NCATReport16-03

ASYNTHESISREPORT:VALUEOFPAVEMENTSMOOTHNESSANDRIDEQUALITYTO

ROADWAYUSERSANDTHEIMPACTOFPAVEMENTROUGHNESSONVEHICLE

OPERATINGCOSTS

ByDr.MaryM.RobbinsDr.NamH.Tran,P.E.

April2016

ASYNTHESISREPORT:VALUEOFPAVEMENTSMOOTHNESSANDRIDEQUALITYTOROADWAYUSERSANDTHEIMPACTOFPAVEMENTROUGHNESSONVEHICLEOPERATINGCOSTS

NCATReport16-03By

MaryM.Robbins,Ph.D.NamH.Tran,Ph.D.,P.E.

NationalCenterforAsphaltTechnologyAuburnUniversity,Auburn,Alabama

SponsoredbyNationalAsphaltPavementAssociation TheStateAsphaltPavementAssociations

April2016

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ACKNOWLEDGEMENTS

This authorswish to thank theNational Asphalt Pavement Association (NAPA) and the StateAsphaltPavementAssociationsforsponsoringthisresearchaspartoftheDeterminingServiceLife Based on Comparable IRI research project and for providing technical review of thisdocument.

DISCLAIMER

Thecontentsof this reportreflect theviewsof theauthorswhoareresponsible for the factsandaccuracyofthedatapresentedherein.Thecontentsdonotnecessarilyreflecttheofficialviews or policies of the sponsoring agencies, theNational Center for Asphalt Technology, orAuburn University. This report does not constitute a standard, specification, or regulation.Commentscontainedinthisreportrelatedtospecifictestingequipmentandmaterialsshouldnotbeconsideredanendorsementofanycommercialproductorservice;nosuchendorsementisintendedorimplied.

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TableofContents

1. INTRODUCTION.......................................................................................................................1

1.1 Background......................................................................................................................1

2. RIDEQUALITYANDPAVEMENTROUGHNESS.........................................................................1

2.1 ImportanceofRideQualitytoTravelingPublic...............................................................2

2.2 FactorsRelatedtoPerceivedRideQuality.......................................................................3

3. IMPACTOFPAVEMENTROUGHNESSONVEHICLEOPERATINGCOSTS..................................6

3.1 ImpactofPavementRoughnessonFuelConsumptionCosts..........................................7

3.2 ImpactofPavementRoughnessonTireWearCosts.......................................................8

3.3 ImpactofPavementRoughnessonMaintenanceandRepairCosts...............................9

3.4 ImpactofRoughnessonOilConsumptionCosts.............................................................9

3.5 ImpactofPavementRoughnessonDepreciationCosts................................................10

4. SUMMARY.............................................................................................................................10

REFERENCES.................................................................................................................................11

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ListofFigures

FIGURE2.1PavementConditionRankedThirdMostImportant(10)............................................2

ListofTables

TABLE2.1FactorsAssociatedwithDrivers'PerceptionofRoadRoughnessonUrbanHighways(9)..3TABLE2.2IRICategories(after15,16,17,19)..............................................................................6

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1. INTRODUCTION1.1 BackgroundRide quality is generally associated with users’ level of comfort relative to the traveledroadway, which, in turn, is affected by pavement smoothness or roughness. Pavementroughnessdescribestheirregularitiesinthesurfaceofthepavement.AsHveemstated,“Eversinceroadsandhighwayshavebeenconstructed,thepeoplewhousethemhavebeenkeenlyawareof the relative degrees of comfort or discomfort experienced in traveling” (1).Hveemgoes on towrite, “There is no doubt thatmankind has long thought of road smoothness orroughnessasbeingsynonymouswithpleasantorunpleasant”(1).Recognizingtheimportanceofridequalitytothetravelingpublic,transportationagencieshaveused this as a key performance indicator in the pavement maintenance and rehabilitationprocess, conducted periodically to extend pavement service life and, more importantly, toimprove motorist safety and satisfaction. The importance of pavement ride quality isrecognized by the Federal Highway Administration (FHWA) through its requirements for theHighwayPerformanceMonitoringSystem (HPMS).Agenciesare required to reportpavementroughnessintermsofInternationalRoughnessIndex(IRI)orPresentServiceabilityRating(PSR)where IRI is not reported (somenon-National Highway System routes) as part of theHPMS.AccordingtotheHPMSFieldGuide,IRI isutilizedinanumberofanalyses, includingmodelingpavementdeteriorationandforuseincostallocationstudies(2).As Swanlund points out in Public Roads, research has shown that smooth roads costtransportationagencieslessoverthelifeofthepavementandresultindecreasedhighwayuseroperatingcosts,delayedcosts,decreasedfuelconsumptionanddecreasedmaintenancecosts(3).Thus,“notonlydoourcustomerswantsmoothroadsforcomfort,smoothroadscostlessfor both the owner/agency and the user” (3). Therefore, there is a need to review previousresearchefforts to fullyunderstand the implicationsofpavement roughness. Thepurposeofthis synthesis report was to search, review, and synthesize available information on theimportance of ride quality and pavement smoothness to the traveling public. As part of thisreport,theimpactofpavementroughnessonvehicleoperatingcostswasalsoexaminedas itlikelyinfluencesthepublic’sperceptionofpavementroughness.2. RIDEQUALITYANDPAVEMENTROUGHNESSPavementroughnessmeasurementintermsofpavementserviceabilitywasfirstintroducedbytheAmericanAssociationofStateHighwayOfficials(AASHO)atthecompletionoftheAASHORoad Test in the late 1950s (4). In this measurement, the serviceability of a pavement isexpressedasPSR.PSRisthemeanroughnessratingonascalefrom0to5assignedbyapanelofpassengersdrivingoverthepavementinavehicle.Therelationshipbetweenthepanel-ratedPSR and other non-panel pavement performance measurements is represented by amathematicalmodelknownasthePresentServiceabilityIndex(PSI).Several studies were conducted after the AASHO Road Test to evaluate various non-panelmeasurementsystems.Ofthosestudies,the InternationalRoadRoughnessExperiment(IRRE)commissionedbytheWorkBankandconductedintheearly1980sinBrazilwasessentialtothe

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developmentoftheridequalitymeasurecommonlyusedtodayacrosstheUnitedStates.Thisexperiment was conducted in 1982 by research teams from Brazil, England, France and theUnited States (5). As a result of the IRRE, the International Roughness Index (IRI) wasestablished(6). IRI isanobjectivemeasurementofpavementroughnessandcanbeobtainedusing vehicle-mounted high-speed inertial profilers. Inertial profilers determine the distancebetween a reference point on the profiler and the pavement surface while accounting forverticalmovementofthevehicletocapturethetruerelativeprofile(7).Amathematicalmodelis then applied to the measured relative surface profile to calculate IRI as the suspensiondisplacement per unit of distance traveled, expressed asm/kmor in/mile (8). IRI is awidelyused method for measuring pavement roughness. IRI was found to be the most significantfactor associatedwith changes in drivers’ perceptionof road roughness inWashington State(9).2.1 ImportanceofRideQualitytoTravelingPublicThe importance of ride quality to the traveling public has been illustrated through national,regional, and state surveys. In a nationwide survey conducted in 2000 by the FHWA,respondentswereaskedwhichhighwaycharacteristicsshouldreceivethemostattentionandresources for improvement. Twenty-one percent selected pavement surface conditionsincludingquiet ride,surfaceappearance,durability,andsmoothnessof ride,whichwasratedjustlowerthantrafficflow(28%)andsafety(26%),asshowninFigure2.1(10).AmorerecentnationalsurveyconductedbyEdelmanBerlandfortheAsphaltPavementAlliancein2014askedrespondentstoidentifytheroadattributestheybelievedwereofgreatestimportance(11).Ofthe3,000-plusdrivers surveyed, “69% said theywerewilling to acceptperiodicmaintenancedelaysifitmeanstheygettoenjoyasmoothdrivingexperience”(11).

FIGURE2.1PavementConditionRankedThirdMostImportant(10)

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A five-yearmulti-phaseregionalsurveyofpublicperceptionsof two-lanehighwaypavementswas conducted toestablishpolicies and thresholds forpavement improvement inWisconsin,Iowa,andMinnesota(12).Oneofthekeyfindingsinthefirstphaseoftheregionalsurveywasthat most survey participants “believed the resurfacing should only occur when the ridedeteriorated”. It can be inferred from this finding that roadway-users relate the need forinterventionmostwithridequality.In a statewide survey conducted for an audit of the Washington State Department ofTransportation (WSDOT) in 1997, respondents indicated that “poor road surface” rankedsecond as the state’s biggest transportation problem (9). Authors reported thatrecommendations from the audit included “a need for greater recognition of customerperceptions of pavement conditions” and WSDOT “should consider including pavementroughnessinadditiontoPavementStructuralConditionandrutting,initscandidatepavementprojectthresholds”(9).2.2 FactorsRelatedtoPerceivedRideQualityAs a result of the 1997 audit, WSDOT conducted an experiment to determine the issuesassociatedwithdrivers’perceptionofsurfaceroughnessandactualmeasuredroughness.Thestudy identified the most significant factor associated with drivers’ perception of roadroughnesswasmeasured IRI,althoughotherfactorsalsoaffectdrivers’perception(seeTable2.1)(9).TABLE2.1FactorsAssociatedwithDrivers'PerceptionofRoadRoughnessonUrbanHighways(9)

VariablesAssociatedwithMoreRoughness

VariablesAssociatedwithLessRoughness

− MeasuredIRI − Olderindividuals− Observable"maintenance" − Sportutilitytestvehicles− Presenceofjoints/abutments − Minivantestvehicles− Ageofsurface − Femaleusers− In-vehiclenoise − FrequentusersofSR520− Vehiclespeed − Highincomeusers − Maleusers − FrequentusersofI-405 Aspartofthefive-yearmulti-phaseregionalstudy,asurveywasconductedwhereparticipantswereaskedtotravel todesignatedstretchesofroadwayand indicatethe level towhichtheywere satisfiedwith the pavement, felt itwas better thanmost, or should be improved (12).RelationshipsdrawnbetweendriversatisfactionandpavementroughnessrevealedthatwhenrespondentsinWisconsinweresatisfied,thevaluesofIRIrangedfromaslowas0.7m/km(44.3in/mile)to3.3m/km(209 in/mile).Although itwasconcludedthat“thepavement indicesdonotexplainsatisfactiontoanygreatdegree,” the IRIvaluesatwhich70%of therespondentswere satisfied were 1.7 m/km (108 in/mile), and 1.2 m/km for Wisconsin and Iowa,

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respectively.Furthermore,inbothstates,70%oftherespondentsagreedthatoftheroadwaysdriven,thosewithanIRIof2.8m/km(177in/mile)shouldbeimproved.A similar studycompleted in2014 forNorthCarolinaDepartmentofTransportation (NCDOT)asked241participants to rate the smoothness of thepavements in each regionof the state(13). Participants rated the pavement by both categorical (acceptable or unacceptable) andnumerical (0 – 5) scales. Similar to the 1997 audit conducted inWashington, perceived ridequalitywasinfluencedmostbymeasuredIRIvalues,speedlimit,andseatinglocationwithinthesurveyvehicle.RelationshipsbetweenmeasuredIRIvalueandcategoricalratingsindicatedthatpavementswithIRIof103in/mileor lessweremost likelyratedasacceptable,androadwayswithIRIgreaterthan151in/mileweremostlikelytoberatedasunacceptable.FHWArecognizedtheimportanceofridequalitywiththeirMobilityGoal,establishedaspartofthe 1998 FHWANational Strategic Plan to increase the percentage ofmiles on theNationalHighwaySystem(NHS)withacceptableridequalityto93%within10years(14).AcceptableridequalityforInterstatesandtheNHSwasdefinedasIRIlessthanorequalto170in/mile,andthusIRIgreater than170 in/milewasconsideredunacceptable. In2002, theprimaryperformancegoal was revised to a goal of 95% of vehicle miles traveled on the NHS, and a secondaryperformancegoalwasalsoestablishedtosetagoalforvehiclemilestraveledonroadwayswithgoodridequality.GoodridequalitywasdefinedbyanIRIof95in/mileorlessandthusrefiningthe IRIcategoriesasgood (≤95 in/mile),acceptable (>95 in/mileand≤170 in/mile),andanimpliedcategoryofunacceptable(>170in/mile).Thesecategorieswererevisedagainin2015when FHWAproposed criteria for determining good, fair, and poor ratings for the proposednationalperformancemeasuresrequiredunderMovingAheadforProgressinthe21stCentury(MAP-21) (15). Proposed categories for IRI are separatedbyurban andnon-urbanareas. Fornon-urbanareas,IRIisdefinedasgood(<95in/mile),fair(95to170in/mile),andpoor(>170in/mile). Forurbanareas (definedbyapopulationof1,000,000), IRI isdefinedasgood (<95in/mile),fair(95–220in/mile),andpoor(>220in/mile).Although thresholds and goals were established at the national level, not all agencies haveadopted them. For example,WSDOT considers an IRI greater than 221 in/mile unacceptable(16).Louisianarespondedtoasurveyissuedaspartofa2009studyonpavementratingsandscores across the nation, with limits defining acceptable IRI for different classifications ofroadways (17). These values were reported as IRI less than 171 in/mile, 201 in/mile, 226,in/mile,and226in/milefor Interstatehighways,NHS,statehighways,andregionalhighways,respectively.While these thresholdsmaybehigher than thoseestablishedby FHWA, severalagencieshaveset internalgoals thatare tighter than thenationalgoals.Aspartof the samesurvey, agencieswere asked to report any specific state legislation or internal goals. Severalagencies reported goals or legislation that specifically pertained to IRI, and they aresummarizedfromthe2009study(17)asfollows:

• ThegoalreportedforArkansaswastorehabilitateallpavementswithIRI>96in/mile.• MaineaimedtokeepIRIontheirroadwayslessthan168in/mile.• Maryland reported separategoals for interstateand secondary routesas IRI less than

119in/mileand171in/mile,respectively.

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• TheinternalgoalsetforMontanawasreportedashavinglessthan5%ofthemileswithIRIgreaterthan148in/mile.

• Atthetimeofthesurvey,Vermont’sproposedgoalwastohavea“minimumrideindexof 50% or IRI of 171 in/mile”. It can be inferred from this that Vermont intends toachieveroadwayswithIRIlessthan171in/mile.

• Virginia’sinternalgoalwasreportedasachievinglessthan15%oftheirroadwayswithIRIgreaterthan139in/mile.

Referring back to the 1997 audit of WSDOT, it was recommended that WSDOT “considerincludingpavementroughnessinadditiontoPavementStructuralConditionandrutting, in itscandidate pavement project thresholds” (9). A WSDOT report on the state’s pavementroughness in 2010 reveals thatWSDOT currently considers an IRI-based roughness index forprogrammingprojects anda cracking indexand rutting index todeterminewhen resurfacingshould be conducted (16). However, because roughness is generally considered a “lagging”indicatorthatincreaseswhenotherproblemssuchascrackingandruttinghavebecomesevere,rehabilitations for asphalt pavements in Washington are typically triggered by cracking orruttingindices.IRIasalaggingindicatormaybeexplainedbyWSDOT’sunacceptableIRIvalueof 221 in/mile. It should be noted, however, that WSDOT reports that only 9.1% of theirpavementnetworkwasunacceptable in2012byFHWA’scriteria(18).WSDOTalsopointsoutthat the FHWAcategories for IRIweredeveloped for InterstateHighways for thepurposeofcomparingrelativeperformancefromstate-to-state(16).Asidefromthresholdsfordefiningacceptableroughness,WSDOT,likemanyagencies,assignedqualitativedescriptionstorangesofIRIforassessmentoftheridequalityoftheirpavements.Table 2.2 shows WSDOT’s comparison of the IRI categories with those of FHWA. The IRIcategoriesforFHWAshowninWSDOT’scomparisonweredevelopedbyFHWAinearlieryearsand presented in the 1999 Conditions and Performance Report when the use of IRImeasurements were not as widespread (19). These categories, listed in Table 2.2, wereintendedfortranslationbetweenPSRandIRI.Toputthesecategoriesincontext,therecentlyproposedcategoriesforMAP-21havealsobeenshown.IRIcategoriesforotheragencieshavebeen reported in a comparison of pavement condition performance measures originallyconductedin2008andpresentedinthe2009studyonpavementscores(17).Thoseagencies’established categoriesof roughness are also listed in Table2.2. It is evident thatmany stateagenciesareusingIRItodescriberidequality,andthevalueswithineachcategoryvarytosomedegreefromstate-to-state.

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TABLE2.2IRICategories(after15,16,17,19) IRICategoriesofRoughness(in/mile)Agency VeryGood Good Fair Poor VeryPoorFHWA(1999) <60 61–95 96–120 121–170 >170FHWA(2015)Non-urbanUrban*

<95<95

95–17095–220

>170>220

WSDOT ≤95 96–170 171–220 221–320 >320Arkansas 1–95 96–170 >170 Indiana <101 101–169 ≥170 Nebraska <55 55–157 158–211 212–267 >268NevadaInterstateNon-interstate

1–59

60–94

95–11995-170

NorthDakota <80 81–129 130–177 >177 *Urbanhaspopulation≥1,000,0003. IMPACTOFPAVEMENTROUGHNESSONVEHICLEOPERATINGCOSTSRidequalityaffectsnotonlyusers’levelofcomfortbutalsotheircosts.AccordingtoBiehler,aformer president of the American Association of State and Highway Transportation Officials(AASHTO),“TheAmericanpublicpaysforpoorroadconditionstwice—firstthroughadditionalvehicleoperatingcostsandtheninhigherrepairandreconstructioncosts”(20).Hegoesontoelaborate,“Drivingonroughroadsacceleratesvehicledepreciation,reducesfuelefficiency,anddamagestiresandsuspension”(20).According toa2015 factsheetonsurface transportationpublishedbynational transportationresearchgroupTRIP,88%ofpersonmilesoftravelareconductedinprivatevehicles(21).Giventhislargepercentage,thepublicislikelytonoticeroughpavements.TRIPusedIRIasreportedby the FHWA in 2013 to surmise that 28% of the “nation’smajor urban roads” (interstates,freeways,andothermajorroutesinurbanareas)wereinpoorcondition(IRI>170in/mile)and41%were inmediocre or fair condition (IRI = 120 – 170 in/mile) (22). TRIP also found that“driving on roads in need of repair costs the average driver $516 annually in extra vehicleoperating costs,” with additional vehicle operating costs ranging between $549 and $1,044annuallyforurbanareas(populations>250,000).ThesecostswereestimatedbyapplyingtherecentHighwayDevelopmentandManagement(HDM)model,HDM-4,andresultsfroma1994TexasTransportation Institute (TTI) report totheaveragenumberofmilesdrivenonroads inneed of repair, determined from the FHWA IRI data reported in 2013, and current vehicleoperatingcostsreportedbyAAAin2012(22).Vehicleoperatingcosts(VOC)generallyconsistsoffuel,tirewear,maintenanceandrepair,oilconsumption costs, and canalso includevehicledepreciation costs. Ithas longbeen thoughtthat VOC is influenced by pavement condition, pavement type, roadway geometry, andoperatingspeedinadditiontovehicletypeandvehicletechnology.Anumberofstudieshave

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beencompletedonsuchtopics,includinganinvestigationdatingbackto1877(23).Sincethen,numerousstudieshavealsobeencompletedontheeffectofpavementcondition,specificallyoncomponentsofVOC.FocushasbeenplacedontheeffectofroughnessonthecomponentsofVOC(excludingthefrequentlychangingvehicletechnologyandtype):fuelconsumption,tirewear, repair and maintenance, and oil consumption costs. The following sections include asynthesisoftherelevantliterature.3.1 ImpactofPavementRoughnessonFuelConsumptionCostsExtensive work has been conducted in the area of fuel consumption relative to pavementroughness. Although not all studies evaluated the additional costs due to increased fuelconsumption on rough pavements, it can be inferred that an increase in fuel consumptionwould result in increased costs. Much of the early studies were conducted in developingcountries and/or on unpaved, gravel or earthen roadway surfaces and, in some cases,roughness levels were beyond those typically seen in the U.S. (24-27). For example, fourprimarycoststudieswereconductedbetween1970and1982 inKenya, theCaribbean, India,and Brazil. These studies indicated that therewas an effect of pavement roughness on fuelconsumption,althoughtheeffectwasmuchmorepronouncedinthemodelsdevelopedintheBrazilianstudythantheotherthree(25).Therangeof IRIusedfortheseexperimentsrangedfromaslowas2.0m/km(intheCaribbean)toashighas22.1m/km(inKenya)(26).Applyingthe conversion factor of 1m/km = 63.4 in/mile to these studies results in a range of 126.8in/mileto1401.1in/mile,whichfallsintothecategoryofpooronthelowendandverypooronthehighendaccordingtotheFHWA(seeTable2.2).Four studies were published in ASTM’s 1990 STP1031, in which an effect of pavementroughness on fuel consumption and/or rolling resistance was reported in four additionalcountries(Belgium,France,SouthAfrica,andSweden)(28-31).LaterconvertedtoIRIbyotherresearchers, the percent change in fuel consumption per unit of IRI (m/km) for these fourstudieswasreportedforacarandrangedfrom0.7%to1.7%(32).Results from a Wisconsin study revealed a nonlinear increase in fuel consumption with anincrease in roughness (33). It was found that a 3% increase in fuel consumption resultedbetween the smoothest (serviceability index (SI) = 4.4) and roughest (SI = 0.9) pavementstested. A South African study found a strong correlation between roughness and rollingresistance(andthus,fuelconsumption)(34).Despitethesereportedcorrelationsbetweenfuelconsumptionandroughness,aseparateU.S.studyfoundnostatisticallysignificantdifferencesatthe95%levelinfuelconsumptiononpavedsections(35).Therefore,itwasconcludedthatfortherangeofconditionsintheU.S.,thetypeorconditionofpavedroadsdoesnotinfluencefuelconsumption.Theauthorsacknowledgedthatthesefindingsconflictwithpreviousstudies(36,37) thatreportedacorrelationbetweenroughnessandfuelconsumption,citingthatthesectionsused in thoseexperimentsdidnot realistically representoperating conditions in theU.S.duetothepotholes,patches,andbadlybrokenportionsoftheroadwayincludedinthosestudies. Later,BarnesandLangworthyelected to forgo theeffectofpavement roughnessonfuelconsumptioncostsindevelopingper-milecostsfortrucksandpassengercarsinMinnesota(38,39).Theauthorsstated thatpreviousstudieswere indevelopingcountriesandonroads

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withmuchworseconditionsthanintheU.S.;therefore,BarnesandLangworthyconcludedthatroughnesseffectsonfuelconsumptionwerenotapplicable.AlthoughBarnes and Langworthy chose not to include the effect of pavement roughness onfuel consumption, several recent studies in the U.S. have reported a positive correlationbetweenpavementroughnessandfuelconsumption.ChattiandZaabarconcludethatthemostimportantpavementconditionfactorrelativetofuelconsumptionissurfaceroughnessintermsofIRI(40).Additionally,studiesatWesTrack(41),Florida(42),andMissouri(43)revealthatforroughness levels and conditions seen in the U.S., pavement roughness influences fuelconsumption.ThesestudiesreportedhigherfuelefficiencyonFloridapavementswithlowerIRI(42) and improvements in fuel consumption by up to 4.5% atWesTrack (41) and 2.461% inMissouri (43) on smoother pavements. It has also been shown at the National Center forAsphaltTechnology’sPavementTestTrackthat fuelconsumption increaseswith increased IRI(44).Furthermore,thewidelyadoptedHDM-4model forcomputingtotal transportcostswascalibrated for U.S. conditions, reflecting roughness levels and improvements in vehicletechnology(40).InevaluatingtheHDM-4fuelconsumptionmodelandtheeffectofpavementroughnesson fuel consumption, ananalysisof covariancewas conducted showingpavementroughness to be statistically significant. In calibrating the HDM-4 fuel consumption model,Chatti and Zaabar determined that a 1 m/km (63.4 in/mile) increase in IRI effects fuelconsumption by approximately 2% (40). By decreasing IRI by 1 m/km, Chatti and Zaabarestimateasmuchas$24billioncouldbesavedinfuelcostsperyearintheU.S.basedona3%reduction in fuel consumption for255millionpassengercarsandgaspriceswhen the reportwascompiledin2012.3.2 ImpactofPavementRoughnessonTireWearCostsTheinfluenceofpavementcondition,specificallypavementroughness,ontirewearcostshasbeenanimportantpartofvehicleoperatingcoststudies.In1985,ZaniewskiandButlerstated“thereisdirectphysicalevidencebeingcompiledaroundtheworldthatshowsthatpavementroughness influences vehicle operating costs” (45). In developing countries, varying levels ofinfluencewerereportedforroughnessontirewear,suchthatmodelsdevelopedforKenyaandthe Caribbeanhad a rate of increase in tirewear due to pavement roughness of nearly twotimeshigher than the rateutilized inmodelsdeveloped forBrazil and India (27).A tirewearpredictionmodelwasdevelopedWatanatada,Dhareshwar,andRezende-Lima(27),usingdataprimarily fromtheBrazil study (46).Although limiteddatawereavailable, themodel forcarsandutilities“wascalibratedasasimplelinearfunctionofroadroughness.”Itwasalsoreportedthatbasedonthisdevelopedmodel, roughnesshadasmalleffect intirewear foraconstantloadlevelonalevelroadandamuchgreatereffectonsteeproads.In1982,aU.S.study(35)developedadjustmentfactorsbasedonrelationshipsdevelopedintheearliercoststudyconductedinBrazil.Costadjustmentfactorsproportionatetothechangeintireconsumptionasthesurfacechangesfromabaselineconditionofaserviceabilityindex(SI)of 3.5 were developed (35). These cost adjustment factors were such that the adjustmentfactors at SI = 3.5 were 1.00 and increased with decreasing SI, indicating that rougherpavementsresultinhighertireexpenserelatedtotirewear.Inasimilarfashion,inaMinnesota

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studyanincrementalincreaseof10%foreverydecreaseinPSIof0.5(startingat5%foraPSIof3.0anddownto2.0)wasusedtoaccountfortheeffectofpavementroughnessonallvehicleoperating costs, with the exception of fuel consumption costs (38, 39). For themost recentstudy in the U.S., in which the HDM-4 tire wear model was calibrated for U.S. conditions,increasingIRIby1m/km(63.4in/mile)wasfoundtoincreasetirewearby1%at88km/h(55mph)(40).Inturn,decreasingIRIby1m/kmcouldsave$340millionperyearintirewearcosts(40).3.3 ImpactofPavementRoughnessonMaintenanceandRepairCostsIt has been shown through vehicle fatigue response testing that pavement roughness doesinfluence the responseof vehicle suspensionandcan result inaccelerated fatigueatPSI lessthan1.0(47).Consistentwiththisfinding,correlationsbetweenmaintenanceandrepaircostsandpavementroughnesshavebeenobservedinseveralstudiesintheU.S.andabroad.Earlierstudies in developing countries found a positive effect of roughness on vehiclemaintenancecosts(25).Inallfourcountries,vehiclepartsconsumptionwasmodeledbyroadroughnessandvehicle age. The effect of roughness on vehicle parts consumption for cars and light goodsvehicleswasmodeledbyalinearrelationshipintheCaribbeanandKenya,whileanexponentialfunction for road roughness was used for the Brazilian and Indian models (25). While thisexponentialrelationshipresults inverylargeincreasesinpartsconsumptionsforroughroads,therearenotabledifferencesbetweenthestudiesincludingsurfacetype(paved,earthen,andgravel), and vehiclemodels and their inherent deterioration rate. Simplemodels correlatingsparepartsandmechanics’ laborwithroadcharacteristicsweredevelopedusingtheBraziliandata(27).Sparepartsconsumptionwasfoundtobedependentonroadroughnessandvehicleage,andtheeffectscombined“multiplicatively”.Whenagewasheldconstant,therelationshipbetween parts consumption and roughness was found to be generally non-linear. Costadjustment factorsweredeveloped for anearlyU.S. study (35, 45) aswell as amore recentstudy in Minnesota (38, 39) to account for the effect of roughness. The Minnesota studysuggests that a 1¢ permile increase formaintenance and repair costs results whenmovingfromthesmoothesttotheroughestpavementinthestudy.ThecalibratedHDM-4repairandmaintenancemodelrevealedthatdecreasingIRIby1m/km(63.4in/mile)couldresultinrepairand maintenance savings between $24.5 billion and 73.5 billion per year (40). It should benotedthattheseestimateswerebasedonestimatedannualrepairandmaintenancecostsfor255 million U.S. passenger cars totaling $244.8 billion and IRI greater than 3 m/km (190.2in/mile).3.4 ImpactofRoughnessonOilConsumptionCostsIt was reported early on that engine oil consumption costs were the least important costcomponentintheaggregatevehicleoperatingcosts(24).Echoingthissentiment,ChesherandHarrison later stated “costs associated with the consumption of engine oils, other oils andgrease,areaminorelementoftransportcosts”(25).Thismayexplainthe limitedresearch intheareaofpavementroughnesseffectsonoilconsumptioncosts.Although limited, researchhasshownthattheeffectforengineoilconsumptioncanbehighforcarsinIndia(20).DespitetheseresultsforIndiancars,ChesherandHarrisonreportedthat“roughnesswaspredictedtohaveaverysmalleffectonIndiantruck[engine]oilconsumption”(25).Additionally,Zaniweski

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et al. updated adjustment factors originally developed by Winfrey (48). These updatedadjustmentfactorsindicatedthatrougherpavementsincreaseengineoilconsumption(35,45).TheadjustmentfactorfortrucksanSIof4.0wasreportedas0.82andabout1.1foranSIof2.0,exhibitinganincreaseinoilconsumptioncostsasSIdecreases(orasroughnessincreases).3.5 ImpactofPavementRoughnessonDepreciationCostsDepreciation was modeled in early international studies indirectly as a function of surfaceconditionsandroughnessbyconsideringvalue-agerelationshipstocalculatevalueduetoage(25). The 1982 U.S. study considered the use-related expense portion of depreciation byapproximatingthereciprocalofthemaximumvehiclelifemileageanddatafromthepreviousBrazilstudytoadjustexpensesforpavementconditionsthroughcostadjustmentfactorsasafunction of serviceability index (35, 45). However, research on the effect of pavementroughness on depreciation costs is limited, with researchers making decisions on vehicleoperating costs based on experience rather than observed causal effects of roughness ondepreciation. This was the case in Minnesota, where researchers stated that “a car that isdriven almost exclusively on smooth highways will last more miles than one that is drivenmostlyonroughpavements”(38).4. SUMMARYThroughstate,regional,andnationwidesurveys, ithasbeenestablishedthatridequalityandpavementsmoothnessareimportanttothetravelingpublic.Pavementsurfaceconditionswereranked third most important in the 2000 FHWA nationwide survey (10) and ranked as thesecondbiggesttransportationprobleminthe1997WSDOTstatewidesurvey(9).Certainlevelsofpavement roughnessareassociatedwithnotonlyacomfortableoranuncomfortable ridebutalsoaneed formaintenanceor rehabilitation.A regional surveyconducted inWisconsin,Iowa,andMinnesotafoundmostparticipants“believedtheresurfacingshouldonlyoccurwhenthe ride deteriorated” (12). Studies in both Washington and North Carolina revealed thatmeasured IRI was one of the most influential factors in drivers’ perception of pavementroughness(9,13).Public perception is also likely to be influenced by increased vehicle operating costs due torougher pavements, therefore, the effect of pavement roughness on components of vehicleoperatingcostswereexplored.Vehicleoperatingcostsincludefuel,tirewear,maintenanceandrepair,oilconsumption,anddepreciationcosts.Extensivestudieshavebeenconductedonthetopic of pavement roughness and its effects on fuel consumption costs. Much of the earlystudies, particularly those conducted indeveloping countries, included roadway surfaces androughness levels that extend well beyond those considered unacceptable in the U.S. Citingthese conditions, researchers elected to forgo the effect of pavement roughness on fuelconsumption costs in developing per-mile costs for trucks and passenger cars inMinnesota(38).However,otherstudiesconductedatWesTrack(41),Florida(42),andMissouri(43)inthe2000s reveal that for roughness levels seen in theU.S., pavement roughness influences fuelconsumption and thus, influences fuel consumption costs. Additionally, the widely adoptedHDM-4modelforcomputingtotaltransportcostswascalibratedforU.S.conditions,reflectingroughness levels and improvements in vehicle technology (40). The calibrated HDM-4 fuel

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consumptionmodelwasusedtodeterminethat1m/km(63.4 in/mile) increase in IRIeffectsfuelconsumptionbyasmuchas2%.In addition to fuel consumption, costs associatedwith tirewear,maintenanceand repair, oilconsumption, and depreciation were also found to be influenced by pavement roughness.Researchintheareasofoilconsumptionanddepreciationcostsarelimited;however,ofthoseearlystudiesthatincludedtheeffectofroughnessonthesecostcomponents,itwasfoundthatindevelopingcountries,anincreaseinoilconsumptionoccurredwithanincreaseinpavementroughness (25). Maintenance and repair costs were found to increase with increases inroughness,albeittherateofincreasevariedfromstudytostudy;thiswasalsothecasewithtirewear costs. Applying the calibratedHDM-4model to 255million vehicles, Chatti and Zaabarreportedthatbydecreasingpavementroughnessby1m/km(63.4in/mile),asmuchas$73.5billion inmaintenance and repair costs could be saved annually in the U.S. (40). Chatti andZaabaralsoreportedthatthesamereductioninIRIcouldtranslatetoasavingsof$340millionperyearintirewearcostsforpassengervehicles.AlthoughtheestimatesconductedbyChattiandZaabarforcostsavingsrelatedtoreductioninIRIaresomewhatgeneralizedandinsomecases, based on roughness values at or above FHWA’s failure criteria, they illustrate thepotentialforVOCsavingswithimprovedpavementroughness.It is evident from the state, regional, andnational surveys that ridequality isoneof thekeyconcernsofthetravelingpublic.Furthermore,researchdatingbacktothe1960shasshowntheinfluence of pavement roughness on components of vehicle operating costs, indicatingincreased VOCs with increased roughness. The findings from literature reported hereunderscore Swanlund’s sentiments that “not only do our customers want smooth roads forcomfort, smooth roads cost less for both the owner/agency and the user” (3), as researchsuggestshighervehicleoperatingcostsareassociatedwithroughpavements.

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