Effect of Traffic and Geometric Characteristics of Rural Two Lane Roads on Traffic Safety: a case study of Ilesha-Akure-Owo road, South-West, Nigeria

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1 Effct of Traffc and Gomtrc Charactrstcs of Rural Two Lan Roads on Traffc Safty: a cas study of Ilsha-Akur-Owo road, South-Wst, Ngra * 1 Monsuru O. Popoola, 2 Oladapo S. Abola and 2 Smon O. Odunfa 1 Dpartmnt of Cvl Engnrng, Moshood Abola Polytchnc, Abokuta, Ngra 2 Dpartmnt of Cvl Engnrng, Fdral Unvrsty of Agrcultur, Abokuta, Ngra popoola.monsuru@mapoly.du.ng abolaos@funaab.du.ng smon_olutayo@yahoo.com Abstract Road safty ngnrng nvolvs dntfyng nfluncng factors causng traffc crashs through accdnt data, carryng out dtald accdnt studs at dffrnt locatons and mplmntng rlvant rmdal masurs. Ths study was carrd out to stablsh rlatonshp btwn traffc accdnt charactrstcs (frquncy and svrty) and traffc and road dsgn charactrstcs on a two-lan hghway. Statstcal modls appld n traffc accdnt modlng ar Posson rgrsson, Ngatv Bnomal rgrsson (NB), and Zro- Inflatd Ngatv Bnomal rgrsson (ZINB).; Traffc flow and road gomtry rlatd varabls wr th ndpndnt varabls of th modls. Usng Ilsha-Akur-Owo hghway, South-Wst, Ngra accdnt prdcton modls wr dvlopd on th bass of accdnt data obtand from Fdral Road Safty Commsson (FRSC) durng a 4-yar montorng prod xtndng btwn 2012 and Curv radus (CR), lan wdth (LW), shouldr factor (SF), accss road (CHAR), avrag annual daly traffc (AADT), parntag havy good vhcl (HGV) and traffc sgn postd (TSP) wr th dntfd ffctv factors on crash occurrnc probablty. Fnally, a comparson of th thr modls dvlopd provd th ffcncy of ZINB modls aganst tradtonal Posson and NB modls. Kywords Traffc accdnts, Sngl carragway, accdnt prdcton modl, road gomtrc charactrstcs 1 INTRODUCTION n Ngra today, roads play vry crucal rols on th Iconomc and socal actvts, bcaus t s th most popular mod of transportaton that xsts wthn th country. It s th only modal class of transportaton that conncts all th ara wthn th country, and t s th chapst. Ths also accounts for why approxmatly 80 prcnt of frght and passngr traffc ar movd through th hghway/roadway ntwork systms n Ngra (Wang, 2002). Traffc ngnrs contnu to mphasz th dntfcaton of causal factors for crashs on ndvdual sctons and on dffrnt functonal classs of hghways as an ara of mphass. Gnrally road safty ssus do not rcv xplct consdraton n th dsgn stag, road safty ngnrng hav bn ntroducd as a man for assurng that crash occurrnc masurs amd to lmnat or rduc th safty problms ar fully consdrd. In ths papr, w dal wth rsarch that hav bn conductd for dntfyng th ffctv factors on frquncy and svrty of accdnts on sngl carragway and us crash data of Ilsha-Akur-Owo hghways, South-Wst, Ngra as a cas study. Data on road accdnts was obtand from th Fdral Road Safty Commsson, whl rsults on traffc and gomtrc charactrstcs wr obtand on st n conjuncton wth Pavmnt Evaluaton Unt, Fdral Mnstry of Works, Kaduna, usng GIS was usd for accurat rcord of tm and plac of accdnts and assstanc of polc rports. * Corrspondng Author 2 LITERATURE REVIEW 2.1 ROAD DESIGN AND TRAFFIC RELATED FACTORS Th traffc flow and road gomtry rlatd factors was usd n ths rsarch as ndpndnt varabls of modls. Such varabls ar obsrvd n studs of many othr rsarchrs partcularly n th ara of modlng crashs occurrd on frways and urban or rural hghways (Dln at al, 2006). L & Mannrng, (2002) appld pavmnt condton or qualty, Chang, (2005) and Haur, (2001) adoptd drvr bhavor and wathr condton rlatd varabls as ndpndnt varabls n thr modls. In ths study, traffc volum (AADT), prcntag havy good vhcl (HGV), traffc sgn postd (TSP), and spd (SPD) ar appld as traffc flow rlatd varabls. Malyshkna & Mannrng, (2009) appld th avrag daly traffc (ADT) or annual avrag daly traffc (AADT) n modlng as traffc flow rlatd varabl and also consdr numbr of lans n ach sd, Chang & Wang (2006) usd and appld ADT or AADT pr lan as road gomtry rlatd varabls. Dln t al (2006) and Ma t al, (2008) n modlng varabls usd avrag daly truck traffc (ADTT) or prcntag of trucks to account for th rol of havy vhcls n frquncy and svrty of hghway accdnts or varabls of avrag daly passngr car traffc and avrag daly truck traffc or prcntag of trucks sparatly for dtachmnt of th rol of passngr cars and havy vhcls n accdnt occurrnc. Anastasopoulos & Mannrng, (2009) howvr usd th avrag dgr of curvatur of horzontal curvs, and gradnt or lngth of vrtcal curvs n traffc crash modllng. In ach sgmnt of hghway hav bn appld n modlng n Haur (2001). A lmtd accss road s whr traffc from local or dstrbutor road s lmtd or controlld, and hav lss ntrfrnc on h g hwa y traffc flow. L & Mannrng, (2002), Chang, (2005), and Mlton t al, (2008) appld varabls of numbr of accss roads, numbr of ntrchangs, at- FUOYEJET

2 grad ntrsctons or ramps n spcfd dstancs of frways or hghways. 2.2 HUMAN RELATED FACTORS Human rlatd factors n road traffc accdnts ar all factors rlatd to drvrs and othr road usrs, t ncluds drvr bhavour, vsual and audtory acuty, dcson makng ablty and racton tm. Drug and alcohol us whl drvng s an obvous prdctor of road traffc accdnt, road traffc njury and dath (Adogu and Asuzu, 2009). Human factors hav th largst nflunc on th occurrnc of accdnt vnts and nclud, th ag of th road usrs, drvr sklls, attnton, fatgu, xprnc, us of ntoxcatv substancs and us of cllulartlphons (Ptrdou and Moustak, 2000; Ogdn, 2007). Th contrbutons of human factors ar too manfold and too complx to b controlld drctly by th road nfrastructur dcson makrs (Lum and Ragan, 2005). In ordr to control human factors, law- nforcmnt actons would b ndd,.g. stop-and-sarch opratons by road safty (FRSC) offcals. 2.3 POISSON REGRESSION MODEL Posson rgrsson s on of th most sutabl tchnqus for crash prdcton modlng bcaus hghway crashs ar dscrt rar vnts and crash counts ar non-ngatv ntgr varabls. Posson rgrsson modls also provd an asy lnkag btwn crash occurrnc and th concpt of probablty. Ths s bcaus th numbr of crashs n a gvn spac-tm rgon can b consdrd as a random varabl wth probablts that ar Posson dstrbutd. Shankar t al, (2005) ctd th gnral form of crash modls drvd usng a Posson rgrsson modl that -th obsrvaton of dpndnt varabl y s modld s a random Posson varabls wth man λ P( y ) y y! 2.4 NEGATIVE BINOMIAL REGRESSION MODEL A waknss of th Posson approach s th unduly rstrctv assumpton that th man and th varanc of crash dstrbuton ar qual. In most crash data, howvr, t s sn that th varanc s gratr than th man, gvng rs to th ovr-dsprson phnomnon. To addrss ths problm, th Ngatv Bnomal modl, a varant of th Posson, has oftn bn proposd for crash prdcton modlng. Th Ngatv Bnomal modl allows for addtonal varanc rprsntng th ffct of omttd varabls. In a study for Indana, Brown t al. (2000) dvlopd crash prdcton modls for crash rats on road sgmnts basd on gomtrc and accss control charactrstcs. Th -th obsrvaton of dpndnt varabl y has th followng probablty dstrbuton functon: ( y r) P( y ) y! ( r) r Th condtonal man of for th vctor of obsrvd ndpndnt varabls, x s gvn by: E( y x ) x 2.5 ZERO-INFLATED NEGATIVE BINOMIAL (ZINB) REGRESSION MODEL Th othr problm, whch accdnt data oftn ncountr, s prpondranc of xcss zro data. In othr words, numbr of zro data s mor than xpctd n Posson and NB modls. If on mts wth xcss zro data whl data mnng, uss zro-nflatd (ZI) dstrbuton for data analyss. Snc Posson and ngatv bnomal crash frquncy modls do not account for th dstncton that som sctons of roadway ar truly saf (nar zro-accdnt lklhood) whl othrs ar unsaf but happn to hav zro crashs obsrvd durng th prod of obsrvaton, thy could produc basd coffcnt stmats bcaus of th prpondranc of zro-crash obsrvatons. Th zro-nflatd famly of modls wr dvlopd by (Maou & Lum, 2003) and xtndd by (Shankar t al, 2007) and (Ivan & O'Mara, 1997). Zro-nflatd count modls ar approprat whn som obsrvatons hav no chanc of xprncng th vnt Th partcular ZINB rgrsson modl consdrd n ths study has th followng form: p( Y y ) 1 1 and r v r r r y r y! r f whr 0 < θ 1. (Not that for θ > 1, th probablty of obsrvng zros s dflatd rathr than nflatd.) 3 METHODOLOGY In ths rsarch, th accdnts of sngl carragway of Ilsha-Akur-Owo road, South-Wst, Ngra s modld by thr rgrsson modls Posson, Ngatv bnomal (NB), and Zro-nflatd Ngatv bnomal (ZINB). Two groups modl wr dvlopd, on for crash frquncy and th othr for crash svrty. Indpndnt varabls appld n ths modls nclud traffc-rlatd and gomtry-rlatd varabls. Traffc Flow-rlatd varabls nclud traffc volum, prcntag havy good vhcl and spd and gomtry-rlatd varabls nclud lan wdth, horzontal curvs and accss roads. Ilsha Akur Owo Road, South-Wst, Ngra s a vry mportant road lnkng Osun Stat to Ondo Stat n South-Wst, Ngra; th 110-klomtr, two lan sngl carragway road s havly traffckd, wth sgnfcant y r r k x j j j x 1 v 1,2,3,... n y f 0 y 1,2,3,... r FUOYEJET

3 proporton of havy vhcls and horzontal curvs n th mx. Th rout s dvdd nto 55 sctons; th man factors that nfluncd th choc of lngth of road scton wr th road faturs, and road landmarks. STATA 13.0 was usd for statstcal computatons rlatd to modls. Aftr statstcal analyss, t s found out whch paramtrs affct traffc accdnt occurrnc and whch dos not hav much part n traffc accdnt occurrnc. Rgrsson analyss was run on th data collctd from to buld accdnt prdcton modl. Th study compar Posson and NB rgrsson modls wth ZINB rgrsson modls, th study us sgnfcanc of dsprson paramtr and lklhood rato (LR) tst as crtrons. Vuong statstc s usd for modl comparson, t s on stag of comparson; th othr stag s th us of Akak Informaton Crtra (AIC) for goodnss-of-ft valuaton of modls and thr ft comparson, AIC s calculatd as: AIC 2LL 2k whr LL s log-lklhood, k numbr of paramtrs and n numbr of obsrvatons. Th lowr AIC s, th mor modl ft and modl wth th last AIC s th ft tst on (Haur, 2001). Rsults of th scond stag of comparson oftn approv th frst stag concluson 4 DATA Ilsha Akur Owo Road, South-Wst, Ngra s usd as a cas study rout for modlng two-lan hghway. Th 110km road s dvdd nto 55 sctons, th data on road traffc accdnts (RTA) was obtand from th Fdral Road Safty Commsson (FRSC), Akur unt btwn 2012 and In ths rsarch, modl ar appld to numbr of accdnts cass (crash frquncy) and crash svrty (Mnor, Srous and Fata Injury accdnts), hghway sctons ar basd on road faturs, and road landmarks. Th traffc flow rlatd varabls ncludng sctonal traffc volum (AADT), prcntag havy good vhcl (HGV), spd (SPD) and traffc sgn post (TSP) and gomtrc varabls ncludng lan wdth (LW), Curv radus (CR), vrtcal gradnt (VGRAD), dflcton angl ( ), Shouldr factor (SF) and accss ponts (CHAR), ar appld as ndpndnt varabls of modls n ths nvstgaton. Th summary statstcs of th ndpndnt varabls of th modls s prsntd on Tabl 1. 5 STATISTICAL MODELING Th rsarch ntnds to xplor n ths nvstgaton, by modlng th ffctv factors on frquncy and svrty of crashs on 2-lan sngl carragway. Th wll-known modls of Posson, Ngatv bnomal (NB), and (ZINB) ar appld to dvlopng two groups modl for crash frquncy and crash svrty. For valuatng sgnfcanc of ndpndnt varabls, th statstc for ach paramtr s also constructd as th rato of th paramtr stmat ovr ts standard rror. If th p-valu of a paramtr s lss than ndd lvl of sgnfcanc (0.05 or lss), th corrspondng varabl s sgnfcant and wll stay n modl, othrws t s nglctd and lavs th modl (Ivan & O'Mara, 1997). Th calculatons wr procss by STATA, stmatd paramtrs and thr sgnfcanc valuaton for accdnts wth crash frquncy and crash svrty ar prsntd n Tabl 2, 3 and 4. 6 RESULTS AND DISCUSSION Th crash prdcton modlng for th sngl carragway yldd rsults that ndcatd varyng rsult unlk th othr varabls, th gnral rsults of all th modls, ar dscussd on a varabl-by-varabl bass usng ZINB modl, t s th modl that taks nto consdraton sctons wth zro crash rcord. 6.1 TRAFFIC FLOW CHARACTERISTICS Traffc Volum AADT turnd out to b a sgnfcant varabl n all th crash modls: Hghr AADT s assocatd wth hghr crash frquncy. If AADT ncrass by on unt n two-lan road, th crash svrty would ncras by 6.2% ( In ). Th modl rsults also showd that prcntag havy good vhcl HGV, s an nfluntal factor affctng crashs on sngl carragway road. From th modl rsults, t s sn that a prcntag numbr havy good vhcl s assocatd wth hghr crash frquncy. On unt ncras n HGV, wll ncras probablty of crash svrty by 0.92% ( ). It was dtrmnd that th traffc Sgn Postd tsp, srvd to rduc th frquncy of crashs, as sn from th modl rsults, as th prsnc of traffc sgns along th hghway s xpctd to ngndr mor cautous drvng. TSP plays an mportant rol n safty nhancmnt, a wll-nformd drvr or on mad awar of mmnnt dangr can rspond appropratly to avrt a potntal crash stuaton. A unt ncras n tsp factor, rducs th probablty of crash svrty on two-lan road by 4.1% ( ). 6.2 GEOMETRIC CHARACTERISTICS Th modlng rsults showd that th Lan Wdth varabl, LW, was found to b a sgnfcant factor n all th crash modls for sngl carragway. Th ngatv coffcnt for th lan wdth n all th modls ndcats that wdr lan wdths ar assocatd wth lowr crashs. Th rsult s consstnt wth xpctaton bcaus wdr lans srv as buffr zons offrng mor opportunty for rrant vhcls to rcovr or for vhcls to sk tmporary rfug to avod an rrant oncomng vhcl and thrfor rduc th rsk of collson. If LW dcrass by on unt n ZINB modl, th probablty of crash frquncy on sngl carragway would ncras by 2.9% ( ). Th Shouldr factor varabl was shown to hav a sgnfcant nflunc on crashs at sngl carragway road. Incrasng qualty of shouldr s assocatd wth dcrasng crash frquncy, as rflctd n th ngatv coffcnt. If SF dcrass by on unt n ZINB modl, th probablty of crash frquncy on sngl carragway would ncras by 7.2% ( ). Wdr and good pavd shouldrs nhanc safty by provdng addtonal buffr zon whr oprators of stray vhcls can rgan control, rcovr from rror and rsum normal travl. FUOYEJET

4 Tabl 1. Statstcal attrbuts of roadway ndpndnt varabl Varabls Sngl Carragway (Ilsha-Akur-Owo Road) Man SD Var. Mn. Max. Traffc Charactrstcs Avrag Annual Daly Traffc (AADT) Prcntag Havy Good Vhcl (HGV) Spd (SPD) Traffc Sgn Post (TSP) Gomtrc Charactrstcs Curv Radus (CR) Dflcton Angl ( ) Vrtcal Gradnt (VGRAD) Lan Wdth (LW) Accss Road Ponts (CHAR) Shouldr Factor (SF) Varabl Tabl2. Posson Modl Calbraton Rsults Crash Frquncy Crash Svrty Coffcnt P-Valu Coffcnt P-Valu Constant Inaadt * * hgv * spd * tsp * cr * * vgrad lw * char * * sf * * LL LR ch2(12) Prob>ch Psudo R Varabl Tabl 3. Ngatv Bnomal Modl Calbraton Rsults Crash Frquncy Crash Svrty Coffcnt P-Valu Coffcnt P-Valu Constant Inaadt * * hgv * spd * tsp * cr * * vgrad lw * char * * sf * * LL LR ch2(12) Prob>ch Psudo R FUOYEJET

5 Varabl Tabl 4. Zro Inflatd Ngatv Bnomal Modl Calbraton Rsults Crash Frquncy Crash Svrty Coffcnt P-Valu Coffcnt P-Valu Constant Inaadt * * hgv * spd * tsp * * cr * * vgrad lw * char * * sf * * LL LR ch2(12) Prob>ch Horzontal Curv rsult, CR, was a sgnfcant factor n th ntr sngl carragway crash modl. Th modl rsults suggst that sctons wth a hghr dgr of horzontal curvs hav mor crash occurrnc. Curv radus (CR) was a sgnfcant factor n all th crash modls. Ths fndng suggsts that sctons wth lowr avrag curv radus xprnc mor crashs, f CR dcrass by on unt n ZINB modl, th probablty of crash frquncy on sngl carragway would ncras by 1.8% ( ). A wdr horzontal curv radus provds smoothr transton btwn tangnt scton and lads to rducton of cntrfugal forcs on vhcls ngotatng th curv, thrby rducng th rsk of ovrturnng. Tabl 5. Rsults of modl valuaton and comparson for two-lan road Posson Ngatv Bnomal Zro Inflatd Ngatv Bnomal alpha (16.59) 1.488(18.03) AIC lklhood rato statstc Vuong Statstcs Goodnss of ft MODELS EVALUATION AND COMPARISON What s consdrabl aftr modlng, s not only sgnfcant varabls ssu but goodnss-of-ft valuaton and comparson btwn modls.. not only aftr modlng w wll s whch varabls hav consdrabl ffct on lklhood of crash frquncy and crash svrty, but modl ft and comparson ssu s also consdrd. Aftr rcvng rsults, frst Posson and NB modls ar compard n trms of data dsprson, so th sgnfcanc valuaton of dsprson paramtr n NB modl and LR tst s mplmntd. Vuong tst whch s usful n comparng non-nstd modls (Washngton, t al, 2003), t can b compard wth z-valus. Hnc, f V s gratr than Vc = 1.96 (crtcal valu assumng a 95% confdnc lvl) th tst favors th slcton of th ZINB modl., V s approxmatly 2.02, from ths w can s that dspt zros n th data, th suprorty of th ZINB modl ovr th NB modl s statstcally confrmd. Th othr stp of comparson s goodnss-of-ft valuaton of modls and thr ft comparson, so (AIC) ar mployd. Th rsults of comparson n th scond stag oftn approv th frst. From Tabl 5 datast for two-lan road, Zro Inflatd Ngatv bnomal rgrsson modl also hav th bst modl data ft, t has th last AIC valu of 186.2, compard to Posson modl and Ngatv Bnomal modl that gv valus of and rspctvly. Th rsults of goodnss-of-ft valuaton of modls and thr comparson ar also prsntd n th Tabl 5. 7 CONCLUSIONS In ths rsarch, th four yars accdnt data ( ) from FRSC on Ilsha-Owo-Akur road hghways wr usd for th analyss and valuatons; traffc rlatd factors and road gomtrc charactrstcs varabls usd as ndpndnt varabls of modls to scrutnz th mpact and ffct on traffc crashs. Th statstcal mthodology appld n ths rsarch, s mployng thr wll known rgrsson modls n modlng hghway accdnts comprsng Posson, Ngatv bnomal and Zro-nflatd Ngatv bnomal rgrsson modls. In ths study, two groups modl was dvlopd, on for crash frquncy and on for crash svrty (Mnor, srous and fatal njury accdnts) and concludd that, th lklhoods of no njury and mor svr accdnts ncras wth ncras n traffc volum and xstnc of numbr of horzontal curvs and accss roads, also as shouldr factor and spd. Th lklhood of rducton n crash svrty wth lan wdth ncrmnt, but t dos not hav much ffct on lklhood of mor svr accdnts. Th rsults of rsarch ndcat that, th prcntag of havy good vhcl hav ncrasng mpact on lklhood on crash frquncy and crash svrts on two-lan road. Aftr that, th study consdrd goodnss-of-ft valuaton and comparson btwn modls and concludd that, Zro Inflatd Ngatv Bnomal (ZINB) rgrsson modl s th bst and fttst modl for crash frquncy and crash svrty prdcton. FUOYEJET

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