Driver Attitudes and Choices: Speed Limits, Seat Belt Use, and Drinking-and-Driving

Transcription

1 Drver Atttudes nd Choces: Speed Lmts, Set Belt Use, nd Drnkng-nd-Drvng Young-Jun Kweon Assocte Reserch Scentst Vrgn Trnsportton Reserch Councl Young- Vrgn Trnsportton Reserch Councl 530 Edgemont Rod Chrlottesvlle, VA Phone: Fx: nd Kr M. Kockelmn (Correspondng Author) Assocte Professor of Cvl, Archtecturl nd Envronmentl Engneerng The Unversty of Texs t Austn kkockelm@ml.utexs.edu The Unversty of Texs t Austn 6.9 E. Cockrell Jr. Hll Austn, TX Phone: Fx: The followng pper s pre-prnt nd the fnl publcton cn be found n Journl of the Trnsportton Reserch Forum 45 (3):39-56, Presented t the 82nd Annul Meetng of the Trnsportton Reserch Bord, Jnury 2004 Abstrct A better understndng of tttudes nd behvorl prncples underlyng drvng behvor nd trffc sfety ssues cn contrbute to desgn nd polcy solutons, such s, speed lmts nd set belt legslton. Ths work exmnes the Motor Vehcle Occupnt Sfety Surveys (MVOSS) dt set to llumnte drvers setbelt use, drvng speed choces, drnkng-nd-drvng tendences, long wth ther tttudes towrds speed lmts nd set belt lws. Ordered probt, negtve bnoml, nd lner regresson models were used for the dt nlyss, nd severl nterestng results emerged. For exmple, persons of hgher ncome nd wth college educton prefer hgher speeds, re more lkely to use set belt, nd re more lkely to support set belt lws nd/or hgher speed lmts. However, persons wth college educton lso tend to drnk nd drve more often. Pckup drvers re less lkely to use set belts, less lkely to support set belt lws, yet less lkely to drnk nd drve. The number nd vrety of results fesble wth ths sngle dt set re nstructve s well s ntrgung. Key words Drver tttudes; Drver behvor; Speedng; Motor Vehcle Occupnt Sfety Surveys (MVOSS) - 1 -

2 INTRODUCTION Mny trffc sfety ssues hve been nvestgted usng crsh dt such s the Generl Estmtes System (GES), Ftlty Anlyss Reportng System (FARS), Hghwy Sfety Informton System (HSIS), nd locl polce crsh records. Conclusons cross studes on the mpct of speed lmt chnges (e.g., Chng nd Pnt 1990, Wgenr et l. 1990, nd Ledolter nd Chn 1996) nd the mpct of speed vrton on trffc sfety (e.g., Lve 1985 nd 1989, Levy nd Asch 1989, Dvs 2002, nd Kockelmn et l. 2006) re not defntve, even wth these sophstcted nd lrge dtsets. One possble explnton of ths dscrepncy mong studes on the sme topc s lck of understndng of rod user behvors nd tttudes. The Ntonl Hghwy Trffc Sfety Admnstrton (NHTSA) hs conducted the Motor Vehcle Occupnt Sfety Surveys (MVOSS) bnnully snce 1994 by telephone ntervew. The 2000 MVOSS dt nclude nformton on drver tttudes towrds sfety ssues (e.g., tttudes towrds the current speed lmt), drver behvors (e.g., speed choce nd drvng frequency), nd crsh hstory, s well s on ndvdul nd household chrcterstcs (Boyle nd Schulmn 2001). These re nlyzed here. A better understndng of behvorl prncples nd crcumstnces tht underle drvng behvor nd drver tttudes cn enhnce vrous trffc sfety polces, ncludng speed lmt selecton, set belt legslton, nd drunk-drvng cmpgns. In ths regrd, the MVOSS dt set provdes mny useful peces of nformton for nvestgton. Knowng who the supporters nd opponents of trffc sfety-relted polces re cn be very helpful n crftng nd promotng such polces, such s defnng trget groups for nt-speedng cmpgns nd drver trnng progrms. Ths study nvestgtes severl nterestng ssues reltng to these vrbles. Through these nvestgtons, the study ms to provde behvorl nd psychologcl nsghts nto the U.S. drvng populton. Wht follows here s lterture revew, model nd descrptons, dscusson of results, nd conclusons. LITERATURE REVIEW A few studes hve ddressed certn drvng behvors nd tttudes usng seres of crosssectonl surveys n the U.S. commssoned by Preventon Mgzne. Schechtmn et l. (1999) ttempted to relte drnkng hbts (frequency nd mount) to set belt use, speed lmt obedence, nd drunk drvng over 11 yers. They found no evdence to lnk drnkng hbts wth set belt use nd speed lmt obedence. However, evdence ndcted lnks between frequency nd mount of drnkng wth drunk drvng, s expected. Shnr et l. (1999) used the sme dtsets s Schechtmn et l. (1999) to exmne trends n drvng behvors nd helth mntennce behvors. They found tht the rte of set belt use ncresed from 41.5% n 1985 to 74.1% n 1995, long wth slght reducton n drunk drvng. The nvestgtors lso noted wek relton between drvng behvors nd helth mntennce behvors. Shnr et l. (2001) used more recent Preventon Mgzne survey dt to nvestgte ssoctons between set belt use, speed lmt observnce, drunk drvng, nd four demogrphc chrcterstcs (gender, ge, educton nd ncome). Ther four-wy ANOVA models usng dt ndcted tht femles reported more lw obedence thn mles n ll behvorl ctegores. Rtes of set belt use ncresed wth ge nd educton level for both mles nd femles. Interestngly, hgher educton nd ncome levels were ssocted wth speedng. One - 2 -

3 my rgue tht ths s due to hgher vlues of trvel tme nd drvng newer vehcles wth better sfety fetures, n mny cses. Koushk et l. (1998) reported tht Kuwt drvers n the sme ge group, who dd not wer set belts, volted trffc regultons more thn twce s often s those who wore set belts. They lso found tht set belt non-users were mostly young nd femle mong Kuwts, nd ther drvng behvors frequently nvolved chngng lnes wthout sgnlng nd chngng trvel speed. Ther fndngs confrm tht drvers who re reluctnt to wer set belt tend to be more dngerous drvers nd/or tke more rsks, n generl. Regrdng crsh njury severty, Km et l. (1995) found set belt use mong Hwns contrbuted sgnfcntly to njury reductons nd crsh survvl. He lso rgued tht dscourgng lcohol use plced drvers t less rsk. Speed choce lso hs been nvestgted. Hglund nd Aberg (2000) exmned drvers tttudes towrds speedng nd the nfluence of other drvers on speed choces. Dt were collected on Swedsh hghwys, wth speed lmt of 90 klometers per hour (km/h) (56 mph). They concluded tht drvers decsons regrdng speedng re hghly correlted wth ther vew of other drvers behvors. Drvers usully overestmted the frcton of hgh-speed drvers (.e., those trvelng t lest 10 km/h over the speed lmt); ther estmtes verged 50.7%, whle the observed percentge ws 22.9%. Furthermore, hgh-speed drvers beleved tht hgh proporton (58%) of other drvers lso qulfed s hgh-speed drvers, ndctng flse mpresson of speed consensus. Drvng speeds re nfluenced by vrous fctors, ncludng rodwy geometry, drver tttudes nd envronmentl fctors (e.g., wether nd enforcement). Knellds et l. (1990) studed pssenger cr speeds on horzontl curves of two-lne rurl rods n Greece. A totl of 207 Greek drvers rted the mpct of 14 elements of the rod s envronment (e.g., sght dstnce, pvement condton, nd lne wdth) on ther choce of speed. Drvers who tended to volte speed lmts rted ll types of sgnge (e.g., wrnng sgns) sgnfcntly lower thn speed lmt observers. Speed lmt offenders lso pd less ttenton to rodwy desgn. Lng et l. (1998) found consderble reductons n men speed nd sgnfcnt ncreses n speed vrnce under foggy nd snowy condtons on Interstte 84 n Idho, whle Edwrds (1999) only reported smll reductons n both men nd vrnce under rny nd foggy condtons on the M4 Motorwy n the U.K. V (1997) found sttstclly sgnfcnt nd somewht lrge reductons n verge speeds nd frcton of speeders due to ncresed polce enforcement on Norwy hghwys. Kockelmn et l. (2006) found verge speed ncreses n cross-secton to be double those n before/fter studes of speed-lmt ncreses, nd modeled optml speed choces s trde-off of crsh, speed lmt volton, nd dely costs. They lso found nstntneous speed vrtons (cross ndvdul vehcles) to hrdly depend on speed lmts nd rodwy desgn ttrbutes, nd they concluded tht hgher speed lmts hve ther gretest effect on crsh outcomes, n terms of njury severty. Mny behvors re recorded s dscrete responses (e.g., yes/no) n dt sets. Dscrete-response models re now common n ssessng crsh results. For exmple, Kockelmn nd Kweon (2002) ppled n ordered probt model for predcton of drver njury severty usng the 1998 GES dt nd developed seprte models for sngle-vehcle, two-vehcle nd mult-vehcle crshes. As expected, hgher trvel speeds were predcted to sgnfcntly ncrese njury severty. Femles nd older persons were lso predcted to be t greter rsk for severe njury, f they experence - 3 -

4 crsh s drver. Ther results re smlr to those of O Donnell nd Connor (1996), who used Austrln crsh records nd ordered logt nd probt models. They found tht drvng lght-duty trucks t hgh speeds, not werng set belts, nd hed-on collsons ll ncresed the lkelhood of severe njury nd ftlty. Cooper (1997) used bnry logt models to nvestgte the reltonshp between vrous volton convctons (e.g., exceedng the speed lmt nd dsobeyng sgnls) nd crsh nvolvement bsed on dt for Brtsh Columb, Cnd. In order to reduce serous nd ftl crshes, he concluded tht the focus should be on excessve speeders (40 km/h or more over the speed lmt). Smply exceedng the speed lmt, whle sttstclly sgnfcnt, ws not prmry predctor of ncresed rsk of serous njury. Mny others hve modeled crsh counts (e.g., Mou 1994, Km et l. 1995; Gebers 1998, nd Ivn et l. 1999) usng Posson nd negtve bnoml models. The negtve bnoml model s typclly more pproprte thn the Posson, snce t llows for unobserved heterogenety whle permttng over-dsperson n the dt (rther thn requrng tht vrnce equl men). Thus, t ws used here to exmne the frequency of drnkng nd drvng. MODELS Three dfferent model specfctons were used. Bref generl descrptons of two models ordered probt nd negtve bnoml models re provded here. Stndrd ordnry lest squres (OLS) regresson lso ws performed, but s not descrbed here. Ordered Probt Model In n ordered probt model, the focus s on the probblty of one of mny possble, ordered responses: (1) y = β x + ε, where ε ~ N(0,1) (2) y f = m τ m 1 y = β x τ m, for m = 0 to J 1 where y s the ltent nd contnuous underlyng mesure of response, y s the observed nd coded dscrete mesure of response, x denotes set of explntory vrbles, β denotes set of coeffcent prmeters (to be estmted), τ m denote threshold prmeters (to be estmted, where τ 1 = nd τ J 1 = ), m s the observed coded dscrete response nd J s the number of response levels or ctegores. Fgure 1 presents the correspondence between ltent contnuous response levels, y, nd the observed dscrete response levels, y. Fgure 1: Reltonshp Between Ltent nd Observed Responses y J-2 J-1 τ -1 = τ 0 τ 1 τ 2 τ J 3 τ J 2 τ J-1 = y For exmple, y = 0 f n ndvdul thnks current speed lmts re too low, 1 f these re bout rght, nd 2 f they re felt to be too hgh. Here, J = 3. The ssocted probbltes re s follows: - 4 -

5 Pr( y = 0) = Φ( τ 0 β x ) (3) p = = = Φ Φ Pr( y m) ( τ m β x ) ( τ m 1 β x ) f y = m (1, K, J 2) Pr( y = 1) = 1 Φ( J τ J 2 β x ) where Φ( ) denotes the stndrd norml cumultve dstrbuton functon. The product of these probbltes s the lkelhood functon, whch ssumes ndependent responses cross ndvduls n the smple: n p = 1 L( β, τ y, X) =. Negtve Bnoml Model Count dt re non-negtve, nteger vlues. These chrcterstcs often render lner regresson models npproprte, whle mkng Posson models populr lterntve (wth n exponentl functon of explntory vrbles for the rte term, λ). Posson models do not llow for unobserved heterogenety nd presume equdsperson (such tht men equls vrnce). A negtve bnoml model dds rndom dsturbnce ( ε ) to the rte functon of the Posson model s follows: (4) μ = exp( β x + ε ) = λδ where μ = expected vlue of observtonl unt s count ( y ), λ = exp( β x ), nd δ = exp( ε ). Assumpton of gmm dstrbuton for δ results n negtve bnoml probblty mss functon (PMF), s follows: y y v exp( μ ) μ exp( λδ )( λδ ) v v 1 (5) Pr( y ) = = nd g( δ ) = δ exp( δ v ) for v > 0 y! y! Γ( v ) y where y = 0, 1, 2,, y! = k = k (e.g., 3!=1 2 3), g( δ ) 1 = gmm probblty densty 1 functon (PDF) wth sngle prmeter = 2 v α ( α > 0), nd Vr( y x ) = μ + αμ ; so tht α s the dstrbuton s over-dsperson prmeter. In cses where α =0, the negtve bnoml reduces to Posson dstrbuton. As n exmple pplcton of ths model bsed on the MVOSS dt, one cn nlyze the number of dys tht respondent reports hvng mbbed lcohol nd drven n the pst 30 dys. Reders my consult Cmeron nd Trved (1986) for detls on the negtve bnoml model. DATA The 2000 Motor Vehcle Occupnt Sfety Survey (MVOSS) dt were collected between November 2000 nd Jnury Dt were obtned from 6,072 respondents, ge 16 or older, resdng n ll 50 U.S. sttes nd Wshngton, D.C. The survey emphszed trffc sfety ssues, ncludng drvng frequency, set belt use, nd drvng tttudes. Bsc vrble detls re shown n Tble 1. Due to non-response on certn questons, the smple szes n the fnl nlyses vry from 4,057 to 4,137, dependng on whch explntory vrbles were used. Household ncome, orgnlly ctegorcl vrble, ws mde contnuous by usng pproxmte medn vlues n ech ctegory

6 Tble 1: Descrpton of Vrbles of U.S MVOSS Dt Vrbles Descrptons Men Age Age of respondent (yers) Income Household ncome (n yer 2000 US $) 54,851 Mle 1 = mle Hspnc 1 = Hspnc or Ltno Mrred 1 = mrred (0 = otherwse; e.g. dvorced, wdowed, etc.) College Educted 1 = possess college educton or hgher Employed 1 = employed or self-employed Centrl Cty Resdent 1 = lvng n centrl cty Drve Pssenger Cr 1 = usully drvng pssenger cr Drve Vn 1 = usully drvng vn or mnvn Drve Pckup 1 = usully drvng pckup truck Drve SUV 1 = usully drvng SUV Drve Hevy Truck 1 = usully drvng hevy truck Drve n Other Vehcle 1 = usully drvng n other vehcle (.e., not bove vehcle types) Drvng Frequency 0 = drvng few dys month or yer (2.38%); 1 = drvng few dys every week (9.37%); 2 = drvng every dy or lmost every dy (88.25%) Setbelt Use Frequency Setbelt Lw Support Level Speed Lmt Support Level Percepton of Other Drvers Pressure to Speed 0 = use set belt rrely or never (1.80%); 1 = use set belt some of the tme (4.10%); 2 = use set belt most of the tme (9.52%); 3 = use set belt ll of the tme (82.53%) 0 = not fvor set belt lw t ll (12.20%); 1 = fvor set belt lw some (20.23%); 2 = fvor set belt lw lot (67.57%) 0 = speed lmt s too low (14.45%); 1 = speed lmt s bout rght (77.37%); 2 = speed lmt s too hgh (8.18%) 0 = other drvers re poor (21.59%); 1 = other drvers re fr (43.14%); 2 = other drvers re good (30.09%); 3 = other drvers re excellent or very good (5.19%) 0 = never feels pressure to exceed the speed lmt (18.35%); 1 = rrely feel pressure to exceed the speed lmt (30.32%); 2 = often feel pressure often to exceed the speed lmt (34.91%); 3 = very often feel pressure to exceed the speed lmt (16.41%) Pss Others More 1 = I pss others more often thn they pss me Pssed by Others 1 = others pss me more often thn I pss them Other Pssng Response 1 = I drve the sme s most others Pss Eqully 1 = I pss others s often s others pss me Usul Hghwy Speed Usul drvng speed on hghwys (klometers per hour) Polce Stop n Pst Yer 1 = hve been stopped by polce n the lst 12 months whle drvng Tcket n Pst Yer 1 = hve receved tcket by polce n the lst 12 months whle drvng Drnkng Dys Number of drnkng dys n the pst 30 dys Number of Drnks Averge number of drnks per drnkng dy Drnkng & Drvng Dys Number of drnkng-nd-drvng dys n the pst 30 dys Pst Injury 1 = hve been njured n crsh (s drver, occupnt or non-occupnt) Pst Injury s Drver 1 = hve been njured s drver t some pont n the pst Number of Pst Injures Number of tmes hvng been njured n crsh An njury n the MVOSS dt set s defned s one tht requres medcl ttenton

7 Among the vrbles n the MVOSS dt reltng to trffc sfety, those tht mert specl exmnton re set belt usge, response to speed lmts nd set belt lws, preferred drvng speed, nd drnkng nd drvng. The reltonshp between these vrbles nd set of explntory vrbles ncludng trffc crsh hstory, ndvdul chrcterstcs (e.g., ge nd educton level), recent drnkng hbts (e.g., drnkng frequency nd mount), vehcle type, nd employment ws nvestgted. Seprte nlyses were crred out for ech vrble of nterest, usng dscrete choce model (ordered probt), count dt model (negtve bnoml), nd lner regresson model (for speed choce). It should be mentoned tht severl of these MVOSS vrbles nvolve stted preferences (e.g., support for set belt lws) nd senstve stted behvors (e.g., drnkng dys per month nd speed choce). Respondents my not know ther true response or my choose to color ther response to hde the truth. (Reders my be nterested n Corbett s (2001) nd Brdburn nd Sudmn s (1979) dscussons of these ssues, s well s survey desgn.) Such tendences certnly cn bs results (e.g., bsng estmtes of drnkng nd drvng to the low sde nd support for set belt lws to the hgh sde). For exmple, 82.5% of MVOSS respondents reported usng the shoulder belt ll of the tme, nd 9.5% reported usng ther belt most of the tme. In contrst, the Ntonl Occupnt Protecton Use Survey (NOPUS) dt, collected t 2,063 stes n October nd November of 2000, suggest tht only 55% to 74% of dults (cross dfferent vehcle types) wer shoulder belts (NHTSA 2001). Therefore, there s probbly some over-reportng of set belt use n the MVOSS dt. RESULTS AND DISCUSSIONS Model outputs re provded n Tbles 2 through 6. In ech of these, n ntl model ncludng ll possble explntory vrbles of nterest, s shown n Tble 1 ws estmted, nd fnl model ws then developed, to recognze only those vrbles tht remned sttstclly sgnfcnt t the 10% level (p-vlue 0.10), fter seres of step-wse deletons. Only the fnl model outputs re presented here. Ordered probt nd negtve bnoml models were estmted usng mxmum lkelhood (ML) estmton technques, nd lner model ws estmted usng OLS. Along wth estmtes of coeffcents (nd thresholds for n ordered probt model), lkelhood rto ndex (LRI) or McFdden s pseudo R 2, s provded for MLE models, whch represents the rto of lkelhood vlues of models estmted wth nd wthout explntory vrbles. 1 All model estmtons were performed usng LIMDEP 7.0. Perceptons of Current Speed Lmts A totl of 76.6% of the survey respondents reported beng stsfed wth current speed lmts, 16.2 % felt they were too low, nd 7.2% thought they were too hgh. As pont of comprson, Hglund nd Aberg (2000) reported fgures of 61.1%, 37.0%, nd 1.9%, respectvely, for Swedes on hghwys wth 90 km/h (56 mph) speed lmt. Of course, the MVOSS survey of Amercn drvers sked more generl queston, nd U.S. freewy speeds re often bove 100 km/h. Thus, t s not unusul to expect tht Amercns my be less lkely to wnt hgher lmts thn ther Swedsh counterprts. The perceptons on the current speed lmts were nlyzed usng the ordered probt model, nd Tble 2 shows these results. Postve sgns on estmted coeffcents suggest tht respondents re more lkely to consder current speed lmts to be too hgh, nd thus fvor lowered lmts

8 Conversely, negtve sgns on coeffcents ndcte tht respondents re less lkely to consder current speed lmts to be too hgh, nd thus fvor hgher lmts. Accordng to Tble 2, mles, employed, mrred, nd hgher-ncome drvers tend to fvor hgher speed lmts, n contrst to drvers of lght-duty-trucks ncludng pckup trucks, vns nd sportutlty vehcles (SUVs) tht fvor lower speed lmts. People who fvor set belt lws, those who re pressured to speed up by other drvers, nd those who hve experenced njury crshes s drvers tend to support lowerng speed lmts. In contrst, less frequent setbelt users, those who frequently pss others on the hghwy (the reference group for ll pssng responses), those who hve recently been stopped by polce offcer, those who drnk more often, nd/or those who ndcted hgher drvng speeds tend to fvor speed lmt ncreses Tble 2: Ordered Probt Model Results of Speed Lmt Supports Vrbles Coeff. Std. Err. P-vlue Constnt Age E Age Squred E Income -1.64E E Mle Mrred Employed Drve Vn Drve Pckup Drve SUV Drve n Other Vehcle Setbelt Use Frequency Setbelt Lw Support Level Pressure to Speed Pssed by Others Other Pssng Response Pss Eqully Usul Hghwy Speed E Polce Stop n Pst Yer Drnkng Dys Pst Injury Pst Injury s Drver τ 0 τ Number of Observtons 4,136 LRI b Adjusted LRI c Note: Dependent vrble s respondent s Speed Lmt Support Level: Y = 0 (current speed lmts re too low), 1 (lmts re bout rght) nd 2 (lmts re too hgh). τ 0 nd τ 1 re thresholds for n ordered probt model. b Lkelhood Rto Index = 1 LogL(Model) LogL(Constnt Only) - 8 -

9 c Adjusted Lkelhood Rto Index = 1 ( LogL(Model) k) LogL(Constnt Only) prmeters., where k s the number of estmted Fgure 2 llustrtes some of the model s predctons. In order to crete ths fgure, the vlues for ndctors for mrred nd employed were set to one (s reference ndvdul) nd verge vlues of ll other explntory vrbles were used n the probblty functon of the ordered probt model (Equton 3 wth J=3), Pr( y 2) 1 ( ˆ ˆ = = Φ τ 1 β x ) where ˆ τ 1 nd βˆ re provded n Tble 2. Then, the vlues for ge, ndctors for gender nd vehcle types were vred. Fgures 3 through 6 were creted n smlr wy. Older persons re predcted to respond tht current speed lmts re too hgh; however, ths trend tops out t bout ge 80. The gender effect s much greter thn the vehcle-type effect: femles re more lkely to consder current speed lmts to be too hgh, regrdless of the vehcle types tht they use. As lluded to bove, vn drvers re estmted to be most lkely to fvor lowered lmts, SUV drvers follow, pckup truck drvers re next, nd pssenger cr drvers (the reference group for the vehcle type ndctor vrbles) re lest lkely to support lowered speed lmts. These re fndngs for the reference person who s mrred nd employed nd hs verge condtons for ll other fctors (e.g., hs drunk lcohol on 3.6 occsons [dys] durng the pst month). Probblty (Current Speed Lmt s Too Hgh) Fgure 2: Probblty of Consderng the Current Speed Lmt to be Too Hgh femle drvng mnvns femle drvng SUVs femle drvng pckups femle drvng pssenger crs nd mle drvng SUVs mle drvng mnvns mle drvng pckups mle drvng pssenger crs Age (yers) Note: (1) Reference ndvdul s mrred nd employed, nd exhbts verge vlues of ll other explntory vrbles ncluded n Tble 2. (2) The curves for femles drvng pssenger crs nd mles drvng SUVs re too close to one nother to dstngush vsully; thus, they re presented s sngle curve. Speed Choces on Hghwys Tble 3 nd Fgure 3 show the OLS model results for predctons of drver speed choces on hghwys. Respondents reports of ther usul hghwy speeds tend to ncrese wth ther household ncome, lcohol consumpton (mount nd frequency), nd recent trffc voltons. Mle, younger, college-educted persons, frequent drvers, those lvng n centrl ctes, nd those who hve been recently stopped or cted by polce lso tend to prefer hgher speeds. Bsed on ths fndng, publc nt-speedng cmpgns should trget such drvers. Those who re older, - 9 -

10 employed, Hspnc, drve SUVs, nd regrd others s good drvers prefer lower speeds. The potentl reson why drvers wth hgher household ncome tend to drve fster nd support hgher speed lmts mght be tht they vlue ther trvel tme more hghly nd drve more expensve vehcles wth better ccelerton nd sfety fetures. Tble 3: Lner Regresson Model Results of Speed Choce on Hghwys Vrbles Coeff. Std. Err. P-vlue Constnt Age E Income 5.20E E Income Squred -2.09E E Mle Hspnc College Educted Employed Centrl Cty Resdent Drve SUV Drvng Frequency Percepton of Other Drvers Pssed by Others Other Pssng Response Pss Eqully Polce Stop n Pst Yer Tcket n Pst Yer Drnkng Dys Number of Drnks Number of Observtons 4,136 R-squred Adj. R-squred

11 Fgure 3: Speed Choce on Hghwys Speed on Hghwys (mph) mle/$100k/centrl cty mle/$50k/centrl cty mle/$100/non-centrl cty mle/$50k/non-centrl cty femle/$100k/centrl cty femle/$50k/centrl cty femle/$100/non-centrl cty femle/$50k/non-centrl cty Age (yers) Notes: (1) Reference ndvdul s n employed, non-hspnc, mrred person wth college degree, nd exhbts verge vlues of ll other explntory vrbles ncluded n Tble 3. (2) $50k denotes n nnul household ncome of $50,000. Set Belt Use Werng set belt plys crucl role n dmnshng the severty of crshes for vehcle occupnts (Km et l. 1995, NHTSA 2004, nd Shults et l. 2004). Tble 4 dsplys the results of n ordered probt model for frequency of set belt use whle drvng. The postve sgns of the coeffcents re nterpreted n the sme wy s n Tble 2: s the ssocted explntory vrble s vlue ncreses, respondents re more lkely to wer ther set belts more often. A negtve sgn mens tht s the prtculr ndependent vrble ncreses, respondents re less lkely to wer ther set belts more often. Fgure 4 llustrtes how model estmtes of ndvduls probbltes of respondng tht they lwys wer ther set belt vry wth ge, gender, nd vehcle type (pckup truck, hevy-duty truck, nd ny other vehcle). Usng n ordered probt model for drver-reported frequency of set belt use, greter belt use s expected to occur mong mrred, college-educted women, nd hvng hgher household ncomes (s well s mong those who fvor set belt lw). However, s shown n Fgure 4, the effect of certn vehcle types ws estmted to domnte tht of gender. Thus, mle pssenger cr drvers re estmted to be more lkely to lwys use ther setbelts thn pckup-drvng femles (when they re mrred nd college-educted). In ddton to mles nd pckup or hevy-truck drvers, more frequent drvers, those recently stopped by polce, those more lkely to pss other vehcles (the reference group for pssng responses), more frequent mbbers of lcohol, nd those drnkng nd drvng more frequently re estmted to be less lkely to wer set belt (Tble 4). However, those hvng receved trffc volton n the pst 12 months long wth those who fvor hgher speed lmts, nd hve been njured s drver re more lkely to use set belts often. In generl, Shnr et l. s (2001) setbelt-use fndngs reltng to educton levels re consstent wth ours; however, they estmte tht the postve ncome effect pples only to femles usng Preventon Mgzne s survey dt from 1983 through

12 Tble 4: Ordered Probt Model Results for Set Belt Use Vrbles Coeff. Std. Err. P-vlue Constnt Income 2.84E E Mle Mrred College Educted Drve Pckup Drve Hevy Truck Drvng Frequency Setbelt Lw Support Level Speed Lmt Support Level Pssed by Others Other Pssng Response Pss Eqully Polce Stop n Pst Yer Tcket n Pst Yer Number of Drnks Drnkng & Drvng Dys Pst Injury s Drver τ 0 τ 1 τ Number of Observtons 4,136 LRI b Adjusted LRI c Note: Dependent vrble s Setbelt Use Frequency: Y = 0 (use set belt rrely or never), 1 (use some of the tme), 2 (use most of tme), nd 3 (use ll of the tme). τ 0, τ 1,nd τ 2 re thresholds for n ordered probt model. b Lkelhood Rto Index = 1 LogL(Model) LogL(Constnt Only) c Adjusted Lkelhood Rto Index = 1 ( LogL(Model) k) LogL(Constnt Only) prmeters., where k s the number of estmted

13 Fgure 4: Probblty of Werng Set Belt All the Tme Probblty (Alwys Use Set Belt) femle drvng non-pckups femle drvng pckups mle drvng non-pckups mle drvng pckups mle drvng hevy trucks Household Income ($1000) Notes: (1) Reference ndvdul s mrred nd college-educted, nd exhbts verge vlues of ll other explntory vrbles ncluded n the model of Tble 4. (2) Non-pckups men ny vehcles other thn pckups nd trucks. Support for Set Belt Lws Respondents support for set belt lws lso ws estmted v n ordered probt specfcton, nd the results re presented n Tble 5 nd Fgure 5. The postve sgns of the coeffcents n Tble 5 re nterpreted n the sme wy s n Tbles 2 nd 3: s the vlues of the vrbles ncrese, respondents re more lkely to support set belt lws. Accordng to Tble 5, mles, pckup, SUV nd hevy truck drvers, those wth less ncome nd/or educton, those who drve nd/or use set belt less frequently, nd those wth recent trffc voltons re less lkely to support set belt lw. Those who prefer hgher speed lmts, choose hgher speeds nd/or drnk more re predcted to be less lkely to fvor set belt lw, whle mrred persons, Hspnc, those who vew others s good drvers, those who resde n centrl ctes, nd those who feel pressured to speed up by other drvers showed more support for such lw

14 Tble 5: Ordered Probt Model Results for Support of Set Belt Lws Vrbles Coeff. Std. Err. P-vlue Constnt Age E Age Squred E Income Squred 1.04E E Mle Hspnc Mrred College Educted Centrl Cty Resdent Drve Pckup Drve SUV Drve Hevy Truck Drvng Frequency Setbelt Use Frequency Speed Lmt Support Level Percepton of Other Drvers Pressure to Speed Pssed by Others Pss Eqully Usul Hghwy Speed Tcket n Pst Yer Number of Drnks Pst Injury τ 0 τ Number of Observtons 4,136 LRI b Adjusted LRI c Note: Dependent vrble s Setbelt Lw Support Level: Y = 0 (Not n fvor set of belt lw t ll), 1 (fvor set belt lw somewht) nd 2 (fvor set belt lw lot). τ 0 nd τ 1 re thresholds for n ordered probt model. b Lkelhood Rto Index = 1 LogL(Model) LogL(Constnt Only) c Adjusted Lkelhood Rto Index = 1 ( LogL(Model) k) LogL(Constnt Only) prmeters., where k s the number of estmted

15 As llustrted n Fgure 5, the gender effect exceeds the vehcle-type effect, nd femles re predcted to be more lkely to fvor set belt lws, rrespectve of vehcle type. In ddton, support for set belt lws vres n convex wy wth ge: declnng wth ge up to ge 50, nd then ncresng.. Fgure 5: Probblty of Strongly Fvorng Set Belt Lws 0.75 Probblty (Fvor Set Belt Lw) femle drvng pssenger crs femle drvng SUVs femle drvng pckups mle drvng pssenger crs mle drvng SUVs mle drvng pckups Age (yers) Notes: Reference ndvdul s non-hspnc, mrred, nd college-educted, nd exhbts verge vlues of ll other explntory vrbles ncluded n Tble 5 s model Drnkng nd Drvng Drnkng nd drvng (durng the pst 30 dys) ws exmned usng negtve bnoml regresson model, nd the results re dsplyed n Tble 6 nd Fgure 6. Postve sgns of coeffcents n Tble 6 suggest tht, s the vlues of the ssocted explntory vrbles ncrese, respondents re more lkely to hve mbbed lcohol nd then drven n the pst 30 dys. Negtve sgns hve the opposte nterpretton. In estmtng the number of dys one hd been recently drnkng nd drvng, t ws found tht the number of drnks per event hd lmost twce the effect of the number of drnkng dys n the pst month. Mle, employed persons, collegeeducted persons, nd those recently stopped by polce reported more drnkng nd drvng. More frequent drvng ws ssocted wth more drnkng nd drvng, s one my expect. Mrred people nd those who more often wer set belts were less lkely to drnk nd drve. Those who drve pckups or hevy trucks were less lkely to drnk nd drve thn those drvng other types of vehcles

16 Tble 6: Negtve Bnoml Model Results of the Frequency of Drnkng nd Drvng Vrbles Coeff. Std. Err. P-vlue Constnt Age Age Squred E Mle Mrred College Educted Employed Drve Pckup Drve Hevy Truck Drvng Frequency Setbelt Use Frequency Pssed by Others Polce Stop n Pst Yer Drnkng Dys Number of Drnks α Number of Observtons 4,137 LRI b Adjusted LRI c Note: Dependent vrble s Drnkng & Drvng Dys: Y = the number of dys of drnkng nd drvng n the pst 30 dys. 2 α s the overdsperson prmeter: Vr ( Y X) = μ + αμ. b Lkelhood Rto Index = 1 LogL(Model) LogL(Constnt Only) c Adjusted Lkelhood Rto Index = 1 ( LogL(Model) k) LogL(Constnt Only) prmeters., where k s the number of estmted

17 Fgure 6: Frequency of Drnkng nd Drvng n the Pst 30 Dys Number of Drnkng nd Drvng Dys Mle drvng non-pckups Mle drvng pckups Femle drvng non-pckups Femle drvng pckups Age (yers) Notes: Reference ndvdul s mrred, employed, nd college-educted, nd exhbts verge vlues of ll other explntory vrbles ncluded n the model of Tble 6. Fgure 6 presents the effects of ge, gender nd vehcle types on the number of drnkng nd drvng dys n the pst 30 dys. Mles nd those drvng non-pckup trucks ncludng pssenger crs, SUVs, nd vns re more lkely to drnk nd drve wth hgher frequency thn femles nd those drvng pckup trucks. However, the gender effect s much greter thn the vehcle-type effect. As drver ges, the number of drnkng nd drvng dys tends to ncrese, untl round ge 65. CONCLUSIONS Ths work reles lrgely on dscrete-response (ordered probt) models nd count dt (negtve bnoml) models for nlyss of the MVOSS. A stndrd lner regresson model lso ws used to estmte usul hghwy speed choce. Dependent vrbles ncluded set belt use, frequency of drnkng nd drvng, tttudes towrd speed lmts nd set belt lw, nd speed choces on hghwys n the U.S. There re multtude of results vlble from ths work. For exmple, mles re less lkely to use set belt nd fvor set belt lws, but more lkely to fvor rsed speed lmts, to drve fster on hghwys, nd to drve fter drnkng. In generl, mles re found to exhbt rsker behvors nd less fvorble tttudes towrds sfety polces thn femles. Younger persons tend to prefer hgher speed lmts nd choose hgher drvng speeds on hghwys. Persons round ge 50 re estmted to be mong those lest lkely to support set belt lws; nd those ner 85 yers of ge re most lkely to consder speed lmts to be too hgh, nd therefore re most lkely to support reducton of speed lmts. Interestngly, there s reltvely lttle spred n reported speed preferences: the verge respondent t the ge of 20 prefers to trvel 70 mph on hghwys, but 67 mph t the ge of

18 Wth regrd to vehcle types, those who drve pckups nd hevy trucks re less lkely to use set belt, less lkely to fvor set belt lws, nd less lkely to drve fter drnkng. Drvers of vns, SUVs, nd pckups re more lkely to support lowerng the speed lmts thn pssenger cr drvers. Hgher household ncomes nd eductonl ttnment ncrese the predcted probbltes of set belt use nd one s support of set belt lws; however, hgher-ncome drvers tend to support hgher speed lmts, nd those wth college educton pper to drnk nd drve more often. Hgh ncome nd college-educted drvers my vlue ther lves nd tme more (nd drve more expensve vehcles wth more sfety fetures nd better ccelerton performnce), but the college educted lso pper to vlue ther lcohol consumpton more (thus drvng fter drnkng). Ths summry reltes just few of the results quntfed n the model outputs presented here. It s hoped tht these wll be useful to polcymkers nd trffc engneers n the domn of trffc sfety for the trvelng publc. Antcptng publc recton to drunk-drvng cmpgns, speed lmts, nd set belt regultons s mportnt nd useful. For exmple, efforts to reduce drunk drvng my fnd t useful to focus on drvers who re young, mle, employed, nd collegeeducted (or bout to become college educted). To boost set belt usge rtes, t my be more effectve to trget mles drvng pckups nd hevy-duty trucks. Moreover, polcymkers nd publc nformton offcers n sttes seekng to enct more strngent set belt lws (e.g., shftng from secondry to prmry enforcement lws) 2 my do well to seek support mong those who re mddle-ged, mle, wthout college degree, of lower ncome, nd drvng pckups, SUVs, or trucks. Trgeted messges on sfe drvng prctces cn mke dfference svng lves, tme, money nd other resources. An understndng of humn behvor on the rodwy s vluble step n ths drecton. ACKNOWLEDGEMENTS We re grteful to Aln Block n the Offce of Reserch nd Trffc Records of the Ntonl Hghwy Trffc Sfety Admnstrton for provdng us wth the MVOSS dt set, nd to severl nonymous referees for ther vluble suggestons. We thnk Annette Perrone for her excellent edtorl ssstnce nd the Ntonl Coopertve Hghwy Reserch Progrm (NCHRP) for sponsorng the study (under contrct number 17-23). Endnotes 1 Low LRI (e.g., 0.1) s very common n mny dscrete-response models of humn behvors nd trffc sfety. For exmple, see Grhm et l. (2005), Ktl et l. (2004), nd Nolnd nd Oh (2004). In the feld of trffc sfety reserch, nterpretton of sttstclly sgnfcnt reltonshps s felt to be fr more mportnt thn goodness of ft. 2 A prmry enforcement lw permts the polce to stop the vehcle when t observes the set belt volton, whle secondry lw permts the polce to ssue ctton only fter t stops the vehcle for nother volton nd observes the set belt volton

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