公路通行能力外文翻译文献

公路通行能力外文翻译文献PAGE1公路通行能力外文翻译文献(文档含中英文对照即英文原文和中文翻译)HighwayCapacityAndLevelsofServiceCapacityDefinedAgeneralizeddefinitionofcapacityis:Thecapacityofanyelementofthehighwaysystemisthemaximumnumberofvehicleswhichhasareasonableexpectationofpassingoverthatsection(ineitheroneorbothdirections)duringagiventimeperiodunderprevailingroadwayandtrafficconditions.Asamplingofcapacitiesformodernhighwayelementsisasfollows:FacilityCapacityinPassengerCarsFreewaysandexpresswaysawayfromrampsandweavingsections,perlaneperhour2000Two-Lanehighways,totalinbothdirections,perhour2000Three-lanehighways,totalineachdirection,perhour2000Twelve-footlaneatsignalizedintersection,perhourofgreensignaltime(nointerferenceandidealprogression)1800Intreatingcapacity，TRBCircular212dividesfreewaysintocomponents:basicfreewaysegmentsandthoseinthezoneofinfluenceofweavingareasandrampjunctions.Capacitiesofexpressways，multilanehighways，andtwo-andthree-lanefacilitiesalsohavethetwocomponents:basicandthoseinthezoneofinfluenceofintersections.Eachoftheseistreatedseparatelybelow.Speed-Volume-CapacityRelationshipsforBasicFreewayandMultilaneHighwaySegmentsAknowledgeoftherelationshipsamongspeed，volume，andcapacityisbasictounderstandingtheplaceofcapacityinhighwaydesignandoperation.Figurel3.1，whichgivessucharelationshipforasinglefreewayorexpresswaylane,isusedforillustrativepurposes.Ifalonevehicletravelsalongatrafficlane，thedriverisfreetoproceedatthedesignspeed.ThissituationisrepresentedatthebeginningoftheappropriatecurveattheupperleftofFig.13.1.Butasthenumberofvehiclesinthelaneincreases,thedriver'sfreedomtoselectspeedisrestricted.Thisrestrictionbringsaprogressivereductioninspeed.Forexample，manyobservationshaveshownthat，forahighwaydesignedfor70mph(113km/h)，whenvolumereaches1900passengercarsperhour，trafficisslowedtoabout43mph(69km/h).Ifvolumeincreasesfurther,therelativelystablenormal-flowconditionusuallyfoundatlowervolumesissubjecttobreakdown.ThiszoneofinstabilityisshownbytheshadedareaontherightsideofFig.13.1.Onepossibleconsequenceisthattrafficflowwillstabilizeatabout2000vehiclesperhouratavelocityof30to40mph(48to64km/h)asshownbythecurvedsolidlineonFig.13.1.Often，however,thequalityofflowdeterioratesandasubstantialdropinvelocityoccurs;inextremecasesvehiclesmaycometoafullstop.Inthiscasethevolumeofflowquicklydecreasesastrafficproceedsunderaconditionknownas‘forcedflow.’VolumesunderforcedflowareshownbythedashedcurveatthebottomofFig.13.1.Readingfromthatcurve，itcanbeseenthatifthespeedfallsto20mph(32km/h)，therateofflowwilldropto1700vehiclesperhour;at10mph（16km/h)theflowrateisonly1000;and,ofcourse，ifvehiclesstop，therateofflowis0.Theresultofthisreductioninflowrateisthatfollowingvehiclesallmustsloworstop，andtherateofflowfallstothelevelsshown.Eveninthosecaseswherethecongestionlastsbutafewseconds,additionalvehiclesareaffectedafterthecongestionattheoriginallocationhasdisappeared.A‘shockwave’developswhichmovesalongthetrafficlaneinthedirectionoppositetothatofvehicletravel.Suchwaveshavebeenobservedseveralmilesfromthesceneoftheoriginalpointofcongestion，withvehiclesslowingorstoppingandthenresumingspeedfornoapparentreasonwhatsoever.Effectsoftheimpositionofspeedlimitsof60,50,and40mpharesuggestedbythedottedlinesonFig.13.1.A55-mph(88km/h)curvecouldalsobedrawnmidwaybetweenthe60and50mphdottedcurvestoreflecttheeffectsofthefederallyimposed55-mphlimit,butthisisconjecturalsincethelevelofenforcementvariessowidely.Vehiclespacing，oritsreciprocal,trafficdensity,probablyhavethegreatesteffectoncapacitysinceitgeneratesthedriver'sfeelingoffreedomorconstraintmorethananyotherfactor.Studiesofdriversastheyfollowothervehiclesindicatethatthetimerequiredtoreachapotentialcollisionpoint，ratherthanvehicleseparation，seemstocontrolbehavior.However，thistimevarieswidelyamongdriversandsituations.Fieldobservationshaverecordedheadways(timebetweenvehicles)rangingfrom0.5to2sec,withanaverageofabout1.5s.Thus，thecalculatedcapacityofatrafficlanebasedonthis1.5saverage,regardlessofspeed，willbe2400vehiclesperhour.Butevenunderthebestofconditions,occasionalgapsinthetrafficstreamcanbeexpected，sothatsuchhighflowsarenotcommon.Rather,asnoted，theyarenearerto2000passengercarsperhour.The‘LevelofService’ConceptAsindicatedinthediscussionoftherelationshipsofspeed,volumeordensity,andvehiclespacing,operatingspeedgoesdownanddriverrestrictionsbecomegreaterastrafficvolumeincrease.‘Levelofservice’iscommonlyacceptedasameasureoftherestrictiveeffectsofincreasedvolume.Eachsegmentofroadwaycanberatedatanappropriatelevel，AtoFinclusive，toreflectitsconditionatthegivendemandorservicevolume.LevelArepresentsalmostidealconditions;LevelEisatcapacity;LevelFindicatesforcedflow.Thetwobestmeasuresforlevelofserviceforuninterruptedflowconditionsareoperatingortravelspeedandtheradioofvolumetocapacity达到最大限度的广播，calledthev/cratio.Fortwo-andthree-laneroadssightdistanceisalsoimportant.Abbreviateddescriptionsofoperatingconditionsforthevariouslevelsofserviceareasfollows:LevelA—Freeflow;speedcontrolledbydriver'sdesire，speedlimits,orphysicalroadwayconditions.LevelB—Stableflow;operatingspeedsbeginningtoberestricted;littleornorestrictionsonmaneuverabilityfromothervehicles.LevelC—Stableflow;speedsandmaneuverabilitymorecloselyrestricted.LevelD—Approachesunstableflow;tolerablespeedscanbemaintainedbuttemporaryrestrictionstoflowcausesubstantialdropsinspeed.Littlefreedomtomaneuver，comfortandconveniencelow.LevelE—Volumesnearcapacity;speedtypicallyinneighborhoodof30mph(48km/h);flowunstable;stoppagesofmomentaryduration.Abilitytomaneuverseverelylimited.LevelF—Forcedflow，low-operatingspeeds，volumesbelowcapacity;queuesformed.AthirdmeasureoflevelofservicesuggestedinTRBCircular212istrafficdensity.Thisis，foratrafficlane，theaveragenumberofvehiclesoccupyingamile(1.6km)oflaneatagiveninstant.Toillustrate，iftheaveragespeedis50mph，avehicleisinagivenmilefor72s.Ifthelanecarrying800vehiclesperhour，averagedensityisthen16vehiclespermile;spacingis330ft(100m)，centertocenter.Theadvantageofthedensityapproachisthatthevariouslevelsofservicecanbemeasuredorportrayedinphotographs.From:ClarksonH.OglesbyandR.GaryHicks“Highwayengineering”,1982公路通行能力和服务水平通行能力的定义道路通行能力的广义定义是：在繁忙的道路和交通条件下公路系统任何元素的通行能力是对在指定的时间通过一断面（一个或两个方向）的最大数量的车辆有一个合理的预期。一个现代公路通行能力的的抽样情况如下：设施小客车通行能力远离斜坡和交织路段的高速公路和每小时每个车道的车流量2000两车道公路，每小时两个方向的车流量2000三车道公路，每小时一个方向的车流量2000有信号的交叉路口的十二英寸车道，在绿灯条件下每小时的车流量（没有干扰的理想通行条件下）1800关于通行能力处理量，运输交通委员会发布的公路通行能力手册将高速公路划分为以下部分：基本高速公路路段，这些区域有影响的交织地区和砸道连接处，高速公路，多车道公路。两车道和三车道的通行能力同样有两部分组成：基本路段和这些区域有影响作用的交叉路口。基本高速公路和多车道公路路段速度，车流量和通行能力的关系速度，车流量和通行能力之间关系是了解某一地方公路设计和运行能力的基础。图3.1说明了高速公路中速度、车流量和通行能力之间的关系。如果司机驾驶一辆汽车一直自由的以设计时速独自行驶在一个行车道上，这种情形在左上角的图13.1中以适当的曲线表示出来。但随着车道上车辆数目的增加，司机自由选择速度受到限制。例如，许多研究表明，一个高速公路的设计速度为70英里每小时（113km/h），当车辆容量达到1900辆每小时时，交通速度下降到43英里每小时（69km/h）。如果车辆数进一步增加，则建立在低车辆数目的相关稳定和正常的流动条件将会被打破。这种不稳定的区域如右侧图13.1阴影区域所示。一个可能的结果是交通流量将如图13.1的实曲线所示以30到40英里每小时（48到64km/h）的速度下稳定在大约2000车辆每小时。然而在通常情况下，车流量的质量恶化，车速大幅度下降；在极端情况下车辆可能完全停止。在这种情况下，车流量迅速下降，这种情况下的交通受益被称为“强制性流动”。强制性流动下的车流量如图13.1下部的虚曲线所示。从曲线上可以看出，如果速度下降到20应力每小时（32km/h），车流量将下降到1700辆每小时，以10英里每小时（16km/h）的流量算只有1000辆车；当然如果车辆停止，车辆流速为0。流速减少的结果是以后的车辆都必须减速或者停止，车辆流速下降到显示的水平。即使在这种情况下，交通拥挤的情况人在持续。短暂时间后，拥挤处原来的车辆离开后，其他的车辆又会受到影响。一个沿着相反车道行驶的车辆的冲击波逐渐形成。这样的冲击波已经在视野里从原来的拥挤点达到几英里。伴随着车辆减速或者停止，而且不会明显恢复原来的速度。速度限制在60、50和40英里每小时的影响在图13.1中的虚线表示出来。55英里每小时（88km/h）的曲线也可以画在60和50英里每小时的虚线中来反映联邦政府限制的55英里每小时的速度的影响。但是因为执法水平的宽泛和多样性，这只是推测而已。车辆间距，或者它的倒数，交通密度可能对通行能力有最大的影响，因为它对于司机形成自由或者约束的感觉比其他任何因素都要多。对于司机的研究显示，他们跟随