Chapter 4 DESIGN CONTROLSWord文档下载推荐.docx

Chapter 4 DESIGN CONTROLSWord文档下载推荐.docx

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∙Thephysicalcharacteristicsofthe"

designvehicle"

（i.e.,thelargestvehiclethatislikelytousethefacilitywithconsiderablefrequency）;

invirtuallyallinstances,thehighwaydesignvehicleisanovertheroadtractortrailer

∙Theperformanceofthedesignvehicle（particularlyimportantintermsofaccommodatingheavytrucksinmountainousterrainorbusesandrecreationalvehiclesinareassubjecttohighlevelsoftouristactivity）

∙Thecapabilitiesofthetypicaldriveralongthefacility（i.e.,localresidentsusinglowspeedneighborhoodstreetsversusinterstatetravelersonruralfreeways）

∙Theexistinganddesignyeartrafficdemandstobeplacedonthefacility（e.g.,dailyandpeakhourtrafficvolumes,themixofpassengercarsandtrucksonthefacility）

Twoofthemostimportantofthesefactorsaredesignspeedandpeakhourlevelofservice.Thischapterwillfocusonthesetwoimportantcriteria.Thepeakhourlevelofservice,however,onlyservesasacontrollingfactorforasmallnumberofhighways.Formosthighways,afterthefunctionalclassificationandassociateddesignspeedforaparticularhighwayfacilityhavebeenestablished,thedegreeofflexibilityavailabletothedesignerissignificantlylimited.

Speedlimitsarenotthesameasthedesignspeed.Theyareregulatorylimitsenforceablebylaw.

DesignSpeed

DesignspeedisdefinedbytheAASHTOGreenBookas:

...themaximumsafespeedthatcanbemaintainedoveraspecifiedsectionofhighwaywhenconditionsaresofavorablethatthedesignfeaturesofthehighwaygovern.

Table4.1

RelationshipBetweenDesignControlsandDesignFeatures

DesignControls

DesignFeatures

FunctionalClassification

TrafficData

TerrainLocale

DesignSpeed

Lanewidth,rural

X

Lanewidth,urban

Ruralshoulderwidth,type

Urbanshoulderwidth,type

Guiderailoffset

Degreeofcurve

Grades

Bridgeclearances（horizontal&

vertical）

Stoppingsightdistance

Superelevation

Wideningoncurves

Ruraldesignspeeds

Urbandesignspeeds

Source:

LocationandDesignManual,Vol.1,RoadwayDesign,OhioDOT.

Allgeometricdesignelementsofthehighwayareaffectedinsomewaybytheselecteddesignspeed.Someroadwaydesignelementsarerelateddirectlytoandvaryappreciablywithdesignspeed.Theseincludehorizontalcurvature,superelevation,sightdistance,andgradient（seeTable4.1）.Otherelementsarelessrelatedtodesignspeed,suchaspavementandshoulderwidthandclearancestowallsandtrafficbarriers.Thedesignofthesefeaturescan,however,affectvehicleoperatingspeedssignificantly.Asaresult,morestringentcriteriaforthesefeaturesaregenerallyrecommendedforhighwayswithhigherdesignspeeds.Conversely,lessstringentcriteriaforthesefeaturesmaybemoreappropriateonroadwayswithlowerdesignspeeds.

Theselectionofaparticulardesignspeedisinfluencedbythefollowing:

∙Thefunctionalclassificationofthehighway

∙Thecharacteroftheterrain

∙Thedensityandcharacterofadjacentlanduses

∙Thetrafficvolumesexpectedtousethehighway

∙Theeconomicandenvironmentalconsiderations.

Typically,anarterialhighwaywarrantsahigherdesignspeedthanalocalroad;

ahighwaylocatedinlevelterrainwarrantsahigherdesignspeedthanoneinmountainousterrain;

ahighwayinaruralareawarrantsahigherdesignspeedthanoneinanurbanarea;

andahighvolumehighwaywarrantsahigherdesignspeedthanonecarryinglowtrafficvolumes.

（Left）Lowerdesignspeedsareappropriateforcitystreets.

（Right）Highervolumeruralhighwayfacilitiesshouldusehigherdesignspeeds.

AsdiscussedinChapter3,mostStatesandlocalitieshaveadoptedarangeofacceptabledesignspeedsforeachofthemajorclassesofhighwaysandstreets（i.e.,freeway,otherarterial,collector,andlocal）.Table4.2illustratestypicalminimumdesignspeedsforvarioustypesofhighwayslocatedinlevel,rolling,andmountainousterrain.

Freeways

DesignSpeeds

Terrain

Rural

Urban

Flat

70-80

70

Rolling

60-70

Mountainous

50-60

ArterialHighways

Terrain

30-60

40-60

30-50

30-50

CollectorandLocalRoads

30-40

20-40

20-30

TrafficEngineeringHandbook（FourthEdition）,InstituteofTransportationEngineers,Washington,DC,1992,p.156.Note:

1mile/hr=1.613km/hr

Table4.2TypicalMinimumDesignSpeedsforVariousTypesofHighways（inmph）

ThevaluespresentedinTable4.2areminimumacceptabledesignspeedsforthevariousconditionsofterrainandtrafficvolumesassociatedwithneworreconstructedhighwayfacilities.Designersneedtobalancetheadvantagesofahighervehicleoperatingspeedgainedthroughtheuseofahigherdesignspeedagainsttheflexibilitylostindesign.Itmaybemoreimportanttoretainthemaximumpossibleflexibility,sothatacontextsensitiveroadwaythatismoreintunewiththeneedsofacommunityisdesignedusingalowerdesignspeed.

Asusedhere,theterm"

contextsensitive"

refersprimarilytothelanduseandenvironmentalconditionsadjacenttothehighway.Forexample,foranyparticularhighwayotherthanafreewayormajorarterial,aslandusedensityincreases,thedesignspeedwouldtypicallydecrease.Thedesignspeedofanurbancollectorstreetpassingthrougharesidentialneighborhoodshouldbeappreciablylowerthanthatforaruralhighwaywiththesamefunctionalclassification.Thisalsorecognizesthefactthatbicyclesandpedestrianswouldbemorelikelytousearoutelocatedinanurbanarea.

Similarly,inareasthathavesignificanthistoricinterestorvisualquality,alowerdesignspeedmaybeappropriateinrecognitionofloweraverageoperating,speedsandtheneedtoavoidaffectingthesehistoricoraestheticresources.

TheGreenBookagreeswiththisphilosophy:

Aboveminimumdesignvaluesshouldbeusedwherefeasible,butinviewofthenumerousconstraintsoftenencountered,practicalvaluesshouldberecognizedandused.

Alongarterialstreets,thecontrollingfactorofdesignspeedappliestoalesserdegreethanonruralhighwaysorhightypeurbanfacilities,suchasfreewaysorexpressways.Onmanyofthearterialstreetslocatedinlargeurbanareas,maximumvehicleoperatingspeedsforseveralhoursofthedaymaybelimitedtothoseatwhichtherecurringpeakperiodtrafficvolumescanbeaccommodated.Thus,speedsmaybegovernedbythepresenceofothervehiclestravelingenmassebothinandacrossthethroughtravellanesandbytrafficcontroldevices,ratherthanbythephysicalcharacteristicsofthestreet.

Duringoffpeakperiodsoflowtomoderatetrafficdemand,vehicleoperatingspeedsaregovernedbysuchfactorsasspeedlimits,midblockturns,intersectionturns,numberofdrivewaysandentrances,trafficsignalspacing,andsignaltiming.Asaresult,whenarterialstreetimprovementsarebeingplanned,theselectionoftheappropriatedesignspeedmustbebalancedagainstsuchfactorsasspeedlimits,physicalandeconomicconstraints,andtheprobablerunningspeedsthatcanbeattainedduringoffpeakhours.

AlthoughmostStateshaveadoptedarangeofallowabledesignspeedsappropriateforeachofthevariousfunctionalclassificationsforuseinthedesignofneworreconstructedhighwayfacilities,situationsmayarisewhereeventheuseofthelowesttypicallyacceptablevaluewouldresultinunacceptablyhighconstructionorrightofwaycostsorunacceptableimpactonadjacentproperties.Insuchinstances,thedesignexceptionprocessdiscussedinChapter2canbeemployed.Forexample,thereconstructionofatwolaneruralarterialroutethrougharelativelyflatbutenvironmentallysensitiveareamightneedtoemployadesignspeedof80km/h（50mph）ratherthantherecommendedvalueforthisfunctionalclassificationof100km/h（60mph）showninTable4.2.

PeakHourLevelofService

Onceanappropriatedesignspeedhasbeenselected,theotherbasicdefiningelementsofthehighway（i.e.,thenumberoflinesandthebasicconfigurationofjunctionswithotherhighwayfacilities）canbedeterminedthroughapplicationoftheconceptofacceptablepeakhourlevelofservice.Levelofserviceisagradingsystemforamountofcongestion,usingtheletterAtorepresenttheleastamountofcongestionandFtorefertothegreatestamount.Foracomprehensivetreatmentofthistopic,refertotheHighwayCapacityManual'

.Table4.3presentsabriefdescriptionoftheoperatingcharacteristicsassociatedwitheachlevelofservice.

Ascongestionapproachescapacity,speeddecreases.

Levelof

Service

Description

A

Freeflowwithlowvolumesandhighspeeds.

B

Reasonablyfreeflow,butspeedsbeginningto