Chapter 6 CROSSSECTION ELEMENTSWord格式文档下载.docx

Chapter 6 CROSSSECTION ELEMENTSWord格式文档下载.docx

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BACKGROUND

Thecrosssectionofaroadincludessomeorallofthefollowingelements:

∙Traveledway（theportionoftheroadwayprovidedforthemovementofvehicles,exclusiveofshoulders）

∙Roadway（theportionofahighway,includingshoulders,providedforvehicularuse）

∙Medianarea（thephysicalorpaintedseparationprovidedondividedhighwaysbetweentwoadjacentroadways）

∙Bicycleandpedestrianfacilities

∙Utilityandlandscapeareas

∙Drainagechannelsandsideslopes

∙Clearzonewidth（i.e.,thedistancefromtheedgeofthetraveledwaytoeitherafixedobstacleornontraversableslope）

Consideredasasingleunit,allthesecrosssectionelementsdefinethehighwayrightofway.Therightofwaycanbedescribedgenerallyasthepubliclyownedparceloflandthatencompassesallthevariouscrosssectionelements（seeFigures6.1and6.2）.

Figure6.1

Twolaneruralhighwaycrosssectiondesignfeaturesandterms.

Somedecisionsaboutcrosssectionaremadeduringprojectdevelopment,suchasthecapacityandnumberoflanesforthefacility.Otherdecisions,suchasfunctionalclassification,aremadeearlierintheprocess.Withintheseparameters,theGreenBookguidelinesrecommendarangeofvaluesforthedimensionstouseforcrosssectionalelements.Decidingwhichoftheelementstoincludeandselectingtheappropriatedimensionswithintheserangesistheroleofthedesigner.

Figure6.2

Urbanhighwaycrosssectiondesignfeaturesandterms.

Inselectingtheappropriatecrosssectionelementsanddimensions,designersneedtoconsideranumberoffactors,includingthefollowing:

∙Volumeandcomposition（percenttrucks,buses,andrecreationalvehicles）ofthevehiculartrafficexpectedtousethefacility

∙Thelikelihoodthatbicyclistsandpedestrianswillusetheroute

∙Climaticconditions（e.g.,theneedtoprovidestoragespaceforplowedsnow）

∙Thepresenceofnaturalorhumanmadeobstructionsadjacenttotheroadway（e.g.,rockcliffs,largetrees,wetlands,buildings,powerlines）

∙Typeandintensityofdevelopmentalongthesectionofthehighwayfacilitythatisbeingdesigned

∙Safetyoftheusers

Themostappropriatedesignforahighwayimprovementistheonethatbalancesthemobilityneedsofthepeopleusingthefacility（motorists,pedestrians,orbicyclists）withthephysicalconstraintsofthecorridorwithinwhichthefacilityislocated.

ility.

CROSSSECTIONELEMENTS

TravelLanes

Thenumberoflanesneededforafacilityisusuallydeterminedduringtheconceptstageofprojectdevelopment.Itisusuallythenumberoflanesnecessarytoaccommodatetheexpectedtrafficvolumesatalevelofservicedeterminedtobeappropriateforthefacility（seeChapter4foradiscussionoflevelofservice）.Thenumberoflanescanonlybeaddedinintegerunits,i.e.,atwolanehighwaycanbewidenedtothreeorfourlanes.Eachadditionallanerepresentsanincreaseinthetrafficcarryingcapabilityofthefacility.

Knowingfutureprojectedtraveldemands,thedesigner,usingtheanalysisproceduresintheHighwayCapacityManual,canprovideinputintothedecisionmakingprocessduringprojectdevelopmenttodeterminetheappropriatenumberoftravellanesforthelevelofservicedesired.Communityinputalsoplaysapartinthisdecision.Acommunitymaydecidethroughpublicinvolvementthatalowerlevelofserviceisacceptableforthesituationthanthelevelofservicenormallyprovidedfornewconstructionprojects.

Inurbanandsuburbanareas,signalizedintersectionsareusuallythepredominantfactorcontrollingthecapacityofthehighwayorstreet.Theremaybemorelatitudeindeterminingthenumberoflanesforthesetypesoffacilities.Forexample,atwolanefacilityapproachinganintersectioncanbeexpandedtofourlanes（oneleftturnlane,twothroughlanes,onerightturnlane）attheintersectionitselfandthenreturnedtotwolanesbeyondtheintersection.Theneedtodistributetrafficsafelywilldeterminetheneedforanyexpansionoftheapproachroadway.Theaddedlanesattheintersectioncanbeinavarietyofconfigurationstoservethetraveldesiresofthetraffic.

LaneWidth

Thewidthoftravellanesislimitedbythephysicaldimensionsofautomobilesandtruckstoarangebetween2.7and3.6m（9and12ft）.Generally,asthedesignspeedofahighwayincreases,somustthelanewidthtoallowforthelateralmovementofvehicleswithinthelane.However,constrictedrightofwayandotherdesignrestrictionscanhaveanimpactonthisdecision.ChapterIVoftheGreenBookrecognizestheneedforflexibilityinthesecases:

Althoughlanewidthsof3.6maredesirableonbothruralandurbanfacilities,therearecircumstancesthatnecessitatetheuseoflaneslessthan3.6mwide.Inurbanareaswhererightofwayandexistingdevelopmentbecomestringentcontrols,theuseof3.3mlanesisacceptable.Lanes3.0mwideareacceptableonlowspeedfacilities.Lanes2.7mwideareappropriateonlowvolumeroadsinruralandresidentialareas.

Alandscapedmedian.

（I35E,St.Paul,MN）

Medians

Animportantconsiderationinthedesignofanymultilanehighwayiswhethertoprovideamedianand,ifoneisprovided,whatthedimensionsshouldbe.Theprimaryfunctionsofhighwaymediansareto:

∙Separateopposingtrafficflows

∙Providearecoveryareaforoutofcontrolvehicles

∙AllowspaceforspeedchangesandleftturningandUturningvehicles

∙Minimizeheadlightglare

∙Providewidthforfuturelanes（particularlyinsuburbanareas）

∙Provideaspaceforlandscapeplantingthatisinkeepingwithsafetyneedsandimprovestheaestheticsofthefacility

∙Provideaspaceforbarriers.

Dependingonagencypracticeandspecificlocationrequirements,mediansmaybedepressed,raised,orflushwiththesurfaceofthetraveledway.Mediansshouldhaveadimensionthatisinbalancewiththeotherelementsofthetotalhighwaycrosssection.Thegeneralrangeofmedianwidthsisfrom1.2m（4ft）,usuallyinurbanareas,to24m（80ft）ormore,inruralareas.Anoffsetofatleasta500mm（1.5ft）shouldbeprovidedbetweenanyverticalelementlocatedwithinthemedian,suchasacurborbarrier,andtheedgeoftheadjacenttraveledlane.

Thedesignandwidthofmediansagainrequiretradeoffsfordesigners.Inlocationswherethetotalavailablerightofwayisrestricted,awidemedianmaynotbedesirableifitrequiresnarrowingtheareasadjacenttotheoutsideedgeofthetraveledway.Areasonableborderwidthisrequiredtoserveasabufferbetweenprivatedevelopmentalongtheroadandtheedgeofthetraveledway,andspacemaybeneededforsidewalks,highwaysigns,utilities,parking,drainagechannelsandstructures,properslopesandclearzones,andanyretainednativeplantmaterial.Ontheotherhand,widermediansprovidemorespaceforplantmaterial,offerarefugeforpedestriansatintersections,andhelpsoftenthelookoftheroadway.Includinganddesigningmediansrequirespublicinputtofindthedesignthatmeetstheneedsofthecommunity.

Twowayleftturnlanesimprovesafetyandefficiencyforvehiculartrafficbutdonotaffordasaferefugeforpedestrians.

TheuseoftwowayleftturnlanesonurbanstreetsindenselydevelopedsuburbancommercialareashasincreasedasanalternativetoraisedmedianswithleftturnorUturnbays.Althoughnotasaestheticallypleasingasraised,plantedmedians,continuousleftturnlanescanimprovecapacity.Twowayleftturnlanesgenerallyarenotrecommendedinresidentialareasbecausetheydonotaffordasaferefugeforpedestrians.Also,thenumberofdrivewayscancreateunsafevehiclemaneuvers.

Shouldersincreasesafetyandhighwaycapacityandprovideaplaceforpedestriansandbicyclistswhennosidewalksareprovided.

（Rt.197,MD）

Shoulders

Althoughthephysicaldimensionsofautomobilesandtruckslimitthebasicwidthoftravellanes,thetreatmentofthatportionofthehighwaytotherightoftheactualtraveledway,thatis,the"

roadwayedge,"

providesthedesignerwithagreaterdegreeofflexibility.Thisistrueinbothurbanandruralareas,althoughdifferentdesignelementsaremoreappropriateineachlocation.

Shoulderwidthstypicallyvaryfromaslittleas0.6m（2ft）onminorruralroads,wherethereisnosurfacing,toabout3.6m（12ft）onmajorhighways,wheretheentireshouldermaybestabilizedorpaved.

Thetreatmentofshouldersisimportantfromanumberofperspectives,includingsafety,thecapacityofthehighwaysection,impactonthesurroundingenvironment,andboththeinitialcapitaloutlayandongoingmaintenanceandoperatingcosts.Theshoulderdesignshouldbalancethesefactors.Forexample,adesignermustconsidertheimpactoftheshoulderwidthandotherroadsideelementsonthesurroundingenvironmentand,atthesametime,howthesedimensionswillaffectcapacity.Evenwithamaximumlanewidthof3.6m（12ft）,theabsenceofashoulderorthepresenceofanobstructionattheedgeofthetravellanecanresultinareductionincapacityofasmuchas30percent,comparedtoanareawhereshoulderorclearzoneexiststhatisaminimum1.8m（6ft）wide.Ontheotherhand,significantenvironmental,scenic,orhistoricresourcesmaybeadverselyaffectedbyawidenedshoulder.

Anotherconsiderationistheaccommodationofpedestriansandnonmotorizedvehicles.Inmanypartsofthecountry,highwayshouldersprovideaseparatetraveledwayforpedestrians,bicyclists,andothers（whennosidewalksareprovided）.

（a）

（b