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TeenDriverRiskinRelationtoAgeandNumberofPassengersMay201260714thStreet,NW,Suite201|Washington,DC20005|AAAFoundation.
org|202-638-5944TeenshavethehighestcrashrateofanygroupintheUnitedStates.
AuthorsBrianC.
TefftAllanF.
WilliamsJurekG.
GrabowskiAcknowledgmentsTheauthorsgratefullyacknowledgetheinputoftwopeerreviewers.
Theirhelpfulsuggestionsresultedinsubstantialimprovementstothisstudy.
Anyerrorsarethesoleresponsibilityoftheauthorsandnotofanyindividualwhoreviewedorprovidedsuggestionsonthisreport.
AbouttheSponsorAAAFoundationforTrafficSafety60714thStreet,NW,Suite201Washington,DC20005202-638-5944www.
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orgFoundedin1947,theAAAFoundationinWashington,D.
C.
isanot-for-profit,publiclysupportedcharitableresearchandeducationorganizationdedicatedtosavinglivesbypreventingtrafficcrashesandreducinginjurieswhencrashesoccur.
FundingforthisreportwasprovidedbyvoluntarycontributionsfromAAA/CAAandtheiraffiliatedmotorclubs,fromindividualmembers,fromAAA-affiliatedinsurancecompanies,aswellasfromotherorganizationsorsources.
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TheAAAFoundationforTrafficSafetydoesnotendorseproductsormanufacturers.
1AbstractThepresenceofpassengersinavehiclehasbeenshowntoincreasetheriskoffatalcrashinvolvementforteenagedrivers;however,thestudiesthathavequantifiedthisrelationshipwerebasedondatathatarenowoveradecadeold.
Intheyearssincethesestudies,mostU.
S.
stateshaveenactedgraduateddriverlicensingsystemsthatlimitthenumberofpassengersthatyoungdriversareallowedtocarryduringtheirfirstseveralmonthsofindependentdriving,andthenumberof16-and17-year-olddriversinvolvedinfatalcrasheseachyearhasdecreasedbymorethanhalf.
Theobjectiveofthisstudywastoprovideupdatedestimatesoftherelationshipbetweenthenumberandagesofpassengerspresentandthecrashriskpermiledrivenof16-and17-year-olddrivers.
Dataoncrashesthatoccurredinyears2007–2010anddataonthenumberofmilesdriveninyears2008–2009wereexamined.
Ratesofcrashinvolvementanddriverdeathpermiledrivenwereestimatedfor16-and17-year-olddriverswithnopassengers;withone,two,andthreeormorepassengersyoungerthanage21(andnoolderpassengers);andwithatleastonepassengeraged35orolder.
Comparedwithhavingnopassengers,havingonepassengeryoungerthanage21(andnoolderpassengers)wasassociatedwitha44%increaseina16-or17-year-olddriver'sriskpermiledrivenofbeingkilledinacrash(RelativeRisk[RR]1.
44,95%ConfidenceInterval[CI]1.
01–2.
04).
Havingtwopassengersyoungerthanage21wasassociatedwithadoublingofadriver'sriskofbeingkilledinacrash,comparedwithhavingnopassengers(RR2.
02,95%CI1.
36–2.
99).
Havingthreeormorepassengersyoungerthanage21wasassociatedwithroughlyaquadruplingofadriver'sriskofbeingkilledinacrash,comparedwithhavingnopassengers(RR4.
39,95%CI1.
45–13.
31).
Therelativeriskofbeinginvolvedinanypolice-reportedcrashinthepresenceofyoungpassengersfollowedasimilarpattern;however,theincreasesinrelativeriskofbeinginvolvedinanypolice-reportedcrashweresmallerandwerenotstatisticallysignificant.
Havingatleastonepassengeraged35orolderinthevehiclewasassociatedwitha62%decreaseina16-or17-year-olddriver'sriskpermiledrivenofbeingkilledinacrash(RR0.
38,95%CI:0.
24–0.
60),anda46%decreaseintheriskofbeinginvolvedinanypolice-reportedcrash(RR0.
54,95%CI0.
35–0.
84),comparedwithhavingnopassengers.
Theseresultsshowthatalthoughtheoverallnumberofteendriverfatalitieshasdecreasedsubstantiallyoverthepastseveralyears,carryingyoungpassengersisstillasignificantriskfactorforyoungdrivers.
Incontrast,carryingadultpassengerssignificantlyreducestherisksofcrashinvolvement.
2IntroductionPermiledriven,driversundertheageof20havehigherratesofinvolvementinfatalcrashesthandriversofanyotheragegroupexceptdriversaged80andolder(InsuranceInstituteforHighwaySafety,2012).
Whereastheapparentover-involvementofolderdriversinfatalcrashesispredominantlyattributabletotheirelevatedprobabilityofdyingiftheyareinvolvedinacrash—notexcessiveriskofcrashing—youngdrivers'excessiveinvolvementinfatalcrashesisduetoexcessivecrashrisk(Lietal.
,2003).
Whiledriversaged85andolderhavethehighestrateofdeathpermiledriven,driversaged16-17havethehighestrateofinvolvementincrashesthatresultinthedeathofoccupantsofothervehiclesornon-motorists(Tefft,2008).
Severalstudieshavefoundthatinrelationtotheirexposure(e.
g.
,numberoftripsornumberofmilesdriven),youngdrivershavehigherratesofcrashinvolvement,injury,anddeathwhencarryingpassengers.
Dohertyetal.
(1998)analyzeddatafromtheprovinceofOntario,Canadaonthedrivingexposureandcrashinvolvementofdrivers,andidentifiedcarryingpassengersasariskfactorfordriversaged16-19,andalsofoundthathavingtwoormorepassengerswasassociatedwithgreaterriskthanhavingonlyonepassenger.
Chenetal.
(2000)performedasimilaranalysisofdatafromtheUnitedStatesandfoundthatcarryingpassengerswasassociatedwithincreasedriskofinvolvementinacrashfataltothedriverfordriversaged16-19,andalsofoundthattheriskincreasedwiththenumberofpassengers.
Riceetal.
(2003)analyzeddatafromcrashesinthestateofCaliforniainwhichadriveraged16or17wasinjured,usingaquasi-inducedexposuremethodwhichinvolvedcomparingtherelativefrequencyofcrashesinwhichayoungdriverwasversuswasnotdeemedculpable.
Itwasestimatedthatcarryingteenagemalepassengersormixed-gendercombinationsofteenagepassengerswasassociatedwithsignificantlyincreasedcrashrisk,carryingteenagefemalepassengerswasnotassociatedwithincreasedrisk,andcarryingpassengersaged30orolderwasassociatedwithsignificantlydecreasedrisk.
Allthreeofthesestudiesalsoidentifieddrivingduringnighttimehoursasanindependentriskfactor.
Between1996andthepresent,mostU.
S.
statesimplementedsomeformofgraduateddriverlicensing(GDL)system,inwhichanewdriverinitiallyisallowedtodriveonlyunderthesupervisionofalicensedadultpassenger,andthenreceivesanintermediate(or"provisional")licensethatallowsunsuperviseddrivingbutonlyundercertainconditions.
Thedriverthenreceivesafull-privilegelicenseuponreachingacertainage(e.
g.
,18)oraccumulatingacertainamountofexperience(e.
g.
,12months)drivingwiththeintermediatelicense.
SeveralstudieshaveestimatedthatGDLsystemshavereducedthefatalcrashinvolvementof16-and17-year-olddriversbyroughly20-40%(Shope,2007).
Inmanystates,theintermediatestageoflicensureincludesrestrictionsoncarryingpassengers.
Felletal.
(2011)estimatedthatrestrictionsoncarryingpassengershavebeenassociatedwitha9%reductioninfatalcrashesinwhichdriversaged16-17hadteenagepassengers,andTrempel(2009)estimatedthatlawsthatlimitednewdriverstocarryingatmostoneteenagepassengerreducedcollisioninsuranceclaimsof16-and17-year-olddriversby4.
8%.
Thestudiesdocumentingtherisksassociatedwithcarryingpassengerswerebasedondatathatarenowoveradecadeold:Dohertyetal.
(1998)analyzeddatafrom1988,Chenetal.
(2000)analyzeddatafrom1992–1997,andRiceetal.
(2003)analyzeddatafrom1993–1998.
3In1998,2,58916-and17-year-olddriverswereinvolvedinfatalcrashes.
By2010,thisnumberhadfallenby56%to1,150(FatalityAnalysisReportingSystem,2012).
GiventheproliferationofGDLsystemsandthelargedecreaseintheoverallannualnumberoffatalcrashinvolvementsofyoungdrivers,thegeneralizabilitytothepresenttimeoftheresultsofthepreviousstudiesoftherelationshipbetweenpassengerpresenceandcrashriskisunknown.
Theobjectiveofthisstudywastoprovideupdatedestimatesoftherelationshipbetweenpassengerpresenceandcrashrisk,usingdatafromtheUnitedStatesfromyears2007-2010.
MethodsMainoutcomemeasureThenumberofdriverskilledincrashespermiledrivenandnumberofdriversinvolvedinpolice-reportedcrashes(ofanyseverity)permiledrivenwereestimatedfordriversaged16-17years,inrelationtothenumberandagesofpassengersinthevehicle.
DataDriverdeathsDataon16-and17-year-olddriverskilledincrasheswereobtainedfromtheNationalHighwayTrafficSafetyAdministration(NHTSA)FatalityAnalysisReportingSystem(FARS),whichcomprisesdataonallmotorvehiclecrashesthatoccuronpublicroadwaysintheUnitedStatesandresultinadeathwithin30daysofthecrash.
Datafromcrashesthatoccurredinyears2007–2010wereanalyzed.
Onlycrashesinwhichtheteenagedriverwasoperatingapassengervehicle(car,pickuptruck,van,minivan,orsportutilityvehicle[SUV])wereincluded;crashesinwhichthedriverwasoperatingamotorcycle,all-terrainvehicle,orothertypeofvehiclewereexcluded.
Therewere2,266recordsof16-and17-year-olddriverskilledincrashesoverthestudyperiod.
Police-reportedcrashesDataon16-and17-year-olddriversinvolvedinpolice-reportedcrasheswereobtainedfromtheNHTSA'sGeneralEstimatesSystem(GES),astratifiedsampleofallpolice-reportedcrashesintheUnitedStates.
RecordsinGESareweightedtorepresentallpolice-reportedcrashesintheUnitedStates.
Datafromcrashesthatoccurredinyears2007–2010wereanalyzed.
Onlycrashesinwhichtheteenagedriverwasoperatingapassengervehiclewereincluded;crashesinwhichthedriverwasoperatinganothertypeofvehiclewereexcluded.
Therewere14,656recordsof16-and17-year-olddriversofpassengervehiclesinvolvedinpolice-reportedcrashesoverthestudyperiod.
MilesdrivenDataonthenumberofmilesdrivenby16-and17-year-olddriverswereobtainedfromtheFederalHighwayAdministration's2009NationalHouseholdTravelSurvey(NHTS).
TheNHTScontainsdataonallofthetripstakenbyallmembersofarepresentativesampleofhouseholdsacrosstheUnitedStates.
IntheNHTS,respondentsentereddataintoatraveldiaryonanassigneddateandreporteditbymeansofatelephoneinterview.
Thesedataincludedthestarttime,endtime,andlengthofeachtrip,thetotalnumberofpassengersinthevehicle,andadditionaldemographicdata(includingage)ofpassengerswhowere4membersofthedriver'shousehold.
DatawerecollectedfromMarch2008throughApril2009andwereweightedtorepresentthetravelofallUnitedStateshouseholdsovera365-dayperiod.
The2009NHTSincludeddatafrom7,188respondentsaged16-17,ofwhom4,799weredrivers,ofwhom2,746madeatotalof9,663tripsasadriverofapassengervehicleontheirassignedtravelday.
DatainspectionCrashesRice&Anderson(2009)notedthatasof2005,FARSdatafromseveralstatesappearedtounder-reportpassengerswhowerenotinjured;Chenetal.
(2000)reportedthatseveralGESprimarysamplingunits(PSUs)alsodidso.
Toinvestigatewhethertherewasevidenceofsystematicunder-reportingofuninjuredpassengersinanystateorPSUduringthestudyperiod,theratiooftheproportionsofpassengerscodedasuninjuredtodrivers(ofanyage,notlimitedtodriversaged16-17)codedasuninjuredwastabulatedbystate(inFARS)andbyPSU(inGES)andwasevaluatedusinganapproximationofPierce'scriterion(1852)describedbyGould(1855).
IntheFARSdata,theaverageratiooftheproportionofpassengersuninjuredtotheproportionofdriversuninjuredwas0.
96(s.
d.
0.
20);recordsfromthestateofVirginia,witharatioof0.
04,wereexcludedfromthestudy(n=68).
IntheGESdata,theaverageratiowas0.
93(s.
d.
0.
19);recordsfromPSUs27(ratio=0.
04),73(ratio=0.
14)and93(ratio=0.
22)wereexcluded(n=951).
Alsoexcludedwererecordsfromindividualvehiclesinwhichtheagesofpassengersorthetotalnumberofpassengerswasunknown(FARS:n=7;GES:n=447).
Intotal,recordsof75fatally-injureddrivers(3.
3%oforiginalpopulation)and1,398driversinvolvedinpolice-reportedcrashes(11.
5%oforiginalweightedsample)wereexcluded.
Thisleft2,191recordsoffatally-injureddriversand13,258recordsofdriversinpolice-reportedcrashesforthemainanalysis.
MilesdrivenOftheoriginal9,663drivingtripsof16-and17-year-oldsintheNHTSsample,110(0.
7%ofweightedtrips)hadunknownlengths.
Anadditional23trips(0.
2%ofweightedtrips)hadlengthsdeemedimplausibleonthebasisoftheircalculatedaveragespeed(greaterthan100milesperhour)—thesewerereplacedwithmissingvalues.
Missingvaluesoftriplengthwerereplacedfor109tripswithvaluespredictedfromlinearregressionoftriplengthontripduration;24tripswhoselengthanddurationwerebothmissingwereexcluded,leaving9,639triprecordsforthemainanalysis.
TheweightsoftheNHTSdatawereadjustedtoalignthepopulationof16-and17-year-oldsestimatedfromtheNHTStothepopulationof16-and17-year-oldsintheUnitedStatesasreportedbytheUnitedStatesCensusBureau(2011).
Adjustmentsweremadeseparatelybyageandsex.
AnalysisCrash-involved16-and17-year-olddriverswereclassifiedaccordingtothenumberandagesofpassengerspresentinthevehicle.
Classificationswere:Nopassengers;1,2,or3+passengersunder21yearsofage(andnopassengersaged21orolder);Atleastonepassengeraged35orolder(anynumberofpassengersofotherages);Other(oldestpassengerinvehicleaged21-34).
5Thedrivingexposureof16-and17-year-olddriverswasalsogroupedaccordingtothesamecombinationsofpassengers.
However,theNHTSonlycollectstheagesofpassengerswhoaremembersofthedriver'shousehold;theagesofpassengerswhowerenotmembersofthedriver'shouseholdwereunknown.
Forthepurposeofthestudy,allpassengersofunknownage(i.
e.
,allpassengerswhowerenotmembersofthedriver'shousehold)wereassumedtohavebeenyoungerthan21yearsofage.
Providedthatatleastsomepassengersofunknownagewereaged21orolder,thismethodoverestimatesthenumberofmilesdrivenwithonlypassengersunderage21.
Similarly,ifanyofthepassengersofunknownagewereaged35orolder,thismethodunderestimatesthenumberofmilesdrivenwithpassengersaged35orolder.
Ratesofdriverskilledandofdriversinvolvedinpolice-reportedcrashespermiledrivenwereestimatedbydividingtheannualaveragenumberofcrash-involveddriverswitheachofthecombinationsofpassengersdefinedpreviouslybythenumberofmilesdrivenby16-and17-year-olddriverswiththesamecombinationofpassengers.
Relativerisks(ratiosofrates)werecomputedfordriverswitheachpassengercombinationrelativetodriverswithnopassengers.
Ratesandrelativeriskswerealsocomputedseparatelybysingleyearofdriverage,driversex,timeofday,andsinglevs.
multiple-vehiclecrashes.
Ratesandrelativeriskswerenotcomputedfordriverswithpassengersaged21-34becausethedataonmilesdrivencontainedtoofewtripsonwhichyoungdrivershadpassengersinthisagegrouptoproducereliableestimates(n=29).
Notethatduetotheassumptionthatallpassengersofunknownagewereunderage21,ratesandrelativerisksestimatedfordriverswithpassengersunderage21representlowerboundsforthetrueratesandthetruerelativerisks;similarly,ratesandrelativerisksestimatedfordriverswithpassengersaged35orolderrepresentupperbounds.
Standarderrorsofthenumbersofdriversinvolvedinallpolice-reportedcrasheswereestimatedusinggeneralizedstandarderrorspublishedbyNHTSA(2011).
StandarderrorsofthenumberofdriversinvolvedinfatalcrasheswereestimatedusingPoissonapproximations.
StandarderrorsofthenumberofmilesdrivenwereestimatedusingjackknifereplicateweightsprovidedintheNHTSdatafile(FHWA,2012).
ThestandarderrorsofratesandrelativeriskswereestimatedonthelogscaleusingfirstorderTaylorseriesapproximations;confidenceintervalsforrisksandrelativeriskswereestimatedonthelogscaleusingnormalapproximations.
ResultsTable1showstheproportionsoffatally-injureddriversandofalldriversinvolvedinpolice-reportedcrashesinrelationtothenumberandagesofpassengerspresent.
Ofthe2,191driversaged16-17whowerekilledincrashesoverthestudyperiod,54.
5%hadnopassengers,2.
9%hadatleastonepassengeraged35yearsorolder,24.
2%hadonepassengeryoungerthan21yearsofage(andnoolderpassengers),9.
3%had2passengersyoungerthanage21,5.
9%had3ormorepassengersyoungerthanage21,and3.
2%hadatleastonepassengeraged21-34inthevehicle(butnopassengersaged35orolder).
Variationsinpassengergroupingsbydriverage,sex,andtimeofdayweresmall;driversinsingle-vehiclecrasheswerelessthanhalfaslikelyasdriversinmulti-vehiclecrashestohavehadapassengeraged35yearsorolder.
Amongall16-and17-year-olddrivers6involvedinpolice-reportedcrashes,theproportionswith1,2,or3+passengersunderage21werelowerthanamongfatally-injureddrivers,andtheproportionswithnopassengersorwithapassengeraged35orolderwerehigherthanamongfatally-injureddrivers.
Theannualnumberof16-and17-year-olddriverskilledincrashesdecreasedby47%overthestudyperiod,from755in2007to398in2010.
Thenumberofdriversinvolvedinpolice-reportedcrashesdecreasedby23%,from468,903in2007to361,433in2010.
However,thedistributionsofdriverskilledincrashesandofalldriversinvolvedincrashesinrelationtopassengergroupingswererelativelystablefromyeartoyear(Table1).
Table1alsoshowstheshareofmilesdrivenby16-and17-year-olddriversinrelationtothenumberandagesofpassengerspresent.
Comparedto17-year-olddrivers,16-year-olddriversdroveasubstantiallygreatershareoftheirtotalmileswithahouseholdmemberaged35orolderinthevehicle.
Driversaged16alsodroveasubstantiallygreatershareoftheirmilesthandid17-year-oldswithonepassengerunderage21.
Table1suggeststhatdriverswerefarmorelikelytohavehadthreeormorepassengersunderage21whentripsoccurredbetweenthehoursof10PMand5:59AMthanwhentripsoccurredatotherhours;however,thiswasbasedononlyasmallrawnumberoftrips(n=11)andthusshouldnotbeover-interpreted.
Table2showsthenumberof16-and17-year-olddriverskilledincrashesper100millionmilesdrivenandnumberinvolvedinpolice-reportedcrashesper1millionmilesdriveninrelationtothenumberandagesofpassengerspresent.
Fordriverswithnopassengers,theper-miledriverdeathrateandcrashinvolvementratewerebothsubstantiallyhigherfor16-year-olddriversthanfor17-year-olds.
Thedeathrateofmaledriverswithnopassengerswassomewhathigherthantherateforfemaledriverswithnopassengers,whereasthecorrespondingratesofinvolvementinanycrashwereslightlyhigherforfemalesthanformales(neitherdifferenceapproachedstatisticalsignificance).
Thedeathratepermiledrivenof16-and17-year-olddriverswithnopassengerswasover6timesashighbetween10PMand5:59AMasbetween6AMand9:59PM.
Theoverallcrashinvolvementratepermiledrivenwaselevatedonlyslightlyduringthesehours,andthedifferencedidnotapproachstatisticalsignificance.
Figure1andTable3showtheper-milerisksofbeingkilledinacrashandofbeinginvolvedinapolice-reportedcrashinrelationtothenumberandagesofpassengerspresent,relativetotheriskswithnopassengerspresent,fordriversaged16-17.
Ingeneral,havingpassengersunderage21wasassociatedwithanincreaseinrisk,andhavingadultpassengersaged35+wasassociatedwithadecreaseinrisk(Figure1;Table3).
Comparedwithhavingnopassengers,havingonepassengeryoungerthanage21(andnoolderpassengers)wasassociatedwitha44%increaseina16-or17-year-olddriver'sriskpermiledrivenofbeingkilledinacrash(RR1.
44,95%CI1.
01–2.
04),havingtwopassengersyoungerthanage21wasassociatedwithadoublingofthedriver'sriskofbeingkilledinacrash(RR2.
02,95%CI1.
36–2.
99),andhavingthreeormorepassengersyoungerthanage21wasassociatedwithaquadruplingintheriskofbeingkilledinacrash(RR4.
39,95%CI1.
45–13.
31).
Therelationshipbetweentherelativeriskofbeinginvolvedinanypolice-reportedcrashandthepresenceofyoungpassengersfollowedasimilarpattern;however,theincreasesinrelativerisksofinvolvementinanypolice-reportedcrashweresmallerandwerenotstatisticallysignificant.
7Havingatleastonepassengeraged35orolderinthevehiclewasassociatedwitha62%decreaseintheriskpermiledrivenofbeingkilledinacrash(RR0.
38,95%CI:0.
24–0.
60)anda46%decreaseintheriskofbeinginvolvedinanypolice-reportedcrash(RR0.
54,95%CI0.
35–0.
84),comparedwithhavingnopassengers.
Table1.
Driverdeaths,driversinvolvedinpolice-reportedcrashes,andmilesdriveninrelationtodriverage,sex,timeofday,andcombinationofpassengersinthevehicle,driversaged16-17yearsdrivingcars,pickuptrucks,vans,minivans,orsportutilityvehicles,UnitedStates,2007-2010.
NopassengersAtleast1passengeraged35+Numberofpassengers<21yearsold(Allpassengers<21yearsold)OtherpassengergroupTotal123+DriverdeathsRowpercentNTotal54.
52.
924.
29.
35.
93.
22,191Driverage1652.
33.
226.
610.
06.
02.
08531756.
02.
822.
68.
85.
84.
01,338Male53.
13.
225.
19.
56.
22.
71,385Female56.
92.
422.
68.
85.
34.
08066AM–9:59PM54.
13.
525.
49.
55.
32.
21,51510PM–5:59AM55.
51.
721.
29.
17.
25.
4651Single-vehiclecrash54.
12.
024.
59.
85.
93.
81,382Multiple-vehiclecrash55.
44.
623.
78.
35.
92.
1809Crashyear200754.
63.
022.
910.
16.
52.
9755200857.
12.
424.
18.
75.
62.
2553200952.
82.
724.
79.
34.
75.
8485201053.
03.
826.
18.
56.
52.
0398Allpolice-reportedcrashesRowpercent(Weighted)WeightedNTotal61.
94.
721.
26.
13.
32.
81,636,618Driverage1659.
56.
521.
26.
43.
43.
1653,5631763.
53.
521.
25.
93.
22.
6983,055Male62.
54.
220.
76.
33.
52.
9878,411Female61.
35.
321.
95.
73.
12.
7758,2086AM–9:59PM62.
34.
821.
25.
93.
12.
71,499,71110PM–5:59AM58.
03.
321.
37.
45.
74.
3136,907Single-vehiclecrash64.
62.
920.
36.
73.
51.
9350,143Multiple-vehiclecrash61.
25.
221.
55.
93.
23.
01,286,476Crashyear200763.
54.
520.
85.
83.
42.
1468,903200861.
25.
221.
15.
73.
73.
2427,289200961.
74.
420.
56.
73.
33.
3378,993201060.
94.
722.
76.
12.
72.
8361,433MilesdrivenaRowpercent(Weighted)Weightedmiles(inmillions)Total63.
89.
019.
75.
41.
60.
620,899Driverage1653.
514.
126.
55.
20.
40.
36,5241768.
46.
716.
65.
42.
10.
814,375Male63.
29.
517.
96.
22.
11.
111,592Female64.
48.
322.
04.
40.
90.
09,3076AM–9:59PM63.
79.
319.
75.
51.
10.
719,22210PM–5:59AM62.
06.
420.
24.
27.
2-1,564Data:FatalityAnalysisReportingSystem(2012),GeneralEstimatesSystem(2012),NationalHouseholdTravelSurvey(2009).
a.
Indataonmilesdriven,passengeragewasunknownwhenthepassengerwasnotamemberofthesubjectdriver'shousehold.
Forthepurposeofthestudy,non-householdpassengerswereassumedtobeunderage21.
Thus,percentagesshownfordriverswithpassengeraged35+andwithotherpassengergrouprepresentlowerbounds,andpercentagesshownfordriverswith1,2,or3+passengers<21yearsoldrepresentupperbounds.
Tripsareweightedtoreflecttotalmilesdrivenovera365-dayperiodfromMay2008throughApril2009.
8Mostestimatesofrelativeriskswithinsubgroupswereimprecise,largelyduetothesparsenessoftheNHTSdataontripswithspecificcombinationsofpassengers;however,theygenerallyfollowedthesamepatternastheoverallresults.
Thepresenceofapassengeraged35+wasassociatedwithasignificantlylargerdecreaseintherelativeriskofinvolvementinsingle-vehiclecrashesthanintherelativeriskofinvolvementinmultiple-vehiclecrashes;thisappliedtobothcrashesinwhichthedriverwaskilled(RatioofRelativeRisks[RRR]0.
44,95%CI0.
32–0.
60)andtoallpolice-reportedcrashes(RRR0.
54,95%CI0.
38–0.
76).
Havingthreeormorepassengersunderage21appearedtoincreasetherelativerisktoagreaterdegreefor16-year-olddriversthanfor17-year-olddrivers;theratiosofrelativerisksofdriverdeathandofinvolvementinanypolice-reportedcrashbothapproachedstatisticalsignificance(driverdeath:RRR4.
1,95%CI:0.
92–18.
0;allpolice-reportedcrashes:RRR4.
2,95%CI:0.
96–18.
3).
Althoughthemagnitudesoftherelativerisksdidappeartovaryacrosssubgroupsforsomepassengercombinations,noothersubgroupdifferencesapproachedstatisticalsignificanceatthe95%confidencelevel.
Table2.
Ratesofdriverdeathandinvolvementinanypolice-reportedcrashinrelationtodriverage,sex,timeofday,andcombinationofpassengersinthevehicle,driversaged16-17,UnitedStates,2007-2010.
NopassengersAtleast1passengeraged35+aNumberofpassengers<21yearsolda(Allpassengers<21yearsold123+DriverdeathsRateper100millionmilesdriven(95%ConfidenceInterval)Total2.
24(1.
88–2.
68)0.
85(0.
56–1.
31)3.
22(2.
38–4.
36)4.
53(3.
19–6.
42)9.
85(3.
30–29.
41)Driverage163.
19(2.
57–3.
97)0.
73(0.
43–1.
27)3.
29(1.
69–6.
39)6.
26(3.
95–9.
92)44.
55(19.
82–100.
14)171.
90(1.
53–2.
37)0.
96(0.
50–1.
87)3.
17(2.
48–4.
05)3.
77(2.
46–5.
78)6.
53(1.
95–21.
82)Male2.
51(1.
91–3.
30)1.
02(0.
61–1.
69)4.
20(3.
14–5.
63)4.
61(2.
94–7.
25)8.
96(2.
03–39.
62)Female1.
91(1.
57–2.
33)0.
62(0.
32–1.
19)2.
22(1.
30–3.
81)4.
37(2.
69–7.
10)12.
28(6.
33–23.
84)6AM–9:59PM1.
67(1.
39–2.
02)0.
75(0.
48–1.
16)2.
54(1.
83–3.
51)3.
41(2.
36–4.
93)9.
43(4.
82–18.
44)10PM–5:59AM9.
31(7.
08–12.
26)2.
75(0.
68–11.
17)10.
90(5.
63–21.
11)22.
57(8.
41–60.
60)10.
43(1.
30–83.
32)Single-vehiclecrash1.
40(1.
17–1.
68)0.
36(0.
21–0.
60)2.
05(1.
50–2.
80)3.
03(2.
11–4.
36)6.
19(2.
05–18.
62)Multiple-vehiclecrash0.
84(0.
69–1.
02)0.
49(0.
31–0.
79)1.
17(0.
84–1.
61)1.
49(1.
00–2.
23)3.
67(1.
20–11.
20)Allpolice-reportedcrashesRateper1millionmilesdriven(95%ConfidenceInterval)Total19.
0(15.
3–23.
5)10.
2(6.
9–15.
1)21.
1(15.
3–29.
2)22.
1(15.
4–31.
8)41.
3(13.
8–123.
5)Driverage1627.
8(21.
8–35.
4)11.
5(7.
4–17.
8)20.
1(10.
3–39.
2)30.
6(19.
4–48.
2)194.
6(87.
7–432.
0)1715.
9(12.
4–20.
3)9.
1(4.
9–16.
8)21.
8(16.
8–28.
5)18.
4(12.
0–28.
3)26.
6(8.
0–88.
6)Male18.
7(13.
9–25.
2)8.
3(5.
2–13.
1)21.
9(16.
0–29.
9)19.
5(12.
3–30.
8)31.
8(7.
2–140.
6)Female19.
4(15.
5–24.
2)13.
1(7.
8–22.
1)20.
3(11.
8–34.
8)26.
8(16.
7–42.
8)67.
2(35.
6–126.
9)6AM–9:59PM19.
1(15.
3–23.
7)10.
2(6.
9–15.
1)21.
0(14.
9–29.
5)21.
1(14.
5–30.
6)53.
8(27.
7–104.
4)10PM–5:59AM20.
5(15.
1–27.
8)11.
4(3.
0–43.
4)23.
0(11.
7–45.
3)38.
8(14.
3–105.
5)17.
4(2.
2–139.
6)Single-vehiclecrash4.
2(3.
4–5.
3)1.
4(0.
86–2.
2)4.
3(3.
1–6.
1)5.
2(3.
5–7.
8)9.
4(3.
1–28.
8)Multiple-vehiclecrash14.
8(11.
9–18.
3)8.
9(6.
0–13.
1)16.
8(12.
1–23.
2)16.
9(11.
7–24.
4)31.
8(10.
6–95.
4)Data:FatalityAnalysisReportingSystem(2012),GeneralEstimatesSystem(2012),NationalHouseholdTravelSurvey(2009).
a.
IntheNationalHouseholdTravelSurveydatausedtocomputecrashratespermiledriven,passengeragewasunknownforpassengerswhowerenotmembersofthesubjectdriver'shousehold.
Forthepurposeofthestudy,allnon-householdpassengerswereassumedtobeunderage21,thusestimatedriskofdrivingwithapassengeraged35+isanupperbound,andestimatedrisksofdrivingwith1,2,or3+passengers<21yearsoldarelowerbounds.
9Figure1.
Relativeriskofdriverdeath(left)andofinvolvementinanypolice-reportedcrash(right)permiledriveninrelationtocombinationofpassengersinthevehicle,driversaged16-17,UnitedStates,2007-2010.
Data:FatalityAnalysisReportingSystem(2012),GeneralEstimatesSystem(2012),NationalHouseholdTravelSurvey(2009).
a.
IntheNationalHouseholdTravelSurveydatausedtocomputecrashratespermiledriven,passengeragewasunknownforpassengerswhowerenotmembersofthesubjectdriver'shousehold.
Forthepurposeofthestudy,allnon-householdpassengerswereassumedtobeunderage21,thustherelativeriskshownfordrivingwithapassengeraged35+isanupperbound.
b.
Relativerisksshownfordrivingwith1,2,and3+passengers<21yearsoldrepresentlowerbounds.
Table3.
Relativerisksofdriverdeathandofinvolvementinanypolice-reportedcrashpermiledriveninrelationtodriverage,sex,timeofday,andcombinationofpassengersinthevehicle,driversaged16-17,UnitedStates,2007-2010.
NoPassengersAtleast1passengeraged35+aNumberofpassengers<21yearsold(Allpassengers<21yearsold)a123+DriverdeathspermiledrivenRelativeRisk(95%ConfidenceInterval)Total1(Reference)0.
38(0.
24–0.
60)1.
44(1.
01–2.
04)2.
02(1.
36–2.
99)4.
39(1.
45–13.
31)161(Reference)0.
23(0.
13–0.
41)1.
03(0.
51–2.
07)1.
96(1.
18–3.
26)13.
95(6.
03–32.
29)171(Reference)0.
51(0.
25–1.
02)1.
66(1.
20–2.
31)1.
98(1.
23–3.
20)3.
43(1.
01–11.
68)Male1(Reference)0.
40(0.
23–0.
72)1.
67(1.
12–2.
50)1.
84(1.
08–3.
12)3.
57(0.
79–16.
19)Female1(Reference)0.
32(0.
16–0.
64)1.
16(0.
65–2.
06)2.
28(1.
35–3.
86)6.
41(3.
21–12.
81)6AM–9:59PM1(Reference)0.
45(0.
28–0.
72)1.
52(1.
04–2.
21)2.
04(1.
35–3.
08)5.
64(2.
81–11.
32)10PM–5:59AM1(Reference)0.
30(0.
07–1.
23)1.
17(0.
57–2.
39)2.
42(0.
87–6.
75)1.
12(0.
14–9.
11)Single-vehiclecrash1(Reference)0.
26(0.
15–0.
44)1.
46(1.
02–2.
10)2.
16(1.
44–3.
25)4.
41(1.
44–13.
48)Multi-vehiclecrash1(Reference)0.
59(0.
35–0.
98)1.
39(0.
95–2.
02)1.
78(1.
14–2.
77)4.
36(1.
40–13.
54)Allpolice-reportedcrashespermiledrivenRelativeRisk(95%ConfidenceInterval)Total1(Reference)0.
54(0.
35–0.
84)1.
11(0.
75–1.
64)1.
16(0.
76–1.
77)2.
17(0.
71–6.
63)161(Reference)0.
41(0.
25–0.
68)0.
72(0.
35–1.
47)1.
10(0.
66–1.
84)7.
00(3.
04–16.
10)171(Reference)0.
57(0.
29–1.
11)1.
38(0.
96–1.
97)1.
16(0.
71–1.
90)1.
67(0.
49–5.
72)Male1(Reference)0.
44(0.
25–0.
76)1.
17(0.
76–1.
80)1.
04(0.
60–1.
80)1.
70(0.
37–7.
74)Female1(Reference)0.
68(0.
38–1.
19)1.
05(0.
58–1.
88)1.
38(0.
82–2.
32)3.
47(1.
77–6.
80)6AM–9:59PM1(Reference)0.
53(0.
34–0.
84)1.
10(0.
73–1.
65)1.
11(0.
72–1.
70)2.
82(1.
40–5.
67)10PM–5:59AM1(Reference)0.
56(0.
14–2.
20)1.
12(0.
54–2.
36)1.
90(0.
67–5.
40)0.
85(0.
10–6.
97)Single-vehiclecrash1(Reference)0.
32(0.
19–0.
54)1.
02(0.
68–1.
53)1.
23(0.
78–1.
94)2.
22(0.
71–6.
94)Multi-vehiclecrash1(Reference)0.
60(0.
38–0.
94)1.
14(0.
77–1.
68)1.
14(0.
75–1.
75)2.
15(0.
70–6.
60)Data:FatalityAnalysisReportingSystem(2012),GeneralEstimatesSystem(2012),NationalHouseholdTravelSurvey(2009).
a.
IntheNationalHouseholdTravelSurveydatausedtocomputecrashratespermiledriven,passengeragewasunknownforpassengerswhowerenotmembersofthesubjectdriver'shousehold.
Forthepurposeofthestudy,allnon-householdpassengerswereassumedtobeunderage21,thusestimatedrelativeriskofdrivingwithapassengeraged35+isanupperbound,andestimatedrelativerisksofdrivingwith1,2,or3+passengers<21yearsoldarelowerbounds.
10DiscussionDriversaged16-17wereshowntobeatincreasedriskpermiledrivenofbeingkilledinacrashwhencarryingyoungpassengers,andtheriskincreasedfurtherasthenumberofyoungpassengersincreased.
Risksofinvolvementinpolice-reportedcrashesofanyseverityappearedtofollowasimilarpattern;however,theincreasesintheriskofinvolvementinanypolice-reportedcrashweresmallerthantheincreasesinriskofdriverdeathanddidnotevenapproachstatisticalsignificance.
Incontrast,youngdrivers'riskofinvolvementinanypolice-reportedcrashesandriskofbeingkilledinacrashwerebothreducedsignificantlywhencarryinganadultpassengeraged35orolder.
Somewhatunexpectedly,itwasestimatedthatthepresenceofoneyoungpassengerincreaseda16-year-olddriver'sriskofbeingkilledinacrashbyonly3%(RR1.
03,95%CI0.
51–2.
07),whereastheriskfora17-year-olddriverincreasedby66%(RR1.
66,95%CI1.
20–2.
31)inthepresenceofoneyoungpassenger.
However,thedifferenceinthesetwoestimatesisnotsignificantlylargerthanwhatmightbeexpectedtooccurbychancealoneunderthehypothesisthattheeffectofhavingoneyoungpassengerdoesnotvarybyage(RatioofRelativeRisks0.
62,95%CI0.
29–1.
34).
Thus,theseestimatesarenotinconsistentwiththehypothesisthattheestimatedeffectfordriversaged16and17combinedappliesequallytodriversaged16anddriversaged17,i.
e.
,thatthepresenceofoneyoungpassengerisassociatedwitha44%increaseintheriskofdeathpermiledriven(RR1.
44,95%CI1.
01–2.
04).
RelationtootherresearchThisstudyconfirmsthattheresultsoriginallyreportedintheseminalstudybyChenetal.
(2000)arestillbroadlyapplicabletoday.
Inthatstudy,Chenetal.
analyzeddataondriverdeathsfromyears1992-1997anddataondrivingexposurefromyears1995-1996toestimaterelativeriskofdriverdeathperdrivingtripinrelationtothenumberofpassengerspresent,andfoundthathavingone,two,orthreeormorepassengersincreasedtheper-tripriskofdriverdeathby39%,86%,and182%,respectively,for16-year-olddrivers,andby48%,158%,and207%,respectivelyfor17-year-olddrivers.
AlthoughChenetal.
didnotformallyinvestigatetheper-triporper-mileriskofinvolvementincrashesthatwerenotfataltothedriver,theyanalyzedtherateofdriverdeathsperpolice-reportedcrashandreportedthathavingteen-agedpassengersandhavingpassengersaged20-29bothwereassociatedwithincreasedriskofdriverdeathintheeventofapolice-reportedcrash,thusimplyingthatthepresenceofyoungpassengersmusthaveincreasedtheper-tripriskofseverecrashestoagreaterextentthanitincreasedtheriskofless-severecrashes.
Thiswasinvestigatedformallyandwasconfirmedinthecurrentstudy.
ThisstudyalsoextendstheresultsofChenetal.
bytakingadvantageofthelimiteddataavailableontheagesofpassengersridingwithyoungdriverstoestimatelowerboundsfortherisksassociatedwithcarryingmultipleyoungpassengersandupperboundsfortherisksassociatedwithcarryingadultpassengers.
Inasimilarstudy,Ouimetetal.
(2010)analyzedfatalcrashdatafromyears1999-2003andtraveldatafrom2001-2002toestimatetherelativeriskoffatalcrashinvolvementpermiledrivenfordriversages15-20inrelationtotheageandsexofpassengerswhentherewasonepassengerinthevehicle.
ThedataanalyzedbyOuimetetal.
,likethedataanalyzedin11thecurrentstudy,onlycontainedinformationabouttheageandsexofpassengerswhoweremembersofthedriver'shousehold.
Theauthorsusedahot-deckimputationmethodtoestimatethedistributionofageandsexamongallpassengers,includingthenon-householdpassengers,andreportedthatthenumberofmilesdrivenwithadultpassengersaged35+wassubstantiallygreaterthanthenumberofmilesdrivenwithpeersofthedriver,whichtheynotedmighthavebeenanartifactoftheirmethod.
Inaddition,theresultsofthatstudymaybeoflimitedgeneralizabilitytodriversages16-17;examinationofthedatausedtoestimatemilesdriveninthecurrentstudyshowsthat79%thetotalmilesdrivenbydriversaged15-20weredrivenbydriversaged18-20.
Astudythatusedin-vehiclecamerastoobserveasampleof40newly-licensedteenagedriversfortheirfirst18monthsoflicenseddriving(Klaueretal.
,2011)reportedresultsverysimilartothoseofthecurrentstudywithrespecttothedistributionsofpassengersinthevehiclesofyoungdrivers.
Inthatstudy,apassengeraged19orolderwaspresentforabout13-14%ofallmilesdrivenbythestudysubjectsduringtheirfirstthreemonthsoflicenseddrivingandforabout7-8%ofallmilesdrivenduringmonths4-18oflicenseddriving;passengersaged13-18werepresentfor26-28%ofallmilesdrivenduringstudysubjects'first12monthsoflicenseddrivingandforabout21-22%ofallmilesdriveninmonths13-18.
Inthecurrentstudy,anadultaged21orolderwaspresentfor15.
2%ofmilesdrivenby16-year-oldsand7.
5%ofmilesdrivenby17-year-olds;passengersyoungerthan21yearsofagewerepresentfor32.
1%ofmilesdrivenby16-year-oldsand24.
1%ofmilesdrivenby17-year-olds.
Thiscloseagreementsuggeststhattheassumptionmadeinthecurrentstudythatallnon-householdpassengerswereunderage21yieldedareasonableapproximationoftheoverallagedistributionofthepassengersridingwithdriversaged16-17.
Thisstudyestimatedthathavinganadultpassengeraged35yearsorolderdecreaseda16-to17-year-olddriver'sriskpermiledrivenofinvolvementinanypolice-reportedcrashby46%anddecreasedthedriver'sriskofbeingkilledinacrashby62%.
Havinganadultpassengerinthevehiclewasshowntobeespeciallyprotectiveagainsttheriskofinvolvementinsingle-vehiclecrashes.
Thismakessense.
Althoughthedataanalyzedinthisstudydonotidentifyonedriveroranotherasbeing"atfault"foranycrash,itislikelythatagreaterproportionofyoungdrivers'single-vehiclecrashinvolvementsthanmultiple-vehiclecrashinvolvementswereatleastpartiallyattributabletosomeactionorerrorcommittedbytheyoungdriver;thus,itislikelythatanadultpassengercouldhelptopreventagreaterproportionofyoungdrivers'single-vehiclecrashesthanmultiple-vehiclecrashes.
ThemagnitudeoftheriskreductionestimatedhereforcarryinganadultpassengeragreesverywellwiththeestimateofRiceetal.
(2003)thatcarryinganadultpassengerwasassociatedwitha70%reductionina16-or17-year-olddriver'sriskofbeinginvolvedinacrashinwhichheorshewasinjuredandwasdeemedculpable.
Whiletherelationshipoftheadultpassengerstothedriverswasnotknownineitherstudy,itislikelythatinmanycasestheywerethedriver'sparents.
Inastudyinwhichin-vehiclecamerasandotherdatacollectionequipmentwereusedtomonitor42teen-ageddriversfortheirfirst18monthsoflicensedindependentdriving,Simons-Mortonetal.
(2011)foundthatthepresenceofanadultpassengerwasassociatedwitha74%reductionintherateofinvolvementincrashesornearcrashespermiledrivenanda68%reductioninhighg-forceevents(e.
g.
,hardacceleration,braking,orswerving)comparedwithdrivingalone.
Theauthorsdidnotconcludewhetherthiseffectwasdueto12teensmoderatingtheirbehaviorinthepresenceofadultpassengers,whethertheadultactivelyhelpedthedriver(e.
g.
,bypointingouthazards),both,orsomethingelse.
InasimilarstudyconductedfortheAAAFoundationforTrafficSafety,Goodwinetal.
(2012)usedin-vehiclecamerastoobservethedrivingofadifferentsampleof52newly-licensedteenagedriversandfoundthatelectronicdeviceusewas86%lessfrequentandotherdistracted-drivingbehaviorswere75%lessfrequentwhenaparentorotheradultwaspresentinthevehiclethanwhenthedriverwasalone,illustratingthatteenagedriversmodifyatleastsomeaspectsoftheirbehaviorinthepresenceofadultpassengers.
Thisstudydoesnotshedlightonthemechanismsbywhichthepresenceofyoungpassengersincreasestheriskoffatalcrashinvolvementforyoungdrivers.
WilliamsandTefft(2012)analyzeddatafromfatalcrashesthatinvolved16-or17-year-olddriversinyears2005-2010andreportedthatincomparisontofatal-crashinvolveddriverswithnopassengers,agreaterproportionofthosewithteenagepassengerswerespeeding,drinkingalcohol,andcodedonthepolicecrashreportashavingcontributedtothecrashinatleastsomeway,andreportedthattheproportionsspeeding,drinkingalcohol,andcontributingtothecrashincreasedasthenumberofteenagepassengersincreased.
Goodwinetal.
(2012b)usedin-vehiclecamerastocollectdatafromasampleofnewly-licensedyoungdriversinNorthCarolina,andfoundthatdriversweremorelikelytospeed,tailgate,and"showoff"whenmultipleteenagepassengerswerepresent.
Theauthorsalsonotedthattheyobservedfewinstancesinwhichthepassengersactivelyencouragedthesebehaviors,suggestingthatitwasthepassengers'merepresencethatelicitedsuchbehaviorfromthedriver.
Incontrast,inanotherstudyalsousingin-vehiclecamerasandotherdatacollectionequipmenttostudyasampleofnewly-licensedyoungdrivers,Simons-Mortonetal.
(2011)foundthatelevatedg-forceevents(e.
g.
,hardbraking,swerving)weresomewhatlessfrequentwhenteenagepassengerswerepresentthanwhenthedriverwasalone.
Thecontrastbetweentheresultsofthesestudiessuggeststherelationshipbetweenthepresenceofyoungpassengersandthebehaviorofayoungdriveriscomplex.
LimitationsThenationalsurveythatwasusedtoestimatedrivingexposureinrelationtotheageandnumberofpassengersdidnotcollectdataontheagesofpassengerswhowerenotmembersofthedriver'shousehold.
Forthepurposeofthestudy,itwasassumedthatallnon-householdpassengerswerelessthan21yearsofage.
Whilethisisunlikelytobecorrect,itprovidesclearandusefulinsights:risksreportedinthisstudyforyoungdriverscarryingonlypassengersunderage21representlowerboundsforthetruerisksassociatedwithcarryingpassengersunderage21,andrisksreportedhererepresentupperboundsforthetruerisks(equivalently:lowerboundsforthetrueriskreductions)associatedwithcarryingpassengersaged35orolder.
Thedrivingexposuredatacontainedtoofewtripswithhouseholdpassengersaged21-34toestimaterelativerisksfordriverswithpassengersinthisagerange.
Chenetal.
(2000)reportedthatthepresenceofpassengersaged20-29wasassociatedwithincreasedcrashseverity(greateraveragenumberofdriverdeathsperpolice-reportedcrash),andWilliams&Tefft(2012)foundthatmajorriskfactorssuchasspeedingandalcoholusewereasprevalentormoreprevalentinfatalcrashesof16-and17-year-olddriverswithpassengersaged20-29aswithmultipleteenagepassengers.
However,inthedataanalyzedinthecurrentstudy,only3.
2%offatally-injured16-and17-year-olddriversand2.
8%ofall16-13and17-year-olddriversinvolvedinpolice-reportedcrasheshadanypassengersaged21-34andnopassengersaged35orolder.
Althoughitisclearthathavingyoungpassengersinthevehicleincreasesriskforyoungdriversandhavingadultpassengersdecreasesrisk,theresultsofthisstudydonotprovideevidenceoftheactualage(ofthepassenger)atwhichthepassenger'spresenceceasestoincreaseadriver'sriskortheagethatitbecomesaprotectivefactor.
Theupperagecutoffforyoungpassengerswasplacedatage21toalignwithmostexistingstateGDLpassengerrestrictions.
Theloweragecutoffof35wasselectedforadultpassengerstocapturepassengersoldenoughthattheycouldplausiblyhavebeenthedriver'sparents.
Thesecutoffpointswereselectedapriori;theywerenotoutcomesofthestudy.
Amongfatally-injureddriversinthisstudy,25%hadafull-privilegedriver'slicense,59%hadanintermediateorprovisionallicensewithsomerestrictions,4%hadalearner'spermitthatalloweddrivingonlywithalicensedadultpassenger,and12%wereunlicensed.
However,dataonthetypeoflicensethatthedriverpossessedwasnotavailableinthedataonallpolice-reportedcrashesnorinthedataondrivingexposure;thus,theresultsreportedherearebasedonalldrivingdonebydriversaged16and17irrespectiveofthetypeofdriver'slicensethattheypossessed.
Mostofthemilesdrivenbydriversintheexposuredatalikelyhadalicensethatalloweddrivingwithoutanadultinthecar;however,somemayhavehadlearner'spermits,andsomemayhaveevenbeenunlicensed.
Asnotedpreviously,theannualnumberofcrashesanddeathsof16-and17-year-olddriversdecreasedsharplyoverthisperiod,whichsuggeststhattheremayhavebeenchangesindrivingexposureoverthisperiodaswell.
However,dataondrivingexposurewereavailableforonlytheone-yearperiodfromapproximatelyMay2008–April2009.
Themainanalyseswerebasedoncrashdatafromyears2007–2010becausethenumbersofdriverdeathswithspecificcombinationsofpassengerswereprohibitivelysmalltoproducestableestimatesusingonlyoneyearofdata.
Totestthesensitivityoftheresultstopossiblesystematicchangesinexposureoverthestudyperiod,themainanalyseswerereplicatedusingcrashdatafromtheone-yearperiodfromMay2008throughApril2009.
Usingcrashdatafromonlythisperiod,theestimatedrelativerisksofdriverdeathassociatedwithhavinganadultpassenger,1passengerunderage21,2passengersunderage21,and3ormorepassengersunderage21were0.
45,1.
47,1.
71,and3.
81,respectively,comparedwith0.
38,1.
44,2.
02,and4.
39whenestimatedusingcrashdatafrom2007–2010;relativerisksofinvolvementinanypolice-reportedcrashvariedevenlessinrelationtothetimeperiodofthecrashdataanalyzed.
Estimatesoftheamountofdrivingdonebyyoungdriverswithvariouscombinationsofpassengersreliedondrivers'self-reportsofthetripsthattheytook,thelengthsofthosetrips,andthepassengerspresentinthevehicle,allofwhichcouldbesubjecttobothrandomerrorandbias.
Althoughanyerrorinthereportednumberorlengthoftripswouldaffectestimatesofabsoluterisks(e.
g.
,crashespermiledriven),neitherrandomerrorsnorsystematicerrorunrelatedtothecombinationofpassengerspresent(e.
g.
,under-reportingofmilesdrivenby25%uniformlyacrossallpassengergroups)wouldbiastherelativeriskofdrivingwithaspecifiedcombinationofpassengersvs.
drivingalone.
Biascouldstillbepresent,however,iferrorsinestimatedmilesdrivenvariedbypassengercombination(e.
g.
,iftripswithmultipleteenagepassengersweremorelikelytobeunreported),ifthereportingofpassengerpresencewasitselfsubjecttoerror,orifthedrivingpatternsofteen14driverswhoparticipatedintheNHTSdifferedfromthedrivingpatternsofteendriverswhodidnotparticipate.
Finally,bydesign,thisstudycouldnotdemonstratethattherelationshipbetweenthepresenceofpassengersandtheriskofcrashinvolvementwascausal.
Whileitappearsthathavingmultipleyoungpassengersincreasesriskandhavingadultpassengersdecreasesrisk,thisstudycannotruleoutthepossibilitythatteendriverswhocarrymultipleyoungpassengerstendtohavehighercrashriskindependentofthepresenceofthepassengers,andsimilarly,thatteendriverswhodrivewithadultpassengerstendtohavelowerriskindependentofthepresenceofthepassengers.
However,theresultsofthisstudy,inconjunctionwithotherstudiesthathaveexamineddifferentpopulationsusingdiversemethods,suggestthatthepresenceofyoungpassengersdoesindeedincreasethecrashriskofyoungdrivers,andthatthepresenceofadultpassengersdecreasesrisk.
ConclusionThisstudyshowsthat,permiledriven,16-and17-year-olddriversaremorelikelytobekilledinacrashwhentheyhaveyoungpassengersintheirvehiclethanwhentheyaredrivingalone.
Theirriskwasfoundtoincreasebyanestimated44%whenonepassengerunderage21(andnoolderpassengers)waspresentintheteendriver'svehicle,approximatelydoublewhentwopassengersunderage21werepresent,andmorethanquadruplewhenthreeormorepassengersunderage21werepresent.
Theeffectofyoungpassengersontheriskofinvolvementinanypolice-reportedcrashappearedtofollowasimilarpattern;however,increasesintheriskofanypolice-reportedcrashweresmallerandwerenotestimatedpreciselyenoughtoevenapproachstatisticalsignificance.
Itisclearthatdiscouragingteendriversfromcarryingpassengersand/ordiscouragingteenagersfromridingwithyounginexperienceddriverswouldbenefitthesafetyofteenagersbothasdriversandaspassengers.
Havinganadultpassengeraged35orolderwasassociatedwithnearlya50%reductionin16-or17-year-olddrivers'riskofinvolvementinanycrashandovera60%reductionintheriskofbeingkilledinacrash.
Parentsclearlycanplayamajorroleinprotectingtheirteen-agersbyridingwiththeirteens,evenafterlicensure,tocontinuetosupportthedevelopmentofsafedrivinghabits.
Inaddition,parentscanhelptoprotecttheirteensfromtherisksassociatedwithteenagedriverscarryingteenagepassengersbyenforcingapplicablestatepassengerrestrictions,andbysupplementingstatelawswiththeirownrulesregardingcarryingteenagepassengersorridingwithteendrivers.
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org|202-638-5944TeenshavethehighestcrashrateofanygroupintheUnitedStates.
AuthorsBrianC.
TefftAllanF.
WilliamsJurekG.
GrabowskiAcknowledgmentsTheauthorsgratefullyacknowledgetheinputoftwopeerreviewers.
Theirhelpfulsuggestionsresultedinsubstantialimprovementstothisstudy.
Anyerrorsarethesoleresponsibilityoftheauthorsandnotofanyindividualwhoreviewedorprovidedsuggestionsonthisreport.
AbouttheSponsorAAAFoundationforTrafficSafety60714thStreet,NW,Suite201Washington,DC20005202-638-5944www.
aaafoundation.
orgFoundedin1947,theAAAFoundationinWashington,D.
C.
isanot-for-profit,publiclysupportedcharitableresearchandeducationorganizationdedicatedtosavinglivesbypreventingtrafficcrashesandreducinginjurieswhencrashesoccur.
FundingforthisreportwasprovidedbyvoluntarycontributionsfromAAA/CAAandtheiraffiliatedmotorclubs,fromindividualmembers,fromAAA-affiliatedinsurancecompanies,aswellasfromotherorganizationsorsources.
ThispublicationisdistributedbytheAAAFoundationforTrafficSafetyatnocharge,asapublicservice.
ItmaynotberesoldorusedforcommercialpurposeswithouttheexplicitpermissionoftheFoundation.
Itmay,however,becopiedinwholeorinpartanddistributedforfreeviaanymedium,providedtheAAAFoundationisgivenappropriatecreditasthesourceofthematerial.
TheAAAFoundationforTrafficSafetyassumesnoliabilityfortheuseormisuseofanyinformation,opinions,findings,conclusions,orrecommendationscontainedinthisreport.
Iftradeormanufacturer'snamesarementioned,itisonlybecausetheyareconsideredessentialtotheobjectofthisreportandtheirmentionshouldnotbeconstruedasanendorsement.
TheAAAFoundationforTrafficSafetydoesnotendorseproductsormanufacturers.
1AbstractThepresenceofpassengersinavehiclehasbeenshowntoincreasetheriskoffatalcrashinvolvementforteenagedrivers;however,thestudiesthathavequantifiedthisrelationshipwerebasedondatathatarenowoveradecadeold.
Intheyearssincethesestudies,mostU.
S.
stateshaveenactedgraduateddriverlicensingsystemsthatlimitthenumberofpassengersthatyoungdriversareallowedtocarryduringtheirfirstseveralmonthsofindependentdriving,andthenumberof16-and17-year-olddriversinvolvedinfatalcrasheseachyearhasdecreasedbymorethanhalf.
Theobjectiveofthisstudywastoprovideupdatedestimatesoftherelationshipbetweenthenumberandagesofpassengerspresentandthecrashriskpermiledrivenof16-and17-year-olddrivers.
Dataoncrashesthatoccurredinyears2007–2010anddataonthenumberofmilesdriveninyears2008–2009wereexamined.
Ratesofcrashinvolvementanddriverdeathpermiledrivenwereestimatedfor16-and17-year-olddriverswithnopassengers;withone,two,andthreeormorepassengersyoungerthanage21(andnoolderpassengers);andwithatleastonepassengeraged35orolder.
Comparedwithhavingnopassengers,havingonepassengeryoungerthanage21(andnoolderpassengers)wasassociatedwitha44%increaseina16-or17-year-olddriver'sriskpermiledrivenofbeingkilledinacrash(RelativeRisk[RR]1.
44,95%ConfidenceInterval[CI]1.
01–2.
04).
Havingtwopassengersyoungerthanage21wasassociatedwithadoublingofadriver'sriskofbeingkilledinacrash,comparedwithhavingnopassengers(RR2.
02,95%CI1.
36–2.
99).
Havingthreeormorepassengersyoungerthanage21wasassociatedwithroughlyaquadruplingofadriver'sriskofbeingkilledinacrash,comparedwithhavingnopassengers(RR4.
39,95%CI1.
45–13.
31).
Therelativeriskofbeinginvolvedinanypolice-reportedcrashinthepresenceofyoungpassengersfollowedasimilarpattern;however,theincreasesinrelativeriskofbeinginvolvedinanypolice-reportedcrashweresmallerandwerenotstatisticallysignificant.
Havingatleastonepassengeraged35orolderinthevehiclewasassociatedwitha62%decreaseina16-or17-year-olddriver'sriskpermiledrivenofbeingkilledinacrash(RR0.
38,95%CI:0.
24–0.
60),anda46%decreaseintheriskofbeinginvolvedinanypolice-reportedcrash(RR0.
54,95%CI0.
35–0.
84),comparedwithhavingnopassengers.
Theseresultsshowthatalthoughtheoverallnumberofteendriverfatalitieshasdecreasedsubstantiallyoverthepastseveralyears,carryingyoungpassengersisstillasignificantriskfactorforyoungdrivers.
Incontrast,carryingadultpassengerssignificantlyreducestherisksofcrashinvolvement.
2IntroductionPermiledriven,driversundertheageof20havehigherratesofinvolvementinfatalcrashesthandriversofanyotheragegroupexceptdriversaged80andolder(InsuranceInstituteforHighwaySafety,2012).
Whereastheapparentover-involvementofolderdriversinfatalcrashesispredominantlyattributabletotheirelevatedprobabilityofdyingiftheyareinvolvedinacrash—notexcessiveriskofcrashing—youngdrivers'excessiveinvolvementinfatalcrashesisduetoexcessivecrashrisk(Lietal.
,2003).
Whiledriversaged85andolderhavethehighestrateofdeathpermiledriven,driversaged16-17havethehighestrateofinvolvementincrashesthatresultinthedeathofoccupantsofothervehiclesornon-motorists(Tefft,2008).
Severalstudieshavefoundthatinrelationtotheirexposure(e.
g.
,numberoftripsornumberofmilesdriven),youngdrivershavehigherratesofcrashinvolvement,injury,anddeathwhencarryingpassengers.
Dohertyetal.
(1998)analyzeddatafromtheprovinceofOntario,Canadaonthedrivingexposureandcrashinvolvementofdrivers,andidentifiedcarryingpassengersasariskfactorfordriversaged16-19,andalsofoundthathavingtwoormorepassengerswasassociatedwithgreaterriskthanhavingonlyonepassenger.
Chenetal.
(2000)performedasimilaranalysisofdatafromtheUnitedStatesandfoundthatcarryingpassengerswasassociatedwithincreasedriskofinvolvementinacrashfataltothedriverfordriversaged16-19,andalsofoundthattheriskincreasedwiththenumberofpassengers.
Riceetal.
(2003)analyzeddatafromcrashesinthestateofCaliforniainwhichadriveraged16or17wasinjured,usingaquasi-inducedexposuremethodwhichinvolvedcomparingtherelativefrequencyofcrashesinwhichayoungdriverwasversuswasnotdeemedculpable.
Itwasestimatedthatcarryingteenagemalepassengersormixed-gendercombinationsofteenagepassengerswasassociatedwithsignificantlyincreasedcrashrisk,carryingteenagefemalepassengerswasnotassociatedwithincreasedrisk,andcarryingpassengersaged30orolderwasassociatedwithsignificantlydecreasedrisk.
Allthreeofthesestudiesalsoidentifieddrivingduringnighttimehoursasanindependentriskfactor.
Between1996andthepresent,mostU.
S.
statesimplementedsomeformofgraduateddriverlicensing(GDL)system,inwhichanewdriverinitiallyisallowedtodriveonlyunderthesupervisionofalicensedadultpassenger,andthenreceivesanintermediate(or"provisional")licensethatallowsunsuperviseddrivingbutonlyundercertainconditions.
Thedriverthenreceivesafull-privilegelicenseuponreachingacertainage(e.
g.
,18)oraccumulatingacertainamountofexperience(e.
g.
,12months)drivingwiththeintermediatelicense.
SeveralstudieshaveestimatedthatGDLsystemshavereducedthefatalcrashinvolvementof16-and17-year-olddriversbyroughly20-40%(Shope,2007).
Inmanystates,theintermediatestageoflicensureincludesrestrictionsoncarryingpassengers.
Felletal.
(2011)estimatedthatrestrictionsoncarryingpassengershavebeenassociatedwitha9%reductioninfatalcrashesinwhichdriversaged16-17hadteenagepassengers,andTrempel(2009)estimatedthatlawsthatlimitednewdriverstocarryingatmostoneteenagepassengerreducedcollisioninsuranceclaimsof16-and17-year-olddriversby4.
8%.
Thestudiesdocumentingtherisksassociatedwithcarryingpassengerswerebasedondatathatarenowoveradecadeold:Dohertyetal.
(1998)analyzeddatafrom1988,Chenetal.
(2000)analyzeddatafrom1992–1997,andRiceetal.
(2003)analyzeddatafrom1993–1998.
3In1998,2,58916-and17-year-olddriverswereinvolvedinfatalcrashes.
By2010,thisnumberhadfallenby56%to1,150(FatalityAnalysisReportingSystem,2012).
GiventheproliferationofGDLsystemsandthelargedecreaseintheoverallannualnumberoffatalcrashinvolvementsofyoungdrivers,thegeneralizabilitytothepresenttimeoftheresultsofthepreviousstudiesoftherelationshipbetweenpassengerpresenceandcrashriskisunknown.
Theobjectiveofthisstudywastoprovideupdatedestimatesoftherelationshipbetweenpassengerpresenceandcrashrisk,usingdatafromtheUnitedStatesfromyears2007-2010.
MethodsMainoutcomemeasureThenumberofdriverskilledincrashespermiledrivenandnumberofdriversinvolvedinpolice-reportedcrashes(ofanyseverity)permiledrivenwereestimatedfordriversaged16-17years,inrelationtothenumberandagesofpassengersinthevehicle.
DataDriverdeathsDataon16-and17-year-olddriverskilledincrasheswereobtainedfromtheNationalHighwayTrafficSafetyAdministration(NHTSA)FatalityAnalysisReportingSystem(FARS),whichcomprisesdataonallmotorvehiclecrashesthatoccuronpublicroadwaysintheUnitedStatesandresultinadeathwithin30daysofthecrash.
Datafromcrashesthatoccurredinyears2007–2010wereanalyzed.
Onlycrashesinwhichtheteenagedriverwasoperatingapassengervehicle(car,pickuptruck,van,minivan,orsportutilityvehicle[SUV])wereincluded;crashesinwhichthedriverwasoperatingamotorcycle,all-terrainvehicle,orothertypeofvehiclewereexcluded.
Therewere2,266recordsof16-and17-year-olddriverskilledincrashesoverthestudyperiod.
Police-reportedcrashesDataon16-and17-year-olddriversinvolvedinpolice-reportedcrasheswereobtainedfromtheNHTSA'sGeneralEstimatesSystem(GES),astratifiedsampleofallpolice-reportedcrashesintheUnitedStates.
RecordsinGESareweightedtorepresentallpolice-reportedcrashesintheUnitedStates.
Datafromcrashesthatoccurredinyears2007–2010wereanalyzed.
Onlycrashesinwhichtheteenagedriverwasoperatingapassengervehiclewereincluded;crashesinwhichthedriverwasoperatinganothertypeofvehiclewereexcluded.
Therewere14,656recordsof16-and17-year-olddriversofpassengervehiclesinvolvedinpolice-reportedcrashesoverthestudyperiod.
MilesdrivenDataonthenumberofmilesdrivenby16-and17-year-olddriverswereobtainedfromtheFederalHighwayAdministration's2009NationalHouseholdTravelSurvey(NHTS).
TheNHTScontainsdataonallofthetripstakenbyallmembersofarepresentativesampleofhouseholdsacrosstheUnitedStates.
IntheNHTS,respondentsentereddataintoatraveldiaryonanassigneddateandreporteditbymeansofatelephoneinterview.
Thesedataincludedthestarttime,endtime,andlengthofeachtrip,thetotalnumberofpassengersinthevehicle,andadditionaldemographicdata(includingage)ofpassengerswhowere4membersofthedriver'shousehold.
DatawerecollectedfromMarch2008throughApril2009andwereweightedtorepresentthetravelofallUnitedStateshouseholdsovera365-dayperiod.
The2009NHTSincludeddatafrom7,188respondentsaged16-17,ofwhom4,799weredrivers,ofwhom2,746madeatotalof9,663tripsasadriverofapassengervehicleontheirassignedtravelday.
DatainspectionCrashesRice&Anderson(2009)notedthatasof2005,FARSdatafromseveralstatesappearedtounder-reportpassengerswhowerenotinjured;Chenetal.
(2000)reportedthatseveralGESprimarysamplingunits(PSUs)alsodidso.
Toinvestigatewhethertherewasevidenceofsystematicunder-reportingofuninjuredpassengersinanystateorPSUduringthestudyperiod,theratiooftheproportionsofpassengerscodedasuninjuredtodrivers(ofanyage,notlimitedtodriversaged16-17)codedasuninjuredwastabulatedbystate(inFARS)andbyPSU(inGES)andwasevaluatedusinganapproximationofPierce'scriterion(1852)describedbyGould(1855).
IntheFARSdata,theaverageratiooftheproportionofpassengersuninjuredtotheproportionofdriversuninjuredwas0.
96(s.
d.
0.
20);recordsfromthestateofVirginia,witharatioof0.
04,wereexcludedfromthestudy(n=68).
IntheGESdata,theaverageratiowas0.
93(s.
d.
0.
19);recordsfromPSUs27(ratio=0.
04),73(ratio=0.
14)and93(ratio=0.
22)wereexcluded(n=951).
Alsoexcludedwererecordsfromindividualvehiclesinwhichtheagesofpassengersorthetotalnumberofpassengerswasunknown(FARS:n=7;GES:n=447).
Intotal,recordsof75fatally-injureddrivers(3.
3%oforiginalpopulation)and1,398driversinvolvedinpolice-reportedcrashes(11.
5%oforiginalweightedsample)wereexcluded.
Thisleft2,191recordsoffatally-injureddriversand13,258recordsofdriversinpolice-reportedcrashesforthemainanalysis.
MilesdrivenOftheoriginal9,663drivingtripsof16-and17-year-oldsintheNHTSsample,110(0.
7%ofweightedtrips)hadunknownlengths.
Anadditional23trips(0.
2%ofweightedtrips)hadlengthsdeemedimplausibleonthebasisoftheircalculatedaveragespeed(greaterthan100milesperhour)—thesewerereplacedwithmissingvalues.
Missingvaluesoftriplengthwerereplacedfor109tripswithvaluespredictedfromlinearregressionoftriplengthontripduration;24tripswhoselengthanddurationwerebothmissingwereexcluded,leaving9,639triprecordsforthemainanalysis.
TheweightsoftheNHTSdatawereadjustedtoalignthepopulationof16-and17-year-oldsestimatedfromtheNHTStothepopulationof16-and17-year-oldsintheUnitedStatesasreportedbytheUnitedStatesCensusBureau(2011).
Adjustmentsweremadeseparatelybyageandsex.
AnalysisCrash-involved16-and17-year-olddriverswereclassifiedaccordingtothenumberandagesofpassengerspresentinthevehicle.
Classificationswere:Nopassengers;1,2,or3+passengersunder21yearsofage(andnopassengersaged21orolder);Atleastonepassengeraged35orolder(anynumberofpassengersofotherages);Other(oldestpassengerinvehicleaged21-34).
5Thedrivingexposureof16-and17-year-olddriverswasalsogroupedaccordingtothesamecombinationsofpassengers.
However,theNHTSonlycollectstheagesofpassengerswhoaremembersofthedriver'shousehold;theagesofpassengerswhowerenotmembersofthedriver'shouseholdwereunknown.
Forthepurposeofthestudy,allpassengersofunknownage(i.
e.
,allpassengerswhowerenotmembersofthedriver'shousehold)wereassumedtohavebeenyoungerthan21yearsofage.
Providedthatatleastsomepassengersofunknownagewereaged21orolder,thismethodoverestimatesthenumberofmilesdrivenwithonlypassengersunderage21.
Similarly,ifanyofthepassengersofunknownagewereaged35orolder,thismethodunderestimatesthenumberofmilesdrivenwithpassengersaged35orolder.
Ratesofdriverskilledandofdriversinvolvedinpolice-reportedcrashespermiledrivenwereestimatedbydividingtheannualaveragenumberofcrash-involveddriverswitheachofthecombinationsofpassengersdefinedpreviouslybythenumberofmilesdrivenby16-and17-year-olddriverswiththesamecombinationofpassengers.
Relativerisks(ratiosofrates)werecomputedfordriverswitheachpassengercombinationrelativetodriverswithnopassengers.
Ratesandrelativeriskswerealsocomputedseparatelybysingleyearofdriverage,driversex,timeofday,andsinglevs.
multiple-vehiclecrashes.
Ratesandrelativeriskswerenotcomputedfordriverswithpassengersaged21-34becausethedataonmilesdrivencontainedtoofewtripsonwhichyoungdrivershadpassengersinthisagegrouptoproducereliableestimates(n=29).
Notethatduetotheassumptionthatallpassengersofunknownagewereunderage21,ratesandrelativerisksestimatedfordriverswithpassengersunderage21representlowerboundsforthetrueratesandthetruerelativerisks;similarly,ratesandrelativerisksestimatedfordriverswithpassengersaged35orolderrepresentupperbounds.
Standarderrorsofthenumbersofdriversinvolvedinallpolice-reportedcrasheswereestimatedusinggeneralizedstandarderrorspublishedbyNHTSA(2011).
StandarderrorsofthenumberofdriversinvolvedinfatalcrasheswereestimatedusingPoissonapproximations.
StandarderrorsofthenumberofmilesdrivenwereestimatedusingjackknifereplicateweightsprovidedintheNHTSdatafile(FHWA,2012).
ThestandarderrorsofratesandrelativeriskswereestimatedonthelogscaleusingfirstorderTaylorseriesapproximations;confidenceintervalsforrisksandrelativeriskswereestimatedonthelogscaleusingnormalapproximations.
ResultsTable1showstheproportionsoffatally-injureddriversandofalldriversinvolvedinpolice-reportedcrashesinrelationtothenumberandagesofpassengerspresent.
Ofthe2,191driversaged16-17whowerekilledincrashesoverthestudyperiod,54.
5%hadnopassengers,2.
9%hadatleastonepassengeraged35yearsorolder,24.
2%hadonepassengeryoungerthan21yearsofage(andnoolderpassengers),9.
3%had2passengersyoungerthanage21,5.
9%had3ormorepassengersyoungerthanage21,and3.
2%hadatleastonepassengeraged21-34inthevehicle(butnopassengersaged35orolder).
Variationsinpassengergroupingsbydriverage,sex,andtimeofdayweresmall;driversinsingle-vehiclecrasheswerelessthanhalfaslikelyasdriversinmulti-vehiclecrashestohavehadapassengeraged35yearsorolder.
Amongall16-and17-year-olddrivers6involvedinpolice-reportedcrashes,theproportionswith1,2,or3+passengersunderage21werelowerthanamongfatally-injureddrivers,andtheproportionswithnopassengersorwithapassengeraged35orolderwerehigherthanamongfatally-injureddrivers.
Theannualnumberof16-and17-year-olddriverskilledincrashesdecreasedby47%overthestudyperiod,from755in2007to398in2010.
Thenumberofdriversinvolvedinpolice-reportedcrashesdecreasedby23%,from468,903in2007to361,433in2010.
However,thedistributionsofdriverskilledincrashesandofalldriversinvolvedincrashesinrelationtopassengergroupingswererelativelystablefromyeartoyear(Table1).
Table1alsoshowstheshareofmilesdrivenby16-and17-year-olddriversinrelationtothenumberandagesofpassengerspresent.
Comparedto17-year-olddrivers,16-year-olddriversdroveasubstantiallygreatershareoftheirtotalmileswithahouseholdmemberaged35orolderinthevehicle.
Driversaged16alsodroveasubstantiallygreatershareoftheirmilesthandid17-year-oldswithonepassengerunderage21.
Table1suggeststhatdriverswerefarmorelikelytohavehadthreeormorepassengersunderage21whentripsoccurredbetweenthehoursof10PMand5:59AMthanwhentripsoccurredatotherhours;however,thiswasbasedononlyasmallrawnumberoftrips(n=11)andthusshouldnotbeover-interpreted.
Table2showsthenumberof16-and17-year-olddriverskilledincrashesper100millionmilesdrivenandnumberinvolvedinpolice-reportedcrashesper1millionmilesdriveninrelationtothenumberandagesofpassengerspresent.
Fordriverswithnopassengers,theper-miledriverdeathrateandcrashinvolvementratewerebothsubstantiallyhigherfor16-year-olddriversthanfor17-year-olds.
Thedeathrateofmaledriverswithnopassengerswassomewhathigherthantherateforfemaledriverswithnopassengers,whereasthecorrespondingratesofinvolvementinanycrashwereslightlyhigherforfemalesthanformales(neitherdifferenceapproachedstatisticalsignificance).
Thedeathratepermiledrivenof16-and17-year-olddriverswithnopassengerswasover6timesashighbetween10PMand5:59AMasbetween6AMand9:59PM.
Theoverallcrashinvolvementratepermiledrivenwaselevatedonlyslightlyduringthesehours,andthedifferencedidnotapproachstatisticalsignificance.
Figure1andTable3showtheper-milerisksofbeingkilledinacrashandofbeinginvolvedinapolice-reportedcrashinrelationtothenumberandagesofpassengerspresent,relativetotheriskswithnopassengerspresent,fordriversaged16-17.
Ingeneral,havingpassengersunderage21wasassociatedwithanincreaseinrisk,andhavingadultpassengersaged35+wasassociatedwithadecreaseinrisk(Figure1;Table3).
Comparedwithhavingnopassengers,havingonepassengeryoungerthanage21(andnoolderpassengers)wasassociatedwitha44%increaseina16-or17-year-olddriver'sriskpermiledrivenofbeingkilledinacrash(RR1.
44,95%CI1.
01–2.
04),havingtwopassengersyoungerthanage21wasassociatedwithadoublingofthedriver'sriskofbeingkilledinacrash(RR2.
02,95%CI1.
36–2.
99),andhavingthreeormorepassengersyoungerthanage21wasassociatedwithaquadruplingintheriskofbeingkilledinacrash(RR4.
39,95%CI1.
45–13.
31).
Therelationshipbetweentherelativeriskofbeinginvolvedinanypolice-reportedcrashandthepresenceofyoungpassengersfollowedasimilarpattern;however,theincreasesinrelativerisksofinvolvementinanypolice-reportedcrashweresmallerandwerenotstatisticallysignificant.
7Havingatleastonepassengeraged35orolderinthevehiclewasassociatedwitha62%decreaseintheriskpermiledrivenofbeingkilledinacrash(RR0.
38,95%CI:0.
24–0.
60)anda46%decreaseintheriskofbeinginvolvedinanypolice-reportedcrash(RR0.
54,95%CI0.
35–0.
84),comparedwithhavingnopassengers.
Table1.
Driverdeaths,driversinvolvedinpolice-reportedcrashes,andmilesdriveninrelationtodriverage,sex,timeofday,andcombinationofpassengersinthevehicle,driversaged16-17yearsdrivingcars,pickuptrucks,vans,minivans,orsportutilityvehicles,UnitedStates,2007-2010.
NopassengersAtleast1passengeraged35+Numberofpassengers<21yearsold(Allpassengers<21yearsold)OtherpassengergroupTotal123+DriverdeathsRowpercentNTotal54.
52.
924.
29.
35.
93.
22,191Driverage1652.
33.
226.
610.
06.
02.
08531756.
02.
822.
68.
85.
84.
01,338Male53.
13.
225.
19.
56.
22.
71,385Female56.
92.
422.
68.
85.
34.
08066AM–9:59PM54.
13.
525.
49.
55.
32.
21,51510PM–5:59AM55.
51.
721.
29.
17.
25.
4651Single-vehiclecrash54.
12.
024.
59.
85.
93.
81,382Multiple-vehiclecrash55.
44.
623.
78.
35.
92.
1809Crashyear200754.
63.
022.
910.
16.
52.
9755200857.
12.
424.
18.
75.
62.
2553200952.
82.
724.
79.
34.
75.
8485201053.
03.
826.
18.
56.
52.
0398Allpolice-reportedcrashesRowpercent(Weighted)WeightedNTotal61.
94.
721.
26.
13.
32.
81,636,618Driverage1659.
56.
521.
26.
43.
43.
1653,5631763.
53.
521.
25.
93.
22.
6983,055Male62.
54.
220.
76.
33.
52.
9878,411Female61.
35.
321.
95.
73.
12.
7758,2086AM–9:59PM62.
34.
821.
25.
93.
12.
71,499,71110PM–5:59AM58.
03.
321.
37.
45.
74.
3136,907Single-vehiclecrash64.
62.
920.
36.
73.
51.
9350,143Multiple-vehiclecrash61.
25.
221.
55.
93.
23.
01,286,476Crashyear200763.
54.
520.
85.
83.
42.
1468,903200861.
25.
221.
15.
73.
73.
2427,289200961.
74.
420.
56.
73.
33.
3378,993201060.
94.
722.
76.
12.
72.
8361,433MilesdrivenaRowpercent(Weighted)Weightedmiles(inmillions)Total63.
89.
019.
75.
41.
60.
620,899Driverage1653.
514.
126.
55.
20.
40.
36,5241768.
46.
716.
65.
42.
10.
814,375Male63.
29.
517.
96.
22.
11.
111,592Female64.
48.
322.
04.
40.
90.
09,3076AM–9:59PM63.
79.
319.
75.
51.
10.
719,22210PM–5:59AM62.
06.
420.
24.
27.
2-1,564Data:FatalityAnalysisReportingSystem(2012),GeneralEstimatesSystem(2012),NationalHouseholdTravelSurvey(2009).
a.
Indataonmilesdriven,passengeragewasunknownwhenthepassengerwasnotamemberofthesubjectdriver'shousehold.
Forthepurposeofthestudy,non-householdpassengerswereassumedtobeunderage21.
Thus,percentagesshownfordriverswithpassengeraged35+andwithotherpassengergrouprepresentlowerbounds,andpercentagesshownfordriverswith1,2,or3+passengers<21yearsoldrepresentupperbounds.
Tripsareweightedtoreflecttotalmilesdrivenovera365-dayperiodfromMay2008throughApril2009.
8Mostestimatesofrelativeriskswithinsubgroupswereimprecise,largelyduetothesparsenessoftheNHTSdataontripswithspecificcombinationsofpassengers;however,theygenerallyfollowedthesamepatternastheoverallresults.
Thepresenceofapassengeraged35+wasassociatedwithasignificantlylargerdecreaseintherelativeriskofinvolvementinsingle-vehiclecrashesthanintherelativeriskofinvolvementinmultiple-vehiclecrashes;thisappliedtobothcrashesinwhichthedriverwaskilled(RatioofRelativeRisks[RRR]0.
44,95%CI0.
32–0.
60)andtoallpolice-reportedcrashes(RRR0.
54,95%CI0.
38–0.
76).
Havingthreeormorepassengersunderage21appearedtoincreasetherelativerisktoagreaterdegreefor16-year-olddriversthanfor17-year-olddrivers;theratiosofrelativerisksofdriverdeathandofinvolvementinanypolice-reportedcrashbothapproachedstatisticalsignificance(driverdeath:RRR4.
1,95%CI:0.
92–18.
0;allpolice-reportedcrashes:RRR4.
2,95%CI:0.
96–18.
3).
Althoughthemagnitudesoftherelativerisksdidappeartovaryacrosssubgroupsforsomepassengercombinations,noothersubgroupdifferencesapproachedstatisticalsignificanceatthe95%confidencelevel.
Table2.
Ratesofdriverdeathandinvolvementinanypolice-reportedcrashinrelationtodriverage,sex,timeofday,andcombinationofpassengersinthevehicle,driversaged16-17,UnitedStates,2007-2010.
NopassengersAtleast1passengeraged35+aNumberofpassengers<21yearsolda(Allpassengers<21yearsold123+DriverdeathsRateper100millionmilesdriven(95%ConfidenceInterval)Total2.
24(1.
88–2.
68)0.
85(0.
56–1.
31)3.
22(2.
38–4.
36)4.
53(3.
19–6.
42)9.
85(3.
30–29.
41)Driverage163.
19(2.
57–3.
97)0.
73(0.
43–1.
27)3.
29(1.
69–6.
39)6.
26(3.
95–9.
92)44.
55(19.
82–100.
14)171.
90(1.
53–2.
37)0.
96(0.
50–1.
87)3.
17(2.
48–4.
05)3.
77(2.
46–5.
78)6.
53(1.
95–21.
82)Male2.
51(1.
91–3.
30)1.
02(0.
61–1.
69)4.
20(3.
14–5.
63)4.
61(2.
94–7.
25)8.
96(2.
03–39.
62)Female1.
91(1.
57–2.
33)0.
62(0.
32–1.
19)2.
22(1.
30–3.
81)4.
37(2.
69–7.
10)12.
28(6.
33–23.
84)6AM–9:59PM1.
67(1.
39–2.
02)0.
75(0.
48–1.
16)2.
54(1.
83–3.
51)3.
41(2.
36–4.
93)9.
43(4.
82–18.
44)10PM–5:59AM9.
31(7.
08–12.
26)2.
75(0.
68–11.
17)10.
90(5.
63–21.
11)22.
57(8.
41–60.
60)10.
43(1.
30–83.
32)Single-vehiclecrash1.
40(1.
17–1.
68)0.
36(0.
21–0.
60)2.
05(1.
50–2.
80)3.
03(2.
11–4.
36)6.
19(2.
05–18.
62)Multiple-vehiclecrash0.
84(0.
69–1.
02)0.
49(0.
31–0.
79)1.
17(0.
84–1.
61)1.
49(1.
00–2.
23)3.
67(1.
20–11.
20)Allpolice-reportedcrashesRateper1millionmilesdriven(95%ConfidenceInterval)Total19.
0(15.
3–23.
5)10.
2(6.
9–15.
1)21.
1(15.
3–29.
2)22.
1(15.
4–31.
8)41.
3(13.
8–123.
5)Driverage1627.
8(21.
8–35.
4)11.
5(7.
4–17.
8)20.
1(10.
3–39.
2)30.
6(19.
4–48.
2)194.
6(87.
7–432.
0)1715.
9(12.
4–20.
3)9.
1(4.
9–16.
8)21.
8(16.
8–28.
5)18.
4(12.
0–28.
3)26.
6(8.
0–88.
6)Male18.
7(13.
9–25.
2)8.
3(5.
2–13.
1)21.
9(16.
0–29.
9)19.
5(12.
3–30.
8)31.
8(7.
2–140.
6)Female19.
4(15.
5–24.
2)13.
1(7.
8–22.
1)20.
3(11.
8–34.
8)26.
8(16.
7–42.
8)67.
2(35.
6–126.
9)6AM–9:59PM19.
1(15.
3–23.
7)10.
2(6.
9–15.
1)21.
0(14.
9–29.
5)21.
1(14.
5–30.
6)53.
8(27.
7–104.
4)10PM–5:59AM20.
5(15.
1–27.
8)11.
4(3.
0–43.
4)23.
0(11.
7–45.
3)38.
8(14.
3–105.
5)17.
4(2.
2–139.
6)Single-vehiclecrash4.
2(3.
4–5.
3)1.
4(0.
86–2.
2)4.
3(3.
1–6.
1)5.
2(3.
5–7.
8)9.
4(3.
1–28.
8)Multiple-vehiclecrash14.
8(11.
9–18.
3)8.
9(6.
0–13.
1)16.
8(12.
1–23.
2)16.
9(11.
7–24.
4)31.
8(10.
6–95.
4)Data:FatalityAnalysisReportingSystem(2012),GeneralEstimatesSystem(2012),NationalHouseholdTravelSurvey(2009).
a.
IntheNationalHouseholdTravelSurveydatausedtocomputecrashratespermiledriven,passengeragewasunknownforpassengerswhowerenotmembersofthesubjectdriver'shousehold.
Forthepurposeofthestudy,allnon-householdpassengerswereassumedtobeunderage21,thusestimatedriskofdrivingwithapassengeraged35+isanupperbound,andestimatedrisksofdrivingwith1,2,or3+passengers<21yearsoldarelowerbounds.
9Figure1.
Relativeriskofdriverdeath(left)andofinvolvementinanypolice-reportedcrash(right)permiledriveninrelationtocombinationofpassengersinthevehicle,driversaged16-17,UnitedStates,2007-2010.
Data:FatalityAnalysisReportingSystem(2012),GeneralEstimatesSystem(2012),NationalHouseholdTravelSurvey(2009).
a.
IntheNationalHouseholdTravelSurveydatausedtocomputecrashratespermiledriven,passengeragewasunknownforpassengerswhowerenotmembersofthesubjectdriver'shousehold.
Forthepurposeofthestudy,allnon-householdpassengerswereassumedtobeunderage21,thustherelativeriskshownfordrivingwithapassengeraged35+isanupperbound.
b.
Relativerisksshownfordrivingwith1,2,and3+passengers<21yearsoldrepresentlowerbounds.
Table3.
Relativerisksofdriverdeathandofinvolvementinanypolice-reportedcrashpermiledriveninrelationtodriverage,sex,timeofday,andcombinationofpassengersinthevehicle,driversaged16-17,UnitedStates,2007-2010.
NoPassengersAtleast1passengeraged35+aNumberofpassengers<21yearsold(Allpassengers<21yearsold)a123+DriverdeathspermiledrivenRelativeRisk(95%ConfidenceInterval)Total1(Reference)0.
38(0.
24–0.
60)1.
44(1.
01–2.
04)2.
02(1.
36–2.
99)4.
39(1.
45–13.
31)161(Reference)0.
23(0.
13–0.
41)1.
03(0.
51–2.
07)1.
96(1.
18–3.
26)13.
95(6.
03–32.
29)171(Reference)0.
51(0.
25–1.
02)1.
66(1.
20–2.
31)1.
98(1.
23–3.
20)3.
43(1.
01–11.
68)Male1(Reference)0.
40(0.
23–0.
72)1.
67(1.
12–2.
50)1.
84(1.
08–3.
12)3.
57(0.
79–16.
19)Female1(Reference)0.
32(0.
16–0.
64)1.
16(0.
65–2.
06)2.
28(1.
35–3.
86)6.
41(3.
21–12.
81)6AM–9:59PM1(Reference)0.
45(0.
28–0.
72)1.
52(1.
04–2.
21)2.
04(1.
35–3.
08)5.
64(2.
81–11.
32)10PM–5:59AM1(Reference)0.
30(0.
07–1.
23)1.
17(0.
57–2.
39)2.
42(0.
87–6.
75)1.
12(0.
14–9.
11)Single-vehiclecrash1(Reference)0.
26(0.
15–0.
44)1.
46(1.
02–2.
10)2.
16(1.
44–3.
25)4.
41(1.
44–13.
48)Multi-vehiclecrash1(Reference)0.
59(0.
35–0.
98)1.
39(0.
95–2.
02)1.
78(1.
14–2.
77)4.
36(1.
40–13.
54)Allpolice-reportedcrashespermiledrivenRelativeRisk(95%ConfidenceInterval)Total1(Reference)0.
54(0.
35–0.
84)1.
11(0.
75–1.
64)1.
16(0.
76–1.
77)2.
17(0.
71–6.
63)161(Reference)0.
41(0.
25–0.
68)0.
72(0.
35–1.
47)1.
10(0.
66–1.
84)7.
00(3.
04–16.
10)171(Reference)0.
57(0.
29–1.
11)1.
38(0.
96–1.
97)1.
16(0.
71–1.
90)1.
67(0.
49–5.
72)Male1(Reference)0.
44(0.
25–0.
76)1.
17(0.
76–1.
80)1.
04(0.
60–1.
80)1.
70(0.
37–7.
74)Female1(Reference)0.
68(0.
38–1.
19)1.
05(0.
58–1.
88)1.
38(0.
82–2.
32)3.
47(1.
77–6.
80)6AM–9:59PM1(Reference)0.
53(0.
34–0.
84)1.
10(0.
73–1.
65)1.
11(0.
72–1.
70)2.
82(1.
40–5.
67)10PM–5:59AM1(Reference)0.
56(0.
14–2.
20)1.
12(0.
54–2.
36)1.
90(0.
67–5.
40)0.
85(0.
10–6.
97)Single-vehiclecrash1(Reference)0.
32(0.
19–0.
54)1.
02(0.
68–1.
53)1.
23(0.
78–1.
94)2.
22(0.
71–6.
94)Multi-vehiclecrash1(Reference)0.
60(0.
38–0.
94)1.
14(0.
77–1.
68)1.
14(0.
75–1.
75)2.
15(0.
70–6.
60)Data:FatalityAnalysisReportingSystem(2012),GeneralEstimatesSystem(2012),NationalHouseholdTravelSurvey(2009).
a.
IntheNationalHouseholdTravelSurveydatausedtocomputecrashratespermiledriven,passengeragewasunknownforpassengerswhowerenotmembersofthesubjectdriver'shousehold.
Forthepurposeofthestudy,allnon-householdpassengerswereassumedtobeunderage21,thusestimatedrelativeriskofdrivingwithapassengeraged35+isanupperbound,andestimatedrelativerisksofdrivingwith1,2,or3+passengers<21yearsoldarelowerbounds.
10DiscussionDriversaged16-17wereshowntobeatincreasedriskpermiledrivenofbeingkilledinacrashwhencarryingyoungpassengers,andtheriskincreasedfurtherasthenumberofyoungpassengersincreased.
Risksofinvolvementinpolice-reportedcrashesofanyseverityappearedtofollowasimilarpattern;however,theincreasesintheriskofinvolvementinanypolice-reportedcrashweresmallerthantheincreasesinriskofdriverdeathanddidnotevenapproachstatisticalsignificance.
Incontrast,youngdrivers'riskofinvolvementinanypolice-reportedcrashesandriskofbeingkilledinacrashwerebothreducedsignificantlywhencarryinganadultpassengeraged35orolder.
Somewhatunexpectedly,itwasestimatedthatthepresenceofoneyoungpassengerincreaseda16-year-olddriver'sriskofbeingkilledinacrashbyonly3%(RR1.
03,95%CI0.
51–2.
07),whereastheriskfora17-year-olddriverincreasedby66%(RR1.
66,95%CI1.
20–2.
31)inthepresenceofoneyoungpassenger.
However,thedifferenceinthesetwoestimatesisnotsignificantlylargerthanwhatmightbeexpectedtooccurbychancealoneunderthehypothesisthattheeffectofhavingoneyoungpassengerdoesnotvarybyage(RatioofRelativeRisks0.
62,95%CI0.
29–1.
34).
Thus,theseestimatesarenotinconsistentwiththehypothesisthattheestimatedeffectfordriversaged16and17combinedappliesequallytodriversaged16anddriversaged17,i.
e.
,thatthepresenceofoneyoungpassengerisassociatedwitha44%increaseintheriskofdeathpermiledriven(RR1.
44,95%CI1.
01–2.
04).
RelationtootherresearchThisstudyconfirmsthattheresultsoriginallyreportedintheseminalstudybyChenetal.
(2000)arestillbroadlyapplicabletoday.
Inthatstudy,Chenetal.
analyzeddataondriverdeathsfromyears1992-1997anddataondrivingexposurefromyears1995-1996toestimaterelativeriskofdriverdeathperdrivingtripinrelationtothenumberofpassengerspresent,andfoundthathavingone,two,orthreeormorepassengersincreasedtheper-tripriskofdriverdeathby39%,86%,and182%,respectively,for16-year-olddrivers,andby48%,158%,and207%,respectivelyfor17-year-olddrivers.
AlthoughChenetal.
didnotformallyinvestigatetheper-triporper-mileriskofinvolvementincrashesthatwerenotfataltothedriver,theyanalyzedtherateofdriverdeathsperpolice-reportedcrashandreportedthathavingteen-agedpassengersandhavingpassengersaged20-29bothwereassociatedwithincreasedriskofdriverdeathintheeventofapolice-reportedcrash,thusimplyingthatthepresenceofyoungpassengersmusthaveincreasedtheper-tripriskofseverecrashestoagreaterextentthanitincreasedtheriskofless-severecrashes.
Thiswasinvestigatedformallyandwasconfirmedinthecurrentstudy.
ThisstudyalsoextendstheresultsofChenetal.
bytakingadvantageofthelimiteddataavailableontheagesofpassengersridingwithyoungdriverstoestimatelowerboundsfortherisksassociatedwithcarryingmultipleyoungpassengersandupperboundsfortherisksassociatedwithcarryingadultpassengers.
Inasimilarstudy,Ouimetetal.
(2010)analyzedfatalcrashdatafromyears1999-2003andtraveldatafrom2001-2002toestimatetherelativeriskoffatalcrashinvolvementpermiledrivenfordriversages15-20inrelationtotheageandsexofpassengerswhentherewasonepassengerinthevehicle.
ThedataanalyzedbyOuimetetal.
,likethedataanalyzedin11thecurrentstudy,onlycontainedinformationabouttheageandsexofpassengerswhoweremembersofthedriver'shousehold.
Theauthorsusedahot-deckimputationmethodtoestimatethedistributionofageandsexamongallpassengers,includingthenon-householdpassengers,andreportedthatthenumberofmilesdrivenwithadultpassengersaged35+wassubstantiallygreaterthanthenumberofmilesdrivenwithpeersofthedriver,whichtheynotedmighthavebeenanartifactoftheirmethod.
Inaddition,theresultsofthatstudymaybeoflimitedgeneralizabilitytodriversages16-17;examinationofthedatausedtoestimatemilesdriveninthecurrentstudyshowsthat79%thetotalmilesdrivenbydriversaged15-20weredrivenbydriversaged18-20.
Astudythatusedin-vehiclecamerastoobserveasampleof40newly-licensedteenagedriversfortheirfirst18monthsoflicenseddriving(Klaueretal.
,2011)reportedresultsverysimilartothoseofthecurrentstudywithrespecttothedistributionsofpassengersinthevehiclesofyoungdrivers.
Inthatstudy,apassengeraged19orolderwaspresentforabout13-14%ofallmilesdrivenbythestudysubjectsduringtheirfirstthreemonthsoflicenseddrivingandforabout7-8%ofallmilesdrivenduringmonths4-18oflicenseddriving;passengersaged13-18werepresentfor26-28%ofallmilesdrivenduringstudysubjects'first12monthsoflicenseddrivingandforabout21-22%ofallmilesdriveninmonths13-18.
Inthecurrentstudy,anadultaged21orolderwaspresentfor15.
2%ofmilesdrivenby16-year-oldsand7.
5%ofmilesdrivenby17-year-olds;passengersyoungerthan21yearsofagewerepresentfor32.
1%ofmilesdrivenby16-year-oldsand24.
1%ofmilesdrivenby17-year-olds.
Thiscloseagreementsuggeststhattheassumptionmadeinthecurrentstudythatallnon-householdpassengerswereunderage21yieldedareasonableapproximationoftheoverallagedistributionofthepassengersridingwithdriversaged16-17.
Thisstudyestimatedthathavinganadultpassengeraged35yearsorolderdecreaseda16-to17-year-olddriver'sriskpermiledrivenofinvolvementinanypolice-reportedcrashby46%anddecreasedthedriver'sriskofbeingkilledinacrashby62%.
Havinganadultpassengerinthevehiclewasshowntobeespeciallyprotectiveagainsttheriskofinvolvementinsingle-vehiclecrashes.
Thismakessense.
Althoughthedataanalyzedinthisstudydonotidentifyonedriveroranotherasbeing"atfault"foranycrash,itislikelythatagreaterproportionofyoungdrivers'single-vehiclecrashinvolvementsthanmultiple-vehiclecrashinvolvementswereatleastpartiallyattributabletosomeactionorerrorcommittedbytheyoungdriver;thus,itislikelythatanadultpassengercouldhelptopreventagreaterproportionofyoungdrivers'single-vehiclecrashesthanmultiple-vehiclecrashes.
ThemagnitudeoftheriskreductionestimatedhereforcarryinganadultpassengeragreesverywellwiththeestimateofRiceetal.
(2003)thatcarryinganadultpassengerwasassociatedwitha70%reductionina16-or17-year-olddriver'sriskofbeinginvolvedinacrashinwhichheorshewasinjuredandwasdeemedculpable.
Whiletherelationshipoftheadultpassengerstothedriverswasnotknownineitherstudy,itislikelythatinmanycasestheywerethedriver'sparents.
Inastudyinwhichin-vehiclecamerasandotherdatacollectionequipmentwereusedtomonitor42teen-ageddriversfortheirfirst18monthsoflicensedindependentdriving,Simons-Mortonetal.
(2011)foundthatthepresenceofanadultpassengerwasassociatedwitha74%reductionintherateofinvolvementincrashesornearcrashespermiledrivenanda68%reductioninhighg-forceevents(e.
g.
,hardacceleration,braking,orswerving)comparedwithdrivingalone.
Theauthorsdidnotconcludewhetherthiseffectwasdueto12teensmoderatingtheirbehaviorinthepresenceofadultpassengers,whethertheadultactivelyhelpedthedriver(e.
g.
,bypointingouthazards),both,orsomethingelse.
InasimilarstudyconductedfortheAAAFoundationforTrafficSafety,Goodwinetal.
(2012)usedin-vehiclecamerastoobservethedrivingofadifferentsampleof52newly-licensedteenagedriversandfoundthatelectronicdeviceusewas86%lessfrequentandotherdistracted-drivingbehaviorswere75%lessfrequentwhenaparentorotheradultwaspresentinthevehiclethanwhenthedriverwasalone,illustratingthatteenagedriversmodifyatleastsomeaspectsoftheirbehaviorinthepresenceofadultpassengers.
Thisstudydoesnotshedlightonthemechanismsbywhichthepresenceofyoungpassengersincreasestheriskoffatalcrashinvolvementforyoungdrivers.
WilliamsandTefft(2012)analyzeddatafromfatalcrashesthatinvolved16-or17-year-olddriversinyears2005-2010andreportedthatincomparisontofatal-crashinvolveddriverswithnopassengers,agreaterproportionofthosewithteenagepassengerswerespeeding,drinkingalcohol,andcodedonthepolicecrashreportashavingcontributedtothecrashinatleastsomeway,andreportedthattheproportionsspeeding,drinkingalcohol,andcontributingtothecrashincreasedasthenumberofteenagepassengersincreased.
Goodwinetal.
(2012b)usedin-vehiclecamerastocollectdatafromasampleofnewly-licensedyoungdriversinNorthCarolina,andfoundthatdriversweremorelikelytospeed,tailgate,and"showoff"whenmultipleteenagepassengerswerepresent.
Theauthorsalsonotedthattheyobservedfewinstancesinwhichthepassengersactivelyencouragedthesebehaviors,suggestingthatitwasthepassengers'merepresencethatelicitedsuchbehaviorfromthedriver.
Incontrast,inanotherstudyalsousingin-vehiclecamerasandotherdatacollectionequipmenttostudyasampleofnewly-licensedyoungdrivers,Simons-Mortonetal.
(2011)foundthatelevatedg-forceevents(e.
g.
,hardbraking,swerving)weresomewhatlessfrequentwhenteenagepassengerswerepresentthanwhenthedriverwasalone.
Thecontrastbetweentheresultsofthesestudiessuggeststherelationshipbetweenthepresenceofyoungpassengersandthebehaviorofayoungdriveriscomplex.
LimitationsThenationalsurveythatwasusedtoestimatedrivingexposureinrelationtotheageandnumberofpassengersdidnotcollectdataontheagesofpassengerswhowerenotmembersofthedriver'shousehold.
Forthepurposeofthestudy,itwasassumedthatallnon-householdpassengerswerelessthan21yearsofage.
Whilethisisunlikelytobecorrect,itprovidesclearandusefulinsights:risksreportedinthisstudyforyoungdriverscarryingonlypassengersunderage21representlowerboundsforthetruerisksassociatedwithcarryingpassengersunderage21,andrisksreportedhererepresentupperboundsforthetruerisks(equivalently:lowerboundsforthetrueriskreductions)associatedwithcarryingpassengersaged35orolder.
Thedrivingexposuredatacontainedtoofewtripswithhouseholdpassengersaged21-34toestimaterelativerisksfordriverswithpassengersinthisagerange.
Chenetal.
(2000)reportedthatthepresenceofpassengersaged20-29wasassociatedwithincreasedcrashseverity(greateraveragenumberofdriverdeathsperpolice-reportedcrash),andWilliams&Tefft(2012)foundthatmajorriskfactorssuchasspeedingandalcoholusewereasprevalentormoreprevalentinfatalcrashesof16-and17-year-olddriverswithpassengersaged20-29aswithmultipleteenagepassengers.
However,inthedataanalyzedinthecurrentstudy,only3.
2%offatally-injured16-and17-year-olddriversand2.
8%ofall16-13and17-year-olddriversinvolvedinpolice-reportedcrasheshadanypassengersaged21-34andnopassengersaged35orolder.
Althoughitisclearthathavingyoungpassengersinthevehicleincreasesriskforyoungdriversandhavingadultpassengersdecreasesrisk,theresultsofthisstudydonotprovideevidenceoftheactualage(ofthepassenger)atwhichthepassenger'spresenceceasestoincreaseadriver'sriskortheagethatitbecomesaprotectivefactor.
Theupperagecutoffforyoungpassengerswasplacedatage21toalignwithmostexistingstateGDLpassengerrestrictions.
Theloweragecutoffof35wasselectedforadultpassengerstocapturepassengersoldenoughthattheycouldplausiblyhavebeenthedriver'sparents.
Thesecutoffpointswereselectedapriori;theywerenotoutcomesofthestudy.
Amongfatally-injureddriversinthisstudy,25%hadafull-privilegedriver'slicense,59%hadanintermediateorprovisionallicensewithsomerestrictions,4%hadalearner'spermitthatalloweddrivingonlywithalicensedadultpassenger,and12%wereunlicensed.
However,dataonthetypeoflicensethatthedriverpossessedwasnotavailableinthedataonallpolice-reportedcrashesnorinthedataondrivingexposure;thus,theresultsreportedherearebasedonalldrivingdonebydriversaged16and17irrespectiveofthetypeofdriver'slicensethattheypossessed.
Mostofthemilesdrivenbydriversintheexposuredatalikelyhadalicensethatalloweddrivingwithoutanadultinthecar;however,somemayhavehadlearner'spermits,andsomemayhaveevenbeenunlicensed.
Asnotedpreviously,theannualnumberofcrashesanddeathsof16-and17-year-olddriversdecreasedsharplyoverthisperiod,whichsuggeststhattheremayhavebeenchangesindrivingexposureoverthisperiodaswell.
However,dataondrivingexposurewereavailableforonlytheone-yearperiodfromapproximatelyMay2008–April2009.
Themainanalyseswerebasedoncrashdatafromyears2007–2010becausethenumbersofdriverdeathswithspecificcombinationsofpassengerswereprohibitivelysmalltoproducestableestimatesusingonlyoneyearofdata.
Totestthesensitivityoftheresultstopossiblesystematicchangesinexposureoverthestudyperiod,themainanalyseswerereplicatedusingcrashdatafromtheone-yearperiodfromMay2008throughApril2009.
Usingcrashdatafromonlythisperiod,theestimatedrelativerisksofdriverdeathassociatedwithhavinganadultpassenger,1passengerunderage21,2passengersunderage21,and3ormorepassengersunderage21were0.
45,1.
47,1.
71,and3.
81,respectively,comparedwith0.
38,1.
44,2.
02,and4.
39whenestimatedusingcrashdatafrom2007–2010;relativerisksofinvolvementinanypolice-reportedcrashvariedevenlessinrelationtothetimeperiodofthecrashdataanalyzed.
Estimatesoftheamountofdrivingdonebyyoungdriverswithvariouscombinationsofpassengersreliedondrivers'self-reportsofthetripsthattheytook,thelengthsofthosetrips,andthepassengerspresentinthevehicle,allofwhichcouldbesubjecttobothrandomerrorandbias.
Althoughanyerrorinthereportednumberorlengthoftripswouldaffectestimatesofabsoluterisks(e.
g.
,crashespermiledriven),neitherrandomerrorsnorsystematicerrorunrelatedtothecombinationofpassengerspresent(e.
g.
,under-reportingofmilesdrivenby25%uniformlyacrossallpassengergroups)wouldbiastherelativeriskofdrivingwithaspecifiedcombinationofpassengersvs.
drivingalone.
Biascouldstillbepresent,however,iferrorsinestimatedmilesdrivenvariedbypassengercombination(e.
g.
,iftripswithmultipleteenagepassengersweremorelikelytobeunreported),ifthereportingofpassengerpresencewasitselfsubjecttoerror,orifthedrivingpatternsofteen14driverswhoparticipatedintheNHTSdifferedfromthedrivingpatternsofteendriverswhodidnotparticipate.
Finally,bydesign,thisstudycouldnotdemonstratethattherelationshipbetweenthepresenceofpassengersandtheriskofcrashinvolvementwascausal.
Whileitappearsthathavingmultipleyoungpassengersincreasesriskandhavingadultpassengersdecreasesrisk,thisstudycannotruleoutthepossibilitythatteendriverswhocarrymultipleyoungpassengerstendtohavehighercrashriskindependentofthepresenceofthepassengers,andsimilarly,thatteendriverswhodrivewithadultpassengerstendtohavelowerriskindependentofthepresenceofthepassengers.
However,theresultsofthisstudy,inconjunctionwithotherstudiesthathaveexamineddifferentpopulationsusingdiversemethods,suggestthatthepresenceofyoungpassengersdoesindeedincreasethecrashriskofyoungdrivers,andthatthepresenceofadultpassengersdecreasesrisk.
ConclusionThisstudyshowsthat,permiledriven,16-and17-year-olddriversaremorelikelytobekilledinacrashwhentheyhaveyoungpassengersintheirvehiclethanwhentheyaredrivingalone.
Theirriskwasfoundtoincreasebyanestimated44%whenonepassengerunderage21(andnoolderpassengers)waspresentintheteendriver'svehicle,approximatelydoublewhentwopassengersunderage21werepresent,andmorethanquadruplewhenthreeormorepassengersunderage21werepresent.
Theeffectofyoungpassengersontheriskofinvolvementinanypolice-reportedcrashappearedtofollowasimilarpattern;however,increasesintheriskofanypolice-reportedcrashweresmallerandwerenotestimatedpreciselyenoughtoevenapproachstatisticalsignificance.
Itisclearthatdiscouragingteendriversfromcarryingpassengersand/ordiscouragingteenagersfromridingwithyounginexperienceddriverswouldbenefitthesafetyofteenagersbothasdriversandaspassengers.
Havinganadultpassengeraged35orolderwasassociatedwithnearlya50%reductionin16-or17-year-olddrivers'riskofinvolvementinanycrashandovera60%reductionintheriskofbeingkilledinacrash.
Parentsclearlycanplayamajorroleinprotectingtheirteen-agersbyridingwiththeirteens,evenafterlicensure,tocontinuetosupportthedevelopmentofsafedrivinghabits.
Inaddition,parentscanhelptoprotecttheirteensfromtherisksassociatedwithteenagedriverscarryingteenagepassengersbyenforcingapplicablestatepassengerrestrictions,andbysupplementingstatelawswiththeirownrulesregardingcarryingteenagepassengersorridingwithteendrivers.
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