交通工程專業(yè)英語(yǔ) 7 25課文

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．一些解決方法實(shí)際上是不可行的(例如一座拱橋缺乏適當(dāng)?shù)幕A(chǔ))，但仍會(huì)有一些重要的選擇將被保留。2．Economicsavoidstheneedforthesearbitraryrulesbyprovidingatangiblecriterion-cost-forchoosingbetweenalternativesolutions．經(jīng)濟(jì)學(xué)通過(guò)在選擇備選方案時(shí)提出一個(gè)切實(shí)的成本標(biāo)準(zhǔn)，從而避免隨意性規(guī)則的使用。3.Itismoreaquestionoftheeconomicattitudeofmindthatasks：Isthistheonlypossiblesolution,andifnot,isthereabetterone?這更多的是一個(gè)從經(jīng)濟(jì)學(xué)角度提出的問(wèn)題：這是唯一可能的解決方法嗎，如果不是，還有更好的方法嗎?4．Questionsofdistributionneverthelessgobeyondmeredifferencesinincome．不過(guò)分配問(wèn)題超過(guò)收人上的微弱差別。5．Itisthereforeoftenappropriatetoseparatetheimpactofatransportimprovementintoitseffectondifferentinterestgroups．inadditiontoitseffectondifferentincomegroups．因此，除了影響不同的收入階層外，通常將交通改善的影響還按其對(duì)不同的利益階層的影響來(lái)加以區(qū)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，通常在雙車(chē)道城市道路或市區(qū)干線上提供超車(chē)視距實(shí)際價(jià)值不大。2．Theminimumsightdistanceavailableonaroadwayshouldbesufficientlylongtoenableavehicletravelingatornearthedesignspeedtostopbeforereachingastationaryobjectinitspath．道路上應(yīng)有的最小視距應(yīng)該足以使那些以等于或接近設(shè)計(jì)速度行駛的車(chē)輛能在到達(dá)行駛路線上的障礙物之前停下來(lái)。orcomplexmakemustdriverswheninadequateoftenaredistancesthese，However．3．instantaneousdecisions,wheninformationisdifficulttoperceive，orwhenunexpectedorunusualmaneuversarerequired．但是，當(dāng)駕駛員必須進(jìn)行復(fù)雜或瞬間的判斷，信息難以感覺(jué)到，或當(dāng)需要進(jìn)行出乎意外的或不同尋常的操縱時(shí)，上述停車(chē)距離往往是不足的。4．Decisionsightdistanceisthedistancerequiredforadrivertodetectanunexpectedorotherwisedifficult-to-perceiveinformationsourceorhazardinaroadwayenvironmentthatmaybevisuallycluttered，recognizethehazardoritsthreatpotential，selectanappropriatespeedandpath，andinitiateandcompletetherequiredsafetymaneuversafelyandefficiently．判斷視距是指駕駛員所需的這樣一個(gè)距離：在可能引起視覺(jué)混亂的道路環(huán)境中發(fā)覺(jué)一個(gè)意外的或難以察覺(jué)的信息源或危險(xiǎn)，或發(fā)覺(jué)其潛在的威脅，從而選擇適當(dāng)?shù)乃俣群吐肪€，并安全、有效地開(kāi)始與完成所需的安全操作。5．Thuspassingmaneuversthatrequirecrossingthecenterlineoffour-laneundividedroadwaysorcrossingthemedianoffour-lanedividedroadwaysarerecklessandshouldbeprohibited．因此，跨越無(wú)分隔設(shè)施的四車(chē)道道路中線或跨越四車(chē)道道路中央分隔帶的超車(chē)運(yùn)行是冒險(xiǎn)的，應(yīng)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．隨著交通量的增加，優(yōu)先控制交叉口的延誤將隨之迅速增加，所以對(duì)于交通量較大的交叉口有必要采用分離式立交的形式。2．Ontheotherhand，iflandpurchaseisnecessary，itisofteneasiertoacquirecornersitesnecessaryforaroundaboutthanthelongnarrowstripsneededwhenparallelwideningoftrafficsignalapproachesiscarriedout．．另一方面，如果必須購(gòu)置土地，與信號(hào)交叉口人口拓寬所需的長(zhǎng)條形土地相比，環(huán)島所需的邊角狀土地更容易獲得。3．Eitherlatestartorearlycut-offfacilitiesoraspecialphasemustbeprovidedcausingreducedoverallcapacityatthejunction．無(wú)論是實(shí)行遲啟交通信號(hào)、早斷交通信號(hào)或者提供一個(gè)專用相位，都會(huì)降低交叉口總的通行能力。4.TheDepartmentofTransportdefinesanormalroundaboutasonewhichhasaone-waycirculatorycarriagewayaroundakerbedcentralisland4mormoreindiameterandusuallywithflaredapproachestoallowmultiplevehicleentry．交通部把通常的環(huán)形交叉口定義成具有圍繞中心島的單向環(huán)形車(chē)行道，環(huán)行道四周連接有允許各種車(chē)輛進(jìn)入的路口加寬的引道，中心島直徑不小于4米并由側(cè)石圍砌而成。5．Itisrecommendedthattheentryradiusintothecirculatingareashouldliebetween6mand100mwithanormalvalueofapproximately20m．米。20米，通常約為100米～6建議進(jìn)入環(huán)形區(qū)的入口半徑應(yīng)為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．a(chǎn)ll-purposeroadsandformotorways.⑤Therelationshipbetweenmainlineandentryflowformerginglanesandtherequirementthatmerginganddivergingflowsshouldnotexceedtheupstreamanddownstreamflowformerginganddiverginglanesrespectively.UsingtheselimitingfactorstheDepartmentofTransporthavedevelopedflowregiondiagramsformerginganddiverginglanedesign.Becauseofthedifferingflowlevelsformotorwaysandall-purposeroads,theflowvaluesmustbemultipliedby1.125whenmotorwaymergesanddivergesarebeingconsidered.DesignofweavingsectionsWeavingtakesplaceinmanysituationsonhighwayswheretrafficstreamsmerge,divergeandcrosswhilsttravelinginthesamegeneraldirection.Thisformofweavingistobefoundonmaincarriagewaysbetweenintersectionsandiscausedbyconflictsinthepathsofentering,leavingandstraightthroughvehicles.Itisalsotobefoundonlinkroadswithinfree-flowintersectionsandintheareaofsomejunctionswhereenteringandleavingvehiclesconflict.Todesigntheweaving-sectionitisnecessarytoknowthedesignspeedofthemainlinecarriagewayupstreamoftheweavingarea;themainlineisnormallytakenastheonecarryingthemajorflow.Themaximumallowablehourlyflowsperlaneusedinthedesignofweavingsectionsvarywithcarriagewaywidthandroadtype.Thedesignofaweavingsectionrelatestheminimumlengthoftheweavingsectiontothetotalweavingflowfordifferingratiosofthemaximumallowablehourlyflowperlanetotheupstreamdesignspeed.Onruralmotorwaysthedesirableminimumweavinglengthisrecommendedas2kilometersbutinextremecaseswhen