Chapter 4 DESIGN CONTROLS.docx
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Chapter4DESIGNCONTROLS
thereareanumberofdesigncontrolsthataffectthegeometryofahighway.(Rt.71,IL)
Chapter4
DESIGNCONTROLS
RefertoChapterIIoftheAASHTOGreenBook
BACKGROUND
Inordertodesignthebasicelementsofahighwayincludingitsalinementandcrosssectionthedesignermusthaveanunderstandingofthebasicdesigncontrolsandcriteriaassociatedwiththehighway.Oneofthemostimportant,highwayfunctionalclassification,wasdiscussedinChapter3.Otherimportantdesigncontrolsinclude,butarenotlimitedtothefollowing:
∙Thedesignspeedofthefacility
∙Theacceptabledegreeofcongestion(i.e.,thedesignyearpeakhourlevelofservice)onthefacility
∙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
X
X
Lanewidth,urban
X
X
Ruralshoulderwidth,type
X
X
Urbanshoulderwidth,type
X
X
Guiderailoffset
X
X
Degreeofcurve
X
Grades
X
X
X
Bridgeclearances(horizontal&vertical)
X
X
Stoppingsightdistance
X
Superelevation
X
Wideningoncurves
X
Ruraldesignspeeds
X
X
X
Urbandesignspeeds
X
X
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
60-70
Mountainous
50-60
50-60
ArterialHighways
Terrain
Rural
Urban
Flat
60-70
30-60
Rolling
40-60
30-50
Mountainous
30-50
30-50
CollectorandLocalRoads
Terrain
Rural
Urban
Flat
30-50
30-40
Rolling
20-40
20-40
Mountainous
20-30
20-30
Source:
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