotaparallel“stoppingsightdistance”considerationforpedestrianssincetheyusuallytravelatlowerspeedsandcanstopwithinafewfeet.However,thedesignermustconsidertheimportanceofpedestrians’abilitytoviewandreacttopotentialconflicts.Thedesignershouldprovideadequatesightlinesatstreetcrossings,aroundcorners,andatotherlocationswherepedestriansinterfacewithotherusers.Forexample,atstreetcrossinglocations,pedestriansshouldbeabletoseeasufficientportionofthetrafficstreamtojudgethesuitabilityofgapsforcrossingthestreet.

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Exhibit 2‐8  Motor Vehicle Stopping Sight Distances 

Design Speed

Stopping Sight Distance (ft) by Percent Grade (%)

Downgrade Upgrade 0 3 6 9 3 6 9

20 115 116 120 126 109 107 104 25 155 158 165 173 147 143 140 30 200 205 215 227 200 184 179 35 250 257 271 287 237 229 222 40 305 315 333 354 289 278 269 45 360 378 400 427 344 331 320 50 425 446 474 507 405 388 375 55 495 520 553 593 469 450 433 60 570 598 638 686 538 515 495 65 645 682 728 785 612 584 561 70 730 771 825 891 690 658 631 75 820 866 927 1003 772 736 704

Source: A Policy on Geometric Design of Streets and Highways, AASHTO, Washington DC, 2011. Chapter 3 Elements of Design Table 3-2

2.7.2 Passing Sight Distance 

Fortwo‐lanehighways,passingmaneuversinwhichfastervehiclesmoveaheadofslowervehiclesmustbeaccomplishedonlanesregularlyusedbyopposingtraffic.Ifpassingistobeaccomplishedsafely,passingsightdistanceisnecessarytoallowthepassingdrivertoseeasufficientdistanceahead,clearoftraffic,tocompletethepassingmaneuverwithoutcuttingoffthepassedvehicleandbeforemeetinganopposingvehiclethatappearsduringthemaneuver.TheAASHTO’sAPolicyonGeometricDesignofHighwaysandStreetsincludesdetailedinformationfortheuseandcalculationofpassingsightdistances.

2.7.3 Decision Sight Distance 

Decisionsightdistanceaddsadimensionoftimetostoppingsightdistancetoallowadrivertodetectandreacttoanunexpectedconditionalongaroadway.Decisionsightdistanceissuggestedwhenthereisevidencethatitwouldbeprudenttoprovidelongersightdistance,suchaswhencomplexdecisionsareneededorwheninformationisdifficulttoperceive.Itisthedistanceneededforadrivertodetectanunexpectedorotherwisedifficult‐to‐perceiveinformationsourceorconditioninaroadwayenvironmentthatmaybevisuallycluttered,recognizetheconditionoritspotentialthreat,selectanappropriatespeedandpath,andinitiateandcompletethemaneuversafelyandefficiently.Exhibit2‐9providesdecisionsightdistancesforarangeofdesignspeeds.

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2.7.4 Intersection Sight Distance 

Sightdistanceisprovidedatintersectionstoallowdriverstoperceivethepresenceofpotentiallyconflictingvehicles.Thisshouldoccurinsufficienttimeforamotoristtostoporadjusttheirspeed,asappropriate,toavoidcollidingintheintersection.Sightdistancealsoallowsdriversofstoppedvehicleswithasufficientviewoftheintersectingroadwaytodecidewhentoenterorcrosstheintersectingroadway.Iftheavailablesightdistanceforanenteringorcrossingvehiclesisatleastequaltotheappropriatestoppingsightdistanceforthemajorroad,thendrivershavesufficientsightdistancetoanticipateoravoidcollisions.However,insomecases,thismayrequireamajor‐roadvehicletosloworstoptoaccommodatethemaneuverbyaminor‐roadvehicle.

Exhibit 2‐9  Decision Sight Distances 

Decision Sight Distance (ft) Avoidance Maneuver

Design Speed A B C D E 30 220 490 450 535 620 35 275 590 525 625 720 40 330 690 600 715 825 45 395 800 675 800 930 50 465 910 750 890 1030 55 535 1030 865 980 1135 60 610 1150 990 1125 1280 65 695 1275 1050 1220 1365 70 780 1410 1105 1275 1445 75 875 1545 1180 1365 1545

Avoidance Maneuver A: Stop on rural road: time (t) = 3.0 sec Avoidance Maneuver B: Stop on urban road: time (t) = 9.1 sec Avoidance Maneuver C: Speed/path/direction change on rural road: time (t) varies between 10.2 and 11.2 sec Avoidance Maneuver D: Speed/path/direction change on suburban road: time (t) varies between 12.1 and 12.9 sec Avoidance Maneuver E: Speed/path/direction change on urban road: t varies between 14.0 and 14.5 sec Source: A Policy on Geometric Design of Streets and Highways, AASHTO, Washington DC, 2011. Chapter 3, Elements of Design

Table 3-3

Toenhancetrafficoperations,intersectionsightdistancesthatexceedstoppingsightdistancesaredesirable.TheHighwayCapacityManualprovidesguidanceongapacceptanceforvehiclesdepartingfromminorapproacheswhichcanbeusedtocalculateonemeasureofintersectionsightdistance.Additionally,AASHTO’sAPolicyontheGeometricDesignofHighwaysandStreetsprovidesprocedurestodeterminedesirablesightdistancesatintersectionsforvariouscasesaredescribedbelowandinclude: CaseA–Intersectionswithnocontrolonanyapproach

CaseB–Intersectionswithstopcontrolontheminorstreet

CaseC–Intersectionswithyieldcontrolontheminorstreet

CaseD–Intersectionswithtrafficsignalcontrol

CaseE–Intersectionswithall‐waystopsigncontrol

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CaseF–Leftturnsfromthemajorroad

2.7.4.1 Intersection Sight Triangle 

Clearsighttrianglesarethoseareasalongtheintersectionapproachlegsthatshouldbeclearofobstructionsthatcanblockroaduser’sviewofoncomingtraffic.Thedimensionsofthetrianglearebasedonthedesignspeedoftheintersectingroadwaysandthetypeoftrafficcontrolusedattheintersection,gradesontheroadways,andtheroadwaywidth.Twotypesofclearsighttrianglesareusedateachintersection:approachsighttrianglesanddeparturesighttriangles.Approachsighttrianglesareapplicableforwhentheminorroaddriverisinmotionwhiledeparturesighttrianglesapplywhentheminorroadvehicleisacceleratingfromastopposition.

2.7.4.2 Identification of Sight Obstructions within Sight Triangles 

Withinasighttriangletherearemanyobstructionsthatcanobscurethedriver’sviewofoncomingvehicles.Thesemayincludebuildings,vegetation,longitudinalbarriersorretainingwalls,sideslopes,etc.Thehorizontalandverticalalignmentoftheintersectingroadwaysandanyvisualobstructionsshouldbeconsidered.Fordesignpurposes,thedriver’seyeisassumedtobe3.5feetabovetheroadway.Theobjectthatisusedfordesignapproximatestheheightofanautomobileandisassumedtobe3.5feetabovetheroadway.Wherethesightdistancevalueusedindesignisbasedonasingle‐unitorcombinationtruckasthedesignvehicle,itisalsoappropriatetousetheeyeheightofatruckdriverincheckingsightobstructions.Therecommendedvalueofatruckdriver’seyeheightis7.6feetabovetheroadwaysurface.

2.7.4.3 Case A – Intersections with No Control on Any Approach 

Whereintersectionmovementsarenotcontrolledbyatrafficcontroldevice(i.e.,signal,STOPorYIELDsign),driversapproachingtheintersectionfromanydirectionmustbeabletoseepotentiallyconflictingvehiclesinsufficienttimetostopbeforereachingtheintersection.Theintersectionsighttriangle,asillustratedinExhibit2‐10isformedbythesightdistancealongtheminorstreet(indicatedasDistanceA)andtheintersectionsightdistancealongthemajorstreet(indicatedasDistanceB).Thecorrespondingdistances,arrayedbydesignspeedarebasedonthedistancetraveledastheapproachingdriverperceivesandreactstothepresenceofapossiblyconflictingvehicle,andbringstheirownvehicletoastop.Forexample,basedonthevaluesExhibit2‐10,anintersectionofamajorstreetwithadesignspeedof40milesperhourwithaminorstreetwithadesignspeedof25milesperhourwouldrequireasightdistancedefinedbyanintersectionsightdistanceof195feet(majorstreet)and115feet(minorstreet).Iftheminorstreetwasona6percentgradethentheintersectionsightdistancewouldbe127feet(115feetmultipliedbythe1.1gradeadjustmentfactor)forthedowngradeand104feetfortheupgrade.

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Exhibit 2‐10    Sight Triangle Case A 

Approach Sight Triangles

Design Speed (mph)

Sight Triangle Legs: Case A – No Traffic Control Length of Legs, both major and minor streets, A and B

(feet)

15 70 20 90 25 115 30 140 35 165 40 195 45 220 50 245 55 285 60 325 65 365 70 405 75 445

For approach grades greater than 3 percent, apply factors below.

Approach Grade %

Approach Grade Adjustments to Sight Distance Design Speed (mph)

15 20 25 30 35 40 45 50 55 60 65+ -6 -5 -4

-3 to +3 +4 +5 +6

1.1 1.0 1.0 1.0 1.0 1.0 1.0

1.1 1.0 1.0 1.0 1.0 1.0 1.0

1.1 1.1 1.0 1.0 1.0 1.0 0.9

1.1 1.1 1.1 1.0 1.0 0.9 0.9

1.1 1.1 1.1 1.0 0.9 0.9 0.9

1.1 1.1 1.1 1.0 0.9 0.9 0.9

1.1 1.1 1.1 1.0 0.9 0.9 0.9

1.2 1.1 1.1 1.0 0.9 0.9 0.9

1.2 1.1 1.1 1.0 0.9 0.9 0.9

1.2 1.1 1.1 1.0 0.9 0.9 0.9

1.2 1.2 1.1 1.0 0.9 0.9 0.9

Source: A Policy on Geometric Design of Streets and Highways, AASHTO, Washington DC, 2011. Chapter 9 Intersections

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2.7.4.4 Case B – Stop Control on Minor Street 

Atanintersectionwithstopcontrolontheminorstreet,asillustratedinExhibit3‐11,thestoppedminor‐streetdrivermustbeabletoseemotorvehiclesandbicyclesapproachingonthemajorstreetfromeitherdirection,atsufficientdistancetoallowcrossingorturningmaneuversfromtheminorstreet.Thelegoftheintersectionsighttriangleontheminorstreet(DimensionA)isthedistancebetweenthedriver’seyeandfrontofvehicle(8feet)plusdistancefromfrontofvehicletoedgeofpavement(6.5feet,prefer10feet)plusthedistancefromedgeofpavementtomiddleoflaneofinterest(e.g.,6feetforarightturn,18feetforaleftturnonanundivided2lanehighway,etc.)Themajorstreetlegofthetriangleistheintersectionsightdistancealongthemajorroad(DimensionB).

Left Turns from Stop Controlled Minor Street 

Formotorvehiclesmakingaleftturn,theintersectionsightdistancealongthemajorstreet(DimensionB)isgivenforanintersectionof2lanestreetsinExhibit3‐11.Forexample,atadesignspeedof35milesperhouronthemajorstreet,andwiththeminorstreetdriver’seyeat14.5feetfromtheedgeofthemajorstreettravellane,theintersectionsightdistance(DimensionB)is390feet.Itisrecommendedthatthisintersectionsightdistance(DimensionB)beappliedalongthemajorstreetinbothdirectionsfromtheintersection.

Right Turns from Stop Controlled Minor Street 

Formotorvehiclesmakingarightturnfromtheminorstreet,theintersectionsightdistancesaregiveninExhibit2‐11.

Through Movement from Stop Controlled Minor Street 

Formotorvehiclescrossingthemajorstreetfromastop‐controlledminorstreet,theintersectionsightdistancesaregiveninExhibit2‐11.

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Exhibit 2‐11  Sight Triangle Case B 

Departure Sight Triangles

Sight Triangle Legs: Case B – Stop Control on Cross Street

Major Street Design Speed (mph)

Length of Sight Triangle Legs (feet)

Minor Street for Vehicles Approaching from Right (AR, feet)

Minor Street for Vehicles Approaching

from Left (AL, feet)

Major Street for Left Turns

(B, feet)

Major Street for Right Turns or

Through (B, feet)

15 20 25 30 35 40 45 50 55 60 65 70 75

32.5 32.5 32.5 32.5 32.5 32.5 32.5 32.5 32.5 32.5 32.5 32.5 32.5

20.5 20.5 20.5 20.5 20.5 20.5 20.5 20.5 20.5 20.5 20.5 20.5 20.5

170 225 280 335 390 445 500 555 610 665 720 775 830

145 195 240 290 335 385 430 480 530 575 625 670 720

Sight triangle legs shown are for passenger car crossing or turning into a two-lane street, with grades (all approaches) 3 percent or less. For other grades and for other major street widths, recalculate using AASHTO Green Book formulas. Source: A Policy on Geometric Design of Streets and Highways, AASHTO, Washington DC, 2011. Chapter 9 Intersections

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2.7.4.5 Case C – Yield Control 

Atintersectionswithyieldcontrolontheminorstreet,theminorstreetdriverarepermittedtoenterorcrossthemajorroadwithoutstopping,iftherearenopotentiallyconflictingvehicles.Yield‐controlledapproachesgenerallyneedgreatersightdistancethanstop‐controlledapproaches.Forfour‐legintersectionswithyieldcontrolontheminorroad,twoseparatepairsofapproachsighttrianglesshouldbeprovided–onesettoaccommodatecrossingthemajorroadandtheothertoaccommodateleftandrightturns.Bothsetsofsighttrianglesshouldbecheckedforpotentialsightobstructions.Forthree‐legintersectionswithyieldcontrolontheminorroad,onlythesighttrianglestoaccommodateleftandrightturnsneedtobechecked.ThemajorandminorstreetlegsofthesighttriangleareshowninExhibit2‐12.

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Exhibit 3‐12  Sight Triangle — Case C 

Sight Triangle — Case C: Yield Control on Cross Street

Design Speed (Major Street, mph)

Crossing without Stopping Sight Triangle Legsa

(feet)

Left and Right Turn without Stopping Sight Triangle Legsa

(feet) Minor Street

(A, feet) Major Streetb

(B, feet) Minor Street

(A, feet) Major Street

(B, feet) 15 20 25 30 35 40 45 50 55 60 65 70 75

80 115 155 200 250 305 360 425 495 570 645 730 720

145 195 240 290 335 385 430 480 530 575 625 670 720

80 115 155 200 250 305 360 425 495 570 645 730 720

180 240 295 355 415 475 530 590 650 710 765 825 885

a Sight triangle legs shown are for passenger car crossing or turning into a two-lane street, with grades (all approaches) 3 percent or less. For other grades and major street widths, recalculate length of legs from AASHTO Green Book formulas.

b Lengths are for design speeds of 20 to 50 mph on minor road. For other minor road design speeds, recalculate length of legs from AASHTO Green Book formulas.

Source: A Policy on Geometric Design of Streets and Highways, AASHTO, Washington DC, 2011. Chapter 9 Intersections

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Case C – Yield Control at Roundabouts 

Atroundabouts,thelocationneedingevaluationofintersectionsightdistanceisattheentries.Theentrysightdistanceevaluationusestwoconflictingapproaches:enteringstream(i.e.,thosevehiclesenteringfromtheimmediateupstreamentry)andcirculatingstream(i.e.,thosevehiclesonthecircularroadway).ThelengthoftheconflictinglegisshowninExhibit3‐13forarangeofconflictingapproachspeeds.Thesightdistancelegsforroundaboutsfollowthecurvatureoftheroadway,thereforedistancesshouldbemeasurednotasstraightlinesbutasdistancesalongthevehicularpath.TheFHWARoundaboutGuiderecommendslimitingthelengthoftheapproachlegofthesighttriangleto49feet.

Exhibit 3‐13  Roundabout Intersection Sight Distance: Computed Length of Conflicting Leg 

Conflicting Approach Speed (mph)

Computed Distance (ft)

10 15 20 25 30

95.4 143.0 190.1 238.6 286.3

Source: Roundabouts: An Informational Guide, FHWA, Washington DC, 2010.

2.7.4.6 Case D – Intersections with Traffic Signal Control 

Atsignalizedintersections,thefirstvehiclestoppedononeapproachshouldbevisibletothedriverofthefirstvehiclestoppedoneachoftheotherapproaches.Whererightturnsonredarepermitted,thesightdistancetriangleforarightturnfromstopapplies(CaseB).Leftturningmotorvehiclesandbicyclesshouldhavesufficientsightdistance,intotheopposingroadway,tobeabletoselectgapssufficienttomaketheirleft‐turnmovement(CaseB).Wherethisdistanceisinsufficient,mostlikelyduetoverticalorhorizontalcurvature,theremediescanincludeconfiningtheleftturntoaprotectedsignalphase,orprohibitingtheleftturn.A“NoTurnonRed”(NTOR)signmayalsobenecessaryifthesightlinetotheleftisnotsufficientasindicatedinCaseB.Further,itmaybenecessarytoremovetheprogrammedflashingoperationofthetrafficsignal.

2.7.4.7 Case E – Intersections with All‐Way Stop Control 

Atintersectionswithall‐waystopcontrol,thefirststoppedvehicleononeapproachshouldbevisibletothedriversofthefirststoppedvehiclesoneachoftheotherapproaches.Forthisreason,all‐waystopcontrolmaybeapreferableoptionatintersectionswhere,duetotopographicorman‐madeconstraints,sightdistancesforothertypesofcontrolcannotbeobtained.

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Section 1, Chapter 2

  Basic Design Controls  1.2‐41 

2.7.4.8 Case F – Left Turns from the Major Road  

Driversturningleftacrossoncomingtrafficofamajorroadwayrequiresufficientsightdistancetodeterminewhenthereistimetocompletethemaneuver.IfstoppingsightdistancehasbeenprovidedcontinuouslyalongthemajorroadandifsightdistanceforCaseB(stopcontrol)orCaseC(yieldcontrol)hasbeenprovidedforeachminor‐roadapproach,sightdistancewillgenerallybeadequateforleftturnsfromthemajorroads.Therefore,noseparatecheckofsightdistanceforCaseFmaybeneeded.However,atthree‐legintersectionsordrivewayslocatedonornearahorizontalcurveorcrestverticalcurveonthemajorroad,theavailabilityofadequatesightdistanceforleftturnsfromthemajorroadshouldbechecked.Inaddition,theavailabilityofsightdistancefromdividedhighwaysshouldbecheckedbecauseofthepossibilityofsightobstructionsinthemedian.IntersectionsightdistancesforCaseFislistedinExhibit2‐14.

Exhibit 3‐14  Case F Intersection Sight Distance 

Design Speed (mph)

Sight Triangle Leg (feet)

15 20 25 30 35 40 45 50 55 60 65 70 75 80

125 165 205 245 285 325 365 405 445 490 530 570 610 650

Intersection sight distances shown are for passenger car making a left turn from an undivided highway. For other conditions and design vehicles, recalculate length of legs using AASHTO Green Book formulas. Source: A Policy on Geometric Design of Streets and Highways, AASHTO, Washington DC, 2011. Chapter 9 Intersections

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1.2‐42  Basic Design Controls   

2.8 For Further Information: 

APolicyonGeometricDesignofHighwaysandStreets,6thEdition,AASHTO,2011.

GuidefortheDevelopmentofBicycleFacilities,AASHTO,2012

UrbanBikewayDesignGuide,NACTO,2011.

GuideforthePlanning,Design,andOperationofPedestrianFacilities,AASHTO,2004

AGuidetoAchievingFlexibilityinHighwayDesign,AASHTO,2004.

HighwayCapacityManual,TransportationResearchBoard,2010.

ManualonUniformTrafficControlDevices,FederalHighwayAdministration,2009.

Real‐TimeHumanPerceptions:TowardaBicycleLevelofService,Landis,Bruce,TransportationResearchRecord1578,WashingtonDC,TransportationResearchBoard,1997.

DevelopmentoftheBicycleCompatibilityIndex:ALevelofServiceConcept,FinalReport,FHWA‐RD‐98‐072,1998.

DevelopmentoftheBicycleCompatibilityIndex:ALevelofServiceConcept,ImplementationManual,FHWA‐RD‐98‐095,1998.

TransitCapacityandQualityofServiceManual,TransportationResearchBoard,TransitCooperativeResearchProgram.Report100,2ndEdition,2003.

TRBSpecialReport254ManagingSpeed:ReviewofCurrentPracticesforSettingandEnforcingSpeedLimits,TransportationResearchBoard,WashingtonDC,1998.

FlexibilityinHighwayDesign,FederalHighwayAdministration,Washington,DC.

AGuidetoBestPracticesinContextSensitiveSolutions,TransportationResearchBoard,NationalCooperativeHighwayResearchProgram.Report480.WashingtonDC,2002.

ADAAGManual:AGuidetotheAmericanswithDisabilitiesActAccessibilityGuidelines,TheU.S.ArchitecturalandTransportationBarriersComplianceBoard(TheAccessBoard).Washington,DC,2002.

StandardsandAnthropometryforWheeledMobility,TheU.S.ArchitecturalandTransportationBarriersComplianceBoard(TheAccessBoard).Washington,DC,2005.


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