交通工程专业英语 7 25课文

EconomicsandTransportationEngineeringUnit7TextAneconomicanalysisshouldnotbecarriedoutasallafterthought.Itshouldbeusedaspartofacontinuousprocess，startingwiththeobjectivesoftheproposedtransportationproject(cantheseobjectivesbesatisfiedinanyotherway?)，runningthroughtheentireplanninganddesignprocess，andonlyending，asafinalsummingup，withtheoverallevaluation.Itshouldconsiderthefollowingquestions：①Projectidentification.Whichprojectsorpoliciesshouldbeconsideredaspossiblesolutionstoaparticulartransportneed?②Establishingroughpriorities．Whichprojectsandpoliciesshouldbeconsideredindetail?③Detailedevaluation．Whichcostsandbenefitsarerelevanttotheevaluation，howshouldtheybemeasured，andhowcantheybestbepresentedasanindexofpriority?④Projectselection．Shouldthisprojectbeselected，shoulditberejected，orshoulditbepostponed?Economicsisoftenoverlookedasanaidtotransportationplanninganddesign．Yetanytransportationengineeringprobleminvolvesawholeseriesofessentiallyeconomicdecisions．Intheareaofdesign，forexample，thetransportationengineermustchoosematerials，selectanoveralldesignconcept，andthencombinethecomponentpartsoftheschemeintoaneffective，andeconomical，whole．Arivercrossingcouldthusbebyferryboat，causeway，orbridge．Inthelattercaseitcouldbeinsteel，concrete，ortimber．Itcouldbesuspended，arched，orsimplysupported．Whatshouldtheengineerchoose?Therangeofchoiceisnotusuallyaslargeasthis．Somesolutionswillbephysicallyinfeasible(e.g.，bylackofsuitablefoundationsforanarchedbridge),butimportantchoiceswillremain．Intheabsenceofarigorouseconomicanalysis，thedesignermaymakethesechoicesonthebasisof：．whichmayormaynotcoveranadequaterangeofoptions，Experience①．②Preconceptionsastowhatisdesirable(e.g.,onesolutionuseslessmaterial)．③Aninnateviewofmathematicalorstructuralelegance(e.g.，anarchisamoreelegantstructuralform)．Economicsavoidstheneedforthesearbitraryrulesbyprovidingatangiblecriterion-costforchoosingbetweenalternativesolutions.Inmanycasesthiscanbedonequitesimplybydrawingenvelopesofcostcurvesof,forexample,steelgirderbridgesversusreinforcedconcretebridgesversusprestressedconcretebridges.Byplottingcost(y)againstspanlength(x),theengineercanchoosewhichtypeismosteconomicoveragivenrangeofspanlengths.Inpractice,theengineerdoesnotusuallycarrytheeconomicanalysistothislevelofdetail,althoughcurveslikethiscommonlyformpartofthestandardhighwaydesignrepertoire.Itismoreaquestionoftheeconomicattitudeofmindthatasks:isthistheonlypossiblesolution,andifnot,isthereabetterone?Itisthisgeneralprinciplewhicheventuallyensuresthatthefinaldesignisthemosteconomicalandtheonewhich,ifjustifiedinaggregateterms,offersbestvalueformoney.EconomicefficiencyanddistributionEconomicsisnotconcernedsolelywiththeoptimumallocationofresources,butwithwiderquestionsofequityandthedistributionofcostsandbenefitsamongindividuals,regions,etc.Itisthusconcernedwiththequestion:whodoeswhat,towhom,andatwhoseexpense?Inasensethisconcerncomplementstheintertemporalconsiderationsnotedabove.Adollartoonepersonisnotnecessarilyworththesameamounttoanother.Sinceeconomicsassumesthatmarginalvaluesdeclineasincomeincreases,interpersonaldifferencesinincomemaythusaffectanyoverallmeasureofconsumerbenefit.Questionsofdistributionneverthelessgobeyondmeredifferencesinincome.Institutionalconstraintsusuallypreventbeneficiaries,definedinthebroadestsense,fromcompensatingpeoplewhoareadverselyaffected.Itisthereforeoftenappropriatetoseparatetheimpactofatransportimprovementintoitseffectondifferentinterestgroups,inadditiontoitseffectondifferentincomegroups.Notes1．Somesolutionswillbephysicallyinfeasible(e．g．，bylackofsuitablefoundationsforanarchedbridge)，butimportantchoiceswillremain．一些解决方法实际上是不可行的(例如一座拱桥缺乏适当的基础)，但仍会有一些重要的选择将被保留。2．Economicsavoidstheneedforthesearbitraryrulesbyprovidingatangiblecriterion-cost-forchoosingbetweenalternativesolutions．经济学通过在选择备选方案时提出一个切实的成本标准，从而避免随意性规则的使用。3.Itismoreaquestionoftheeconomicattitudeofmindthatasks：Isthistheonlypossiblesolution,andifnot,isthereabetterone?这更多的是一个从经济学角度提出的问题：这是唯一可能的解决方法吗，如果不是，还有更好的方法吗?4．Questionsofdistributionneverthelessgobeyondmeredifferencesinincome．不过分配问题超过收人上的微弱差别。5．Itisthereforeoftenappropriatetoseparatetheimpactofatransportimprovementintoitseffectondifferentinterestgroups．inadditiontoitseffectondifferentincomegroups．因此，除了影响不同的收入阶层外，通常将交通改善的影响还按其对不同的利益阶层的影响来加以区分。．Unit8SightDistanceTextTheabilitytoseeaheadisoftheutmostimportantinthesafeandefficientoperationofavehicleonahighway.Onarailroad，trainsareconfinedtoafixedpath，yetablocksignalsystemandtrainedoperatorsarenecessaryforsafeoperation．Ontheotherhand，thepathandspeedofmotorvehiclesonhighwaysandstreetsaresubjecttothecontrolofdriverswhoseability，training，andexperiencearequitevaried．Forsafetyonhighwaysthedesignersmustprovidesightdistanceofsufficientlengththatdriverscancontroltheoperationoftheirvehiclestoavoidstrikinganunexpectedobjectonthetraveledway．Certaintwo-lanehighwaysshouldalsohavesufficientsightdistancetoenabledriverstooccupytheopposingtrafficlanewithouthazard．Two-laneruralhighwaysshouldgenerallyprovidesuchpassingsightdistanceatfrequentintervalsandforsubstantialportionsoftheirlength．Conversely，itnormallyisoflittlepracticalvaluetoprovidepassingsightdistanceontwo-laneurbanstreetsorarterials．Thelengthandintervalofpassingsightdistanceshouldbecompatiblewiththecriteriaestablishedinthechapterpertainingtothatspecifichighwayorstreetclassification．Sightdistanceisdiscussedinfoursteps：thedistancesrequiredforstopping，applicableonallhighways；thedistancesrequiredforthepassingofovertakenvehicles，applicableonlyontwo-lanehighways；thedistancesneededfordecisionsatcomplexlocations；andthecriteriaformeasuringthesedistancesforuseindesign．StoppingsightdistanceSightdistanceisthelengthofroadwayaheadvisibletothedriver．Theminimumsightdistanceavailableonaroadwayshouldbesufficientlylongtoenableavehicletravelingatornearthedesignspeedtostopbeforereachingastationaryobjectinitspath．Althoughgreaterlengthisdesirable，sightdistanceateverypointalongthehighwayshouldbeatleastthatrequiredforabelowaverageoperatororvehicletostopinthisdistance．thedistancetraversedbythevehicle：oftwodistancesthesumdistanceisStoppingsightfromtheinstantthedriversightsanobjectnecessitatingastoptotheinstantthebrakesareappliedandthedistancerequiredtostopthevehiclefromthebrakeapplicationbegins．Thesearereferredtoasbrakesreactiondistanceandbrakingdistance，respectively．DesignsightdistanceStoppingsightdistancesareusuallysufficienttoallowreasonablycompetentandalertdriverstocometoahurriedstopunderordinarycircumstances.However,thesedistancesareofteninadequatewhendriversmustmakecomplexorinstantaneousdecisions,wheninformationisdifficulttoperceive,orwhenunexpectedorunusualmaneuversarerequired.Limitingsightdistancestothoseprovidedforstoppingmayalsoprecludedriversfromperformingevasivemaneuver，whichareoftenlesshazardousandotherwisepreferabletostopping.Stoppingsightdistancesmaynotprovidesufficientvisibilitydistancesfordriverstocorroborateadvancewarningsandtoperformthenecessarymaneuvers.Itisevidentthattherearemanylocationswhereitwouldbeprudenttoprovidelongersightdistances.Inthesecircumstances,decisionsightdistanceprovidesthegreaterlengththatdriversneed.Decisionsightdistanceisthedistancerequiredforadrivertodetectanunexpectedorotherwisedifficult-to-perceiveinformationsourceorhazardinaroadwayenvironmentthatmaybevisuallycluttered,recognizethehazardoritsthreatpotential,selectanappropriatespeedandpath,andinitiateandcompletetherequiredsafetymaneuversafelyandefficiently.Becausedecisionsightdistancegivesdriversadditionalmarginforerrorandaffordsthemsufficientlengthtomaneuvertheirvehiclesatthesameorreducedspeedratherthantojuststop,itsvaluesaresubstantiallygreaterthanstoppingsightdistance.Passingsightdistancefortwo-lanehighwaysMostroadsandnumerousstreetsareconsideredtoqualifyastwo-lanetwo-wayhighwaysonwhichvehiclesfrequentlyovertakeslowermovingvehicles,thepassingofwhichmustbeaccomplishedonlanesregularlyusedbyopposingtraffic.Ifpassingistobeaccomplishedwithsafety,thedrivershouldbeabletoseeasufficientdistanceahead,clearoftraffic,tocompletethepassingmaneuverwithoutcuttingoffthepassedvehicleinadvanceofmeetinganopposingvehicleappearingduringthemaneuver.Whenrequired,adrivercanreturntotherightlanewithoutpassingifheseesopposingtrafficistooclosewhenthemaneuverisonlypartiallycompleted.Manypassingsareaccomplishedwithoutthedriverseeingasafepassingsectionahead,butdesignbasedsuchmaneuversdoesnothavethedesiredfactorofsafety.Becausemanycautiousdriverswouldnotattempttopassundersuchconditions,designonthisbasiswouldreducetheusefulnessofthehighway.Passingsightdistanceforuseindesignshouldbedeterminedonthebasisofthelengthneededtosafelycompletenormalpassingmaneuvers.Whiletheremaybeoccasionstoconsidermultiplepassings,wheretwoormorevehiclespassorarepassed,itisnotpracticaltoassumesuchconditionsindevelopingminimumdesigncriteria.Instead,sightdistanceisdeterminedforasinglevehiclepassingasinglevehicle.Longersightdistancesoccurindesignandtheselocationscanaccommodateanoccasionalmultiplepassing.SightdistanceformultilanehighwaysItisnotnecessarytoconsiderpassingsightdistanceonhighwaysorstreetsthathavetwoormoretrafficlanesineachdirectionoftravel.Passingmaneuversonmultilaneroadwaysareexpectedtooccurwithinthelimitsofeachone-waytraveledway.Thuspassingmaneuversthatrequirecrossingthecenterlineoffour-laneundividedroadwaysorcrossingthemedianoffour-lanedividedroadwaysarerecklessandshouldbeprohibited.Notes1．Conversely，itnormallyisoflittlepracticalvaluetoprovidepassingsightdistanceontwo-laneurbanstreetsorarterials．相反地，通常在双车道城市道路或市区干线上提供超车视距实际价值不大。2．Theminimumsightdistanceavailableonaroadwayshouldbesufficientlylongtoenableavehicletravelingatornearthedesignspeedtostopbeforereachingastationaryobjectinitspath．道路上应有的最小视距应该足以使那些以等于或接近设计速度行驶的车辆能在到达行驶路线上的障碍物之前停下来。orcomplexmakemustdriverswheninadequateoftenaredistancesthese，However．3．instantaneousdecisions,wheninformationisdifficulttoperceive，orwhenunexpectedorunusualmaneuversarerequired．但是，当驾驶员必须进行复杂或瞬间的判断，信息难以感觉到，或当需要进行出乎意外的或不同寻常的操纵时，上述停车距离往往是不足的。4．Decisionsightdistanceisthedistancerequiredforadrivertodetectanunexpectedorotherwisedifficult-to-perceiveinformationsourceorhazardinaroadwayenvironmentthatmaybevisuallycluttered，recognizethehazardoritsthreatpotential，selectanappropriatespeedandpath，andinitiateandcompletetherequiredsafetymaneuversafelyandefficiently．判断视距是指驾驶员所需的这样一个距离：在可能引起视觉混乱的道路环境中发觉一个意外的或难以察觉的信息源或危险，或发觉其潜在的威胁，从而选择适当的速度和路线，并安全、有效地开始与完成所需的安全操作。5．Thuspassingmaneuversthatrequirecrossingthecenterlineoffour-laneundividedroadwaysorcrossingthemedianoffour-lanedividedroadwaysarerecklessandshouldbeprohibited．因此，跨越无分隔设施的四车道道路中线或跨越四车道道路中央分隔带的超车运行是冒险的，应予以禁止。．Unit9RoundaboutIntersectionsTextWheretrafficflowsaresmallthenthecontroloftrafficmovementsatintersectionsmaybeachievedbyprioritycontrol.OnthecontinentofEuropenearsidepriorityisusedwherevehiclesgivewaytotrafficapproachingfromtherightwhileinAustraliaoff-sidepriorityisused.Astrafficflowsincreasedelayswithprioritycontrolbecomeexcessiveandathighlevelsofflowgrade-separatedjunctionsarenecessary.Ajunctionofthisformishoweverextremelyexpensive:inaddition,landrequirementsaregreatandinurbanandsuburbanareasinterferencewithpedestrianflowcanbeconsiderable.Forthesereasonsat-gradeintersectionseitheroftheroundaboutorsignalcontroltypeareextremelyimportantinurbanareas.ThecharacteristicsofthesejunctiontypeshavebeendescribedbyMillardandaconsiderationofthesecharacteristicswillusuallydeterminewhichtypeofjunctionisappropriate.Theyinclude:Inurbansituationslandrequirementsareusuallythedecidingfactor.Ifthisisso,itwillbefoundthatthelandrequiredforalargeislandroundaboutisgreaterthanthatneededfortrafficsignalcontrol.Thisisespeciallytrueifflowsononepairofarmsarelow.Ontheotherhand,iflandpurchaseisnecessary,itisofteneasiertoacquirecornersitesnecessaryforaroundaboutthanthelongnarrowstripsneededwhenparallelwideningoftrafficsignalapproachesiscarriedout.Bothconventionalandmini-roundaboutshavedifficultyindealingwithunbalancedflows,especiallyduringpeakhourswhenthetrafficenteringonanarmisconsiderablygreaterthanthetrafficleavingbyit.Insuchsituationsthereisfrequentlyashortageofgapsinthecirculatingstreamand,underoff-sidepriorityrole,delaysmaybecomeexcessive.Right-turningvehicles(left-handruleoftheroad)causedifficultieswithsignalcontrolwhentheirnumbersarelarge.Eitherlatestartorearlycut-offfacilitiesoraspecialphasemustbeprovidedcausingreducedoverallcapacityatthejunction.Insuchcircumstancesroundaboutsofferadvantages.Trafficsignalcontrolhasdifficultyindealingwiththree-wayjunctions,especiallywheretheflowsarebalanced.Toalesserextentthisistrueofjunctionswithfiveormoreapproaches.Roundaboutshavebeencommonlyusedincentralcityareaswheretraditionallytheywereusedtoresolvetrafficandpedestrianconflictsinthelargeopensquareswhichexistedintheearlypartofthetwentiethcentury.Thecentralislandfrequentlycoveredalargeareaofthesquareandwasutilizedforornamentalflowerbedswhilsttrafficcirculatedaroundthesurroundingcarriageway.Increasingtrafficdemandandthepressuretoallowpedestrianstocrossthecarriagewayatsurfacelevelhaveresultedinmanyoftheseroundaboutsbeingconvertedtosignalcontrolsothatmorepositivecontrolovertrafficmovementsonanareawidebasismaybeexercised.Insuburbanareas,roundaboutsarefrequentlyfoundattheintersectionofradialandringtyperoadswheretheyaresubjectedtopeakhourtrafficdemandsduetocommuterflows.Roundaboutsarealsousedonruralroadswheretrafficflowsortheroadtypedonotjustifytheprovisionofagrade-separatedintersection.Inthesesituationsspeedsarehighontheapproachestoroundaboutsandsafetyisanimportantconsideration.Roundaboutsdealefficientlywithtrafficmovementswhentherearethreeorfourarms.Withthreearms,andwellbalancedflowsaroundaboutisconsideredtobemoreefficientthansignalcontrol.Whenthenumberofarmsexceedsfourthendirectionsigninganddrivercomprehensionbecomedifficult.Withmanyapproacharmsthediameteroftheroundaboutincreases,leadingtopossiblehighercirculatingspeedsandconsequentsafetyproblems.Inadditiontotheresolutionoftrafficconflicts,roundaboutsareemployedwherethereisasignificantchangeinroadtype,achangefromruraltourbanconditionsorwhenasignificantchangeinroaddirectionisrequired.IncurrentUnitedKingdompracticetherearethreebasictypesofroundabout:normalroundabouts,miniroundaboutsanddoubleroundabouts.Thesearevariationsofthesetypestoformringjunctions,grade-separatedroundaboutsandsignalizedroundabouts.TheDepartmentofTransportdefinesanormalroundaboutasonewhichhasaone-waycirculatorcarriagewayaroundakerbedcentralisland4mormoreindiameterandusuallywithflaredapproachestoallowmultiplevehicleentry.Miniroundaboutsaredefinedasaroundabouthavingaone-waycirculatorycarriagewayaroundaflushoraslightlyraisedcircularmarkinglessthan4mindiameterandwithorwithoutflaredapproaches.Theyhavebeenwidelyusedinurbanareaswherethespeedlimitdoesnotexceed30mph.Physicaldeflectionofvehiclepathstotheleft,animportantfactorinroundaboutsafety,maybedifficultinurbanjunctionswithfixedkerblinesandinthesecircumstancesroadmarkingsshouldbeusedtoinducesomevehicledeflection.Thecircularmarkingvariesindiameterfrom1to4mdiameterandisdomedtoamaximumheightof125mm.Ifspacewithinthejunctionisverylimitedthenthecentralislandwillbefrequentlyrunoverbylargervehiclesandinthesecasestheislandisnormallyflushwiththeroadsurface.TheDepartmentofTransportadvisethatpedalcyclistsmayexperiencedifficultyandifthereareasubstantialnumberofcyclistspassingthroughthejunctionthensignalcontrolmaybepreferable.AdoubleroundaboutisdefinedasanindividualjunctionwithtwonormalorminiisItisland.kerbedorroadlinkcentralabyconnectedorcontiguouseitherroundaboutsconsideredthatthisformofjunctionwillhaveadvantagesinthefollowingcircumstances:improvinganexistingstaggeredjunctionwhereitavoidstheneedtorealignoneoftheapproachroads,unusualjunctionssuchasscissorsjunctions,joiningtwoparallelroutesseparatedbyariver,railwayormotorway,existingcrossroadswhereopposingright-turningmovementscanbeseparatedoverloadedsingleroundabouts,andjunctionswithmorethanfourentries.Asolutiontotheproblemoflargeroundaboutswhereentriesareapproachingcapacitymaysometimesbefoundbyconversiontoaringjunctionwheretheusualclockwisecirculationofvehiclesaroundalargeislandisreplacedbytwo-waycirculationwiththree-armminiroundaboutsatthejunctionofeachapproacharmwiththecirculatorycarriageway.Becauseofthetwo-wayflowonthecirculatorycarriagewayandtheneedforcirculatingvehiclestogiveway,adequatesignpostingisessentialforefficientoperation.Roundaboutcontrolisalsoutilizedingradeseparatedintersectionsintheformoftwo-bridgeroundaboutsanddumb-bellroundabouts.Whenroundaboutsarebeingdesigneditisusualtoaddatleastoneadditionallaneattheentryoftheapproachroadstothecirculatingareawithamaximumadditionoftwolanesandamaximumentrywidthoffourlanes.Whenonlylowflowsarepredictedinthefuturethenwideningmaynotbeconsiderednecessarybutaminimumoftwolanesinanentryisdesirable.Theangleatwhichvehiclesenterthecirculatingareaisofconsiderableimportanceintheoperationofaroundabout.Ifvehiclesenteratanangleapproaching90°tothecirculatingflowthenvehiclesareliabletostopquicklyonentryandcauserearendcollisions.Shouldtheyhoweverenteratasmallanglethendriversmustlookovertheirshouldersinanattempttomerge.Safetyatroundaboutsisenhancedbylimitingcirculatingspeedsandthisisachievedingeometricdesignbyentrypathcurvature,whichisameasureofthevehiclepathdeflectiontotheleftimposedonvehiclesenteringthecirculatingarea.Thepathofvehiclestravelingstraightthroughtheroundaboutisdrawnwithaflexiblecurvesothatitisthecenterlineofthemostrealisticpaththatavehiclewouldtakeinitscompletepassagethroughthejunctiononasmoothalignmentwithoutsharptransitions.ThetightestradiusoftheentrypathcurvatureisrecommendedbytheDepartmentofTransporttobenotgreaterthan100m.Thesizeofaroundaboutislargelydeterminedbytheinscribedcirclediameter:theinscribedcircleisthatwhichapproximatelytouchestheouteredgesoffilecirculatingarea.ForanormalroundaboutthesmallestrecommendedinscribedcirclediameterforUnitedKingdomconstructionis28m;ifthisisnotpossibleaminiroundaboutshouldbeused.Tomeetdeflectionrequirementstheinscribedcirclediametershouldnotbelessthan40m.Foraminiroundabouttheinscribedcirclediametershouldnotbegreaterthan28m.Circulatingcarriagewaywidthsshouldbeconstantandnotexceed15m;ingeneralthewidthshouldnotbelessthan,or20percentgreaterthanthemaximumentrywidth.Itisrecommendedthattheentryradiusintothecirculatingareashouldliebetween6mandl00mwithanormalvalueofapproximately20m.Existingradiusshouldnotbelessthan20mandpreferablyhaveavalueof40m.Notes1．Astrafficflowsincreasedelayswithprioritycontrolbecomeexcessiveandathighlevelsofflowgrade-separatedjunctionsarenecessary．随着交通量的增加，优先控制交叉口的延误将随之迅速增加，所以对于交通量较大的交叉口有必要采用分离式立交的形式。2．Ontheotherhand，iflandpurchaseisnecessary，itisofteneasiertoacquirecornersitesnecessaryforaroundaboutthanthelongnarrowstripsneededwhenparallelwideningoftrafficsignalapproachesiscarriedout．．另一方面，如果必须购置土地，与信号交叉口人口拓宽所需的长条形土地相比，环岛所需的边角状土地更容易获得。3．Eitherlatestartorearlycut-offfacilitiesoraspecialphasemustbeprovidedcausingreducedoverallcapacityatthejunction．无论是实行迟启交通信号、早断交通信号或者提供一个专用相位，都会降低交叉口总的通行能力。4.TheDepartmentofTransportdefinesanormalroundaboutasonewhichhasaone-waycirculatorycarriagewayaroundakerbedcentralisland4mormoreindiameterandusuallywithflaredapproachestoallowmultiplevehicleentry．交通部把通常的环形交叉口定义成具有围绕中心岛的单向环形车行道，环行道四周连接有允许各种车辆进入的路口加宽的引道，中心岛直径不小于4米并由侧石围砌而成。5．Itisrecommendedthattheentryradiusintothecirculatingareashouldliebetween6mand100mwithanormalvalueofapproximately20m．米。20米，通常约为100米～6建议进入环形区的入口半径应为Unit10Merging,DivergingandWeavingatGrade-separatedJunctionsandInterchangesTextMerging,divergingandweavingoccuratandbetweenmanytypesofjunctionsinbothurbanandruralsituations.MostfrequentlythesetrafficactionsareassociatedwithjunctionsonrouteswithahighstandardofgeometricdesignsuchasisfoundontheBritishmotorwayandtrunkroadsystem.InBritishpracticeagrade-separatedjunctionisonewherethecommencementorterminationofsliproadsconnectswithanat-graderoundaboutormajor-minorpriorityjunction.Aninterchangegivesuninterruptedmovementfromonemainlinetoanothermainlinecarriagewaybylinkroadsusingmerginganddivergingmovements.Weavingbetweentrafficstreamscanoccurinbothurbanandruralsituationsandisofteninherentinurbantrafficmanagementsystems.Designstandardsformerging,divergingandweavingdesignintheUnitedKingdomaregivenbytheDepartmentofTransport.Acollectivetermusedforlinkandsliproadsis'connectorroads'.Inmanytypesofgrade-separatedjunctionsandinterchanges,vehiclesarerequiredtochangetheirdirectionofmotionbytravelingaroundloops,theminimumradiusoftheseloopsisrelatedtospeed.Forloopsontooroffamotorwaytheminimumradiusis75m;forloopsontoall-purposeroadsthevalueis30m,andoffall-purposeroads50m.Maximumflowlevelsusedinthedesignofmerging,divergingandweavingareasarebasedonflowsobservedintheUnitedKingdom.Thedesignflowusedinmerging,divergingandweavingcalculationsdependsupontheroadtype.FormainurbanroadsasdefinedbytheTrafficAppraisalManualthe30thhighestannualhourlyflowisused;forInter-UrbanandRecreationalroadtypesthe50thand200thhighesthourlyflowsrespectivelyareused.Theseflowsshouldbepredictedfor15yearsaftertheopeningofthehighway.DesignofmerginglanesanddiverginglanesInthedesignofmerginganddiverginglanesatgrade-separatedjunctionsandintersections,thefollowingfactorsareconsidered:①Thenumberoflanesandthedesignhourflowsupstreamanddownstreamofthemerginganddivergingareasonthemaincarriageway.②Thenumberoflanesandthedesignhourflowsonthemergingordiverginglinkorconnector.③Thetrafficcompositionandthegradientofthemainlinecarriagewayandthemergingordiverginglinks.MaximumhourlyflowsperlanebasedonUnitedKingdomexperience;thesedifferfor④．all-purposeroadsandformotorways.⑤Therelationshipbetweenmainlineandentryflowformerginglanesandtherequirementthatmerginganddivergingflowsshouldnotexceedtheupstreamanddownstreamflowformerginganddiverginglanesrespectively.UsingtheselimitingfactorstheDepartmentofTransporthavedevelopedflowregiondiagramsformerginganddiverginglanedesign.Becauseofthedifferingflowlevelsformotorwaysandall-purposeroads,theflowvaluesmustbemultipliedby1.125whenmotorwaymergesanddivergesarebeingconsidered.DesignofweavingsectionsWeavingtakesplaceinmanysituationsonhighwayswheretrafficstreamsmerge,divergeandcrosswhilsttravelinginthesamegeneraldirection.Thisformofweavingistobefoundonmaincarriagewaysbetweenintersectionsandiscausedbyconflictsinthepathsofentering,leavingandstraightthroughvehicles.Itisalsotobefoundonlinkroadswithinfree-flowintersectionsandintheareaofsomejunctionswhereenteringandleavingvehiclesconflict.Todesigntheweaving-sectionitisnecessarytoknowthedesignspeedofthemainlinecarriagewayupstreamoftheweavingarea;themainlineisnormallytakenastheonecarryingthemajorflow.Themaximumallowablehourlyflowsperlaneusedinthedesignofweavingsectionsvarywithcarriagewaywidthandroadtype.Thedesignofaweavingsectionrelatestheminimumlengthoftheweavingsectiontothetotalweavingflowfordifferingratiosofthemaximumallowablehourlyflowperlanetotheupstreamdesignspeed.Onruralmotorwaysthedesirableminimumweavinglengthisrecommendedas2kilometersbutinextremecaseswhen