Effects of Paved Shoulders on Accident Rates for Rural Texas Highways

Transcription

1 3 Transportaton Research Record 819 Hghways. FHWA, Rept. FHWA-RD-76-87, R.H. Eckols and S.H. Rchards. Gdelnes for Drveway Desgn and Operaton: Volme 1--Annotated Bblography. Texas Transportaton Ins t- tte, Texas A&M Unv., College Staton, Rept , Gdelnes for Drveway Desgn and Locaton. ITE, Arlngton, VA, Effects of Paved Sholders on Accdt Rates for Rral Texas Hghways DANIELS. TURNER, DANIEL B. FAMBRO, AND RAMEY. ROGNESS The sholder s one of the most extsvely stded roadway elemts; how ever, ts effectvess n redcng accdts has be the sbject of mch de bate. A stdy s descrbed n whch accdt rates and characterstcs were compared for three dffert types of rral Texas hghways: two-lane roadways wthot paved sholders, two-lane roadways wth fll-wdth paved sholders, and for-lane ndvded roadways wthot paved sholders. Approxmately 3 roadways of each type were selected for the stdy. A rgoros screng procedre was developed to sre that each ste was a "typcal" Texas roadway. A detaled three-year accdt hstory was obtaned for each ste. More than 125 km of hghway and 16 accdts were nclded n the stdy data base. For each roadway type, accdt rates ncreased as traffc volme ncreased. Two-lane hghways wthot paved sholders had the hghest accdt rates and were the most sstve to changes n traffc volme. Two-!ane hghways wth paved sholders had the lowest accdt rates ntl the traffc volme reached 75 vehcles/day. At that pont, for-lane ndvded hghways wthot paved sholders were safer. Based on the stdy fndngs, t was conclded that fll-wdth paved sholders are effectve n redcng the accdt rate on rral hghways. It also appears that the presce of fll-wdth paved sholders may redce the nmber of rral ntersecton accdts. Today's hghway gneers are faced wth the dal dlemma of nflatonary constrcton costs and redced reves, whch necesstates the carefl selecton of new projects based on the optmm se of exstng fnds. The Texas State Departmt of Hghways and Pblc Transportaton (TSDHPT) has tred several nnovatve technqes n order to maxme the se of fscal resorces. One sch technqe has be to provde addtonal capacty at mnmm expse by convertng two-lane roadways wth fll-wdth paved sholders to for-lane roadways wthot sholders. Ths treatmt, whch has become known as the "poor-boy" hghway, tals resrfacng and restrpng or smply restrpng the exstng pavemt. Expses for earthwork, dranage, ntersectons, and strctres are mnmed. A poor-boy hghway s typcally ndvded, has no sholder, and has a paved travel srface from 44 to 48 ft wde. A poor-boy s certanly an nexpsve alternatve to pgradng a two-lane hghway to a for-lane dvded hghway. However, the poor-boy concept s not a standard treatmt, and there s crrtly a lmted amont of sch mleage n the state. Fgre l compares the amont of for-lane roadway wthot sholders wth the amont of two-lane roadways n Texas. As the fgre shows, Texas has km (43 35 mles) of two-lane hghways wthot sholders. They are charactered by small traffc volmes. In fact, 95 perct of these. roads carry fewer than 2 vehcles/day. There s also 21 km (13 5 mles) of two-lane hghways wth fllwdth paved sholders. Most of these roadways carry 1-3 vehcles/day. The state has only 9 km (56 mles) of for-lane roadways wthot sholders. These roadways are most heavly conctrated n the range of 1-5 vehcles/day. For the remander of ths paper, the terms poor-boy and for-lane hghway wthot sholders are sed nterchangeably. SCOPE OF STUDY The TSDHPT does not have a docmted data base for establshng desgn polces and practces for the pgradng of two-lane hghways wthot paved sholders to two-lane hghways wth paved sholders or for pgradng two-lane hghways wth paved sholders to poor-boy hghways. One of the prposes of ths research was to establsh the accdt effects related to the presce or absce of a paved sholder. The plannng dvson of TSDHPT consders any paved sholder less than 6 ft wde as "none or nadeqate" and codes the state's comptered geometrc fles n that manner. The same defnton for sholder was adopted for ths stdy. The stdy was lmted to rral Texas hghways of the types shown n Fgre 2. BACKGROUND The sholder s one of the most extsvely stded roadway elemts; however, safety fndngs have not always be consstt. Ths has cased consderable confson abot the exact relaton betwe sholder characterstcs and accdt experce. Prevos stdes can be placed nto three geral grops: those that fnd that wder sholders have adverse effects, those that ndcate that wder sholders have nclear or nll effects, and those that ndcate that wder sholders have favorable effects. Belmont's stdes n Calforna (1,2) and Blsly and Head's stdy n Oregon ( 3) fo-;;d- ncreases n accdt rate, property damag and accdt severty for certan types of wde-sholder condtons. Investgatons by Perkns (4), Taragn and Eckhardt (,2.), Raff (&_), and Foody and Long <Il ndcated that sholder effects on accdts were margnal or nsgnfcant. Other researchers, ncldng Stohner <.!!.lr Jorgs (2), and Zegeer and Mayes (1), fond a redcton n accdt rates de to wder sholders. These stdes may be consdered a partal lst, snce there are many other research efforts n the feld. It oft seems that dffert conclsons have be reached n nearly every stdy on the sbject. Ths dversty of opnon can be explaned n part by consderaton of the scope of the varos stdes. Many of the researchers examned data n lmted areas of volme, geometry, and operatonal

2 Transportaton Research Record Fgre 1. Total mleage n Texas for dffert hghway classes. 8 3:: ::: ::: LL 6 U) :.: 2 m lj " 6,136 l<m LEGEND 11,428 l<m ---- [J - 2-LANE NO SHOULDER!Im - 2-LANE WITH SHOULDER - 4 LANE NO SHOULDER Note: 1 k m =,62 mle. Fgre 2. Typcal examples of three hghways classes: (top) two lanes wth npaved sholders, (mddle) two lanes wth paved sholders, and (bottom) for lanes wth npaved sholders (poor boy). <I !I! AVERAGE DAILY TRAFFIC (X 1) characterstcs. It s necessary to solate and examne all other roadway and operatonal characterstcs n order to correctly determne the effect of paved sholders on accdt experce (11). The majorty of the dffer ces can be t raced to flaws n gatherng and nterpretng the data. Becase accdts normally occr nfreqtly, very large data samples are reqred to dscern ther effects. It s oft mpossble to fnd stable stes wthot changes n traffc volmes, geometry, or accdt reportng practces. The nteractve natre of accdt elemts also creates complexty and makes analyss more dffclt. The combned effects of the data constrants can be consdered the major reason for the lack of harmonos reslts. Rect research efforts have observed strcter controls n establshng and evalatng data. Hembach, Hnter, a nd Chao (12) ndertook a very thorogh classfcaton and matchng procedre to stdy sholders n North Carolna. They conclded that sgnfcantly less accdt experce s assocated wth homogeos hghway sectons that have.9- to 1.2-m-wde (3- to 4-ft-wde) paved sholders than wth dtcal hghway sectons that do not have sholders. They also fond that analyses of cost-effectvess and nvestmt retrn cold be sed to determne wh to add sholders to hghways. Fambro, Trner, and Rogness (]]) sed a large data sample of Texas roadways to determne whether the presce of paved sholders decreased accdts. Ther. conclson was smlar to that of Hembach n that two-lane roadways wth paved sholders were fond to have fewer accdts than roadways of the same type wthot sholders. Rnde <l!l, sng a before-and-after technqe n Calforna, fond a redcton n accdts de to the addton of sholders. He sggested optmm wdths of pavemt for varos combnatons of roadway type and volme. Seve ral researche r s (12, 15,lll have gone a step frther and have qantfed procedres to determne wh to add sholders and how to estab- 1 sh the cost-effectvess of sch addtons. The crrt empha s s on t he absce o r presce of paved shold.era, rather than t he wdth of sch s holders, has clearly establshed ther postve contrbton n redc1ng accdts. To smmare the lteratre revew, t appears that, after conflctng earler stdes, better control of the data poplaton has establshed the desrablty of provdng paved sholders for rral hghways. Rect research has fond a decrease n accdts de to the presce of paved sholders. ACCIDENT INVESTIGATION The level of safety per formanc e s one of the major c haracterstcs o f a roadway. Traffc e ngneers r otnely perform a varety of acc dt analyses n o rde r to dete rmne s a fety c haracterstc s. Althogh t here a r e ma ny types of a nalyse s, t he most common prposes for sch stdes are to dtfy haardos locatons, examne the aptness of proposed cor rect ve measres, or evalate the effectvess of prevos mprovemts. In addton, stdes are oft performed to compare the safety characterstcs of dffe rt t ypes of roadways. The c haracterstc patterns of traffc accdts exhbt only slght changes from ye ar to year (17, p. 281). As l ong as roadway geometrcs and traffc volmes reman relatvely nform, accdt patterns shold dsplay the same geral trds. Ths, the patterns applcable to specfc r oadways can be sol ated and dtfed f the researcher ses care n selectng and analyng the data. Accdts are assmed to be random occrrces that can be descrbed by statstcal theory and are ths qte sstve to sample se and the bas of the observer. Jorgs's landmark report on accdt analyss procedres (9) ponts ot that small sample ses and r a ndom vaa to n s n nonroadway characterstcs may serosly bas the reslts of a stdy. Jorgs also states that the data mst

3 32 Transportaton Research Record 819 Table 1. Ste-selecton data. Canddate Rl-2-TLOG Segmts Selected Stes No. of Class Nmber Lgth (km) Pottal Stes Nmber Lgth (km) I 'JO IS2 IO I IO 9S II IO SIS S s S2 SI I IO l62.s6 7 5S S :!. 1 1 _fl l!Q Total S Note: l km=.62 mle. cover a sffct perod of tme to gve a statstcally vald sample. To sre that there was a sond statstcal bass for ste selecton and that the sample wold be represtatve of Texas hghways, three years of acc dt data were gathered for a seres of careflly selected locatons. Three years were selected becase Gwynn (18) has shown that sng more than three years of accdt data does not s gnfcantly mprove the statstcal accracy of sch stdes. The steps that were followed to select the stes and collect the data for the accdt nvestgaton are descrbed n detal below. Ste Selecton A matrx of desred characterstcs was created to stratfy the s tes by traffc volme, sholder type, and nmber of lanes. The table below gves the nne classfcatons created to allow a comparatve analyss of the effects of these varables on accdt rates and charactarstcs (MlT = average daly traffc): Type of Type of Class Hghw::: Sholder ADT l Two-lane Unpaved Two-lane Paved For-lane Unpaved To sre a large and represtatve data sample, 1 stes n each class were stded, and each ste contaned 5 or more mles of consstt roadway. Once the geral ste crtera had be defned, all roadways n Texas were screed as pottal stes throgh a compter lstng of the roadway geometrc fle, commonly referred to as the Rl-2- TLOG. The fle wa s careflly revewed to obtan a lst of pottal rral stes that ft the reqremts otlned n the table above. The ntal screng was a sbstantal ndertakng that nvolved an evalaton of more than 29 roadway segmts. The selecton process s smmared n Table l. A lstng of the nmber of segmts of Texas roadway n each stdy classfcaton s gv. Clearly, there was not a nform dstrbton of segmts among the classes. For nstance, t was fond that more than 77 perct of the total elgble klo- meters of roadway fell n classes l and 4 (two lanes, wth and wthot sholder, and ADT of 1-3). Ths, the majorty of canddate stes cold be descrbed as lght-traffc, two-lane rral hghways. On the other hand, only 3 perct of the total klometers conssted of poor-boy hghways (classes 7, 8, 9, and 1). Drng ths porton of the stdy, t became appart that 1 stdy stes cold not be obtaned for some classfcatons. I n fact, only 8 stes were fond to meet the qalfcatons of class 3, 9 stes for class 7, and 4 stes each for classes 8 and 9. Classes 7-9 were restrcted de to the lmted qantty of poor-boy mleage n the state. At ths pont, t was decded to ao a t" :.a6S to the stdy (for lanes, npaved sholder, and 1-3 ADT) to handle low-volme poor-boy roadways. The addton of ths category allowed drect comparson of accdt rates at low traffc volmes for the three types of hghways n the stdy. A great deal of effort was expded n checkng the elgble stes to sre that they were typcal of ther respectve categores. For example, ADT was revewed for each locaton to verfy that no major changes had occrred drng the three-year stdy perod. Pavemt wdths and sholder types were scrtned foe nformty foe both each ste and each class. Dvded roadways were deleted from the nvestgaton. Carefl revews were condcted by sng conty, hghway dstrct, and state maps to solate and remove stes that contaned major ntersectons, towns, or other factors that mght bas the reslts of the stdy. The 85 selected stes were tak from the orgnal grop of more than 29 segmts contaned n the RI-2-TLOG. Table l clearly shows several stages of refnemt n achevng a represtatve sample of rral hghways spread over a wde geographc area. The abndance of low-volme, two-lane roadways (classes l and 4) prodced more than 4 pottal stes, and the research staff was able to choose roadways of consderable lgth that had an optmal geographc dstrbton. On the other hand, the lmted amont of poor-boy mleage (classes 7-1) cased a correspondng redcton n the nmber of stes avalable for stdy. It shold be noted that at least 25 perct of the contes n the state were represted by at least one stdy ste. Collecton of Accdt Data Once the stes had be fnaled, accdt data were gathered for the three most rect years avalable ( ). The comptered accdt fles of TSDHPT were scanned for all accdts that occrred at the 85 stes drng the three-year perod. A total of accdts were :lmclded n the stdy data base. Each comprehsve accdt record contaned 393 characters of nformaton abot the collson and the roadway on whch t occrred. The detaled accdt nformaton allowed a very thorogh examnaton and cross classfcaton of the data base. Records were revewed, and locatons that exhbted nsal condtons were removed. For example, constrcton ones, nsal traffc-control devces, and ralroad crossngs were fond to be major contrbtors to the accdt rate at varos locatons. In each case, the stdy ste was shorted to remove atypcal statons. Geometrc data were tak from the RI-2-TLOG to calclate the vehcle mles of travel for each ste. These fgres were combned wth the nmber of accdts on each roadway to yeld accdt rates. Snce the nmber of ntersectons vared from ste to ste, a separate analyss was performed on nonntersecton accdts to solate and remove

4 Transportaton Research Record Fgre 3. Accdt rates for dffert hghway classes (all accdts).. 3. I-. :::E. IL :.:: U). 2. I-. :::. > <( 1. ::: :::E 2-LANE NO SHOULDER 4-LANE NO SHOULDER /::;, 2-LANE WlTH -SHOUC.DER o, Note: 1 km=.62 mle. 2 4 AVERAGE DAILY 6 TRAFFIC 8 x 1 Fgre 4. Rates of nonntersecton accdts for dffert hghway classes. ar:: LANE NO SHOU\.DER I-. Note: 1 km= 62 mle. D 4-LANE NO SHOULDER 2 ar::. IL :.:: 2. I- "'. :I: "' > 2-LANE WITH SHOULDER 1. [!] <( -8 ::::; AVERAGE DAILY TRAFFIC x 1 the dsproportonate effects that ths type of accdt mght case among stes n the varos classfcatons. Intersecton accdts were dtfed from coded accdt data. Wthot ntersecton accdts, the data base contaned 8815 accdts. The two data sets are referred to here as the all-accdt grop and the non-ntersecton-accdt grop. ACCIDENT RATES Fgres 3 and 4 show the reslts of the nvestgaton of accdt rates for all accdts and for nonntersecton accdts, respectvely. The two fgres are smlar: The non-ntersecton-accdt crves gerally have lower vales and flatter slopes than the all-accdt crves. The mplcatons of the two fgres are dscssed below. All Accdts The most obvos featre of Fgre 3 s that the accdt rate tds to ncrease as ADT ncreases for all three types of roads n the stdy. The hghest accdt rate s assocated wth two-lane roads wthot sholders. Ths type of road also has the crve wth the steepest slope. Ths, t s the most sstve to ncreases n ADT. The crve becomes very steep and approxmates a straght lne at traffc volmes greater than 4 vehcles/day. In ths reg on, the two-lane, no-sholder road s have a dstnctly hgher accdt rate than ether of the other two road classes. It s mportant to note that the lowest accdt rate s assocated wth two-lane roads that have paved sholders. The crve for ths type of road has a gtle slope and s slghtly parabolc n natre. The crve for the thrd type of road n the stdy, for lanes wthot sholders, falls n betwe the crves for the other two types, beng safer than two-lane roads wthot sholders and not as safe as two-lane roads wth sholders. It wold seem that all for-lane roads shold be safer than all two-lane roads, yet Fgre 3 clearly ndcates otherwse. The add t on of paved sholders to two-lane roads makes them safer than forlane roads wthot sholders (poor-boy). The beft of addng paved sholders s ndcated n Fgre 3 by sharply redced accdt rates. There are several logcal reasons foe these safety befts. Paved sholders p:ovde recovery areas for vehcles that accdtally leave the roadway, provde a refge for stalled vehcles, provde acceleraton and deceleraton lanes for rght-trnng vehcles, and nstll a feelng of comfort and secrty n drvers. Conversely, there are reasons why poor-boy hghways have relatvely hgher accdt rates. Wh the paved sholders a re conve-rted to tr::avel lanes, the mmedate recovery one s removed for vehcles that accdtly ext the travel. lanes. The clear one adjact to the pavemt s not extded otward and ths fxed objects are left n the recovery area. The recovery manever becomes more dffclt and conseqtly leads to more accdts of the rn-off-road and fxed-object varetes. In addton, the paved for-lane road corages hgher speeds at lower traffc volmes. Ths s only a pa:tal lst of reasons why two-lane roads wth sholders are safer than ether of the other two types of roads n the stdy. In smmary, two major conclsons can be drawn from Fgre 3. The frst s that accdt rates ncrease wth ADT for all three types of roads n the stdy. The second fndng s the poston of the accdt-rate crves. In descdng order of accdt rates, they are two-lane roads wthot sholdecs, for-lane roads wthot sholders, and two- lane roads wth sholders. onntersecton Accdts The data on nonntersecton accdts were smmared, and the reslts are shown n Fgre 4. Hghways n classes 1-3 (two-lane roads wthot sholders ) agan dsplayed the hghest accdt rates. The fgre shows that, p to an ADT of 5 vehcles/day, the accdt rate ncreases.n a crvlnear manner. The crve for two-lane roads wth sholders (classes 4 and 6) has the same geral shape and s gerally parallel to the nosholder crve; however, the roads wth sholders have only two-thrds as many accdts as the roads wthot sholders. The addton of sholders to two-lane hghways offers defnte safety befts by decreasng nonntersecton accdts. Once agan, the accdt rates for the for-lane roads wthot sholders fell n betwe those of the other two types of roads. The crve has a gtly ascdng slope wth a d stnctly lower rate of ncrease than that for two-lane roads. In fact, the for-lane crve s nearly horontal, and ths mples that the accdt rate s farly nform over the range of traffc volmes nclded n ths stdy. The crves for poor-boy roads and two-lane roads wth sholders appear to converge at hgher volme levels. If the two-lane crve s extded, t may be noted that, at an ADT of abot 75 vehcles/day, the accdt rate s the same for both types of

5 34 Transportaton Research Record 819 roadways. The dashed ex ts on of the lowest crve n Fgre 4 ndcates the locaton of ths theoretcal pont of ntersecton. The data ndcate that two-lane roads wth sholders are safer than poorboys at all ADTs below that level. Based on these observatons, consderaton shold be gv to sng poor-boy hghways for ADTs greater than 75 and two-lane roads wth sholders that carry lower traffc volmes f accdts are the major or only consderaton for pgradng a faclty. Comparson of All Accdts and Nonntersecton Accdts The removal of ntersecton accdts casea ne crves shown n Fgre 3 to gerally move downward and to the rght. Ths sgnfes a lower acc dt rate, as wold be expected. The dffert types of roads do not all experce the same amont of change. For example, two- lane, no-sholder roads ndergn a very notceable alteraton. The crve for nonntersecton accdts s sgnfcantly dsplaced to the rght and has a mch flatter slope. Ths ndcates that sch roads are appartly very sstve to ntersecton accdts, especally as volme ncreases. At an ADT of 2 vehcles/day, there s only abot a 1 perct dfferce n the two crves. Ths dfferce reaches 5 perct at an ADT of 6 vehcles/day. Ths sggests that the constrcton of paved sholders at rral ntersectons may be effectve n redcng the accdt rate on roads wth hgh traffc volmes. The same ntersecton-accdt trd s also prest for poor-boy hghways. The crve for nonntersecton accdts has a mch flatter slope than the crve for all accdts. At low volme levels the crves are very close to the same vale, bt as ADT ncreases the crves dverge. Wh the ADT reaches 8 vehcles/day, nter secton accdts add 3 perct to the accdt rate. Ths agan ponts to ncreased ntersecton-accdt sstvty as volme ncreases. The least amont of change s experced by two-lane roads wth sholders. The crves for all accdts and nonntersecton accdts are vrtally parallel, whch ndcates that ADT has very lttle effect on the rate of ntersecton accdts. Ths, t appears that the presce of sholders has a befcal effect on redcng ntersect on accdts: For the two types of roads wthot sholders, there were large changes n accdt rates betwe the two data sets whereas, for the roads wth sholders, there were only mnor changes betwe data sets. ACCIDENT CHARACTERISTICS The types of accdts that occr on a specfc roadway or at a specfc ste oft yeld cles to the natre of safety-related problems. Correctve procedres are oft formlated based on analyses of accdt characterstcs. The pertnt fndngs of the nvestgaton of accdt characterstcs are snnared below. prevosly n relaton to Fgres 3 and 4--.e., accdt rate rsng as volme ncreases. The hghest rates are assocated wth two-lane hghways wthot sholders and the lowest rates are for two-lane hghways wth sholders. The crves for two-lane hghways are approxmately parallel, and both are parabolc n natre. The crve for forlane roads wthot sholders remans betwe the two-lane crves. The hgh degree of smlarty betwe the crves for property-damage accdt rate and the crves for njry-accdt rate ndcates that there are no nsal changes n the njry pattern as volme ncreases and that analyss of t.r ds n the rate of prope.rty-damage accdts wll sffce for anctly...b f '"'r.cndo!:-;. the!:::j!.1!'-!!,..; dt rate for the prpose of ths stdy. crves for fatal-accdt rate are shown by the dashed lnes n Fgre 5. The data ponts all fall n close proxmty to one another, and there s not a great dstncton betwe the crves for the three types of roads. The slopes are almost flat, whch ndcates that the fatal-accdt rate s frly constant fo.r volmes betwe 1 and 8 vehcles/day. For the two types of roads wthot sholders, there s a slght dverson from ths trd n the area of 5 vehcles/day. The mplcatons of ths dverson are not clear, bt t cold be that two-lane roads wthot sholders have passed a crtcal volme level and have become slghtly more ssceptble to hgh-severty accdts. Tbe lowest rates are for two-lane roads wth sholders, where the crve remans arond.4 fatal accdts per mllon vehcle klometers (.6 accdts per mllon vehcle mles) of travel. Ths crve s vrtally a straght lne. It s dffclt to draw conclsons from the fatal-accdt plot de to the low rates and data convergce. Perhaps the most sgnfcant conclson wold be the nformty of the fatal-accdt rate. The fact that these rates reman constant over a wde volme range whle the other types of accdts ncrease mples that the addtonal accdts at hgh volme levels are not h9h-sever ty accdts. Fgre 6 shows the relaton among the three accdt severty rates for a sngle class of road (two-lane road wth sholder) for nonntersecton accdts. Smlar relatons exst for the other types of roadways n the stdy. The poston of the crves n Fgre 6 ndcates the relatons of rates of total, njry, and fatal accdts. The rate of fatal accdts s tmes lower than the total Fgre 5. Rates of njry and fatal accd1s for dffert hghway cla$ses (non ntersecton accdts). 1.2 Note: 1 km =.62 mle. Severty Analyss 2-LANE NO SHOULDER 4-LANE NO SHOULDER 2-LANE WITH SHOULDER INJURY ACCIDENTS FATAL ACCIDENTS - Traffc accdts are sally placed n one of three severty classfcatons: fatal, njry, or property-damage collsons. The accdt-rate crves shown n Fgres 3 and 4 can be consdered to represt nclsve property-damage crves. Crves for njry and fatal accdts have be plo ted n Fgre 5. The sold lnes depct the rates of njry accdts and fall nto a famlar pattern. The crves seem to dsplay relatons dscssed AVERAGE DAILY TRAFFIC x 1 -

6 Transportaton Research Record Fgre 6. Severty of three types of accdts on two-lane roads wth sholders (nonntersecton accd1s). :. _J :.:.3 _J ::: > O.< ::: _J :... a.1 I I I ""' """"' Note: 1 km=.62 mle. INJURY ACCIDENTS - A,.,..-- -O-----'r;,,. FATAL ACCIDENTS AVERAGE DAILY TRAFFIC X 1 Fgre 7. Rates of rn-off-road accd1s for dffert hghway classes (nonntersecton accd1s). :. 2 :::!. :.: _J : >.4 3 :.. Ill.2. ff e AVERAGE DAILY TRAFFIC 2-LANE NO SHOULDER 4-LANE NO SHOULDER f;. 2-LANE WITH SHOULDER Note: 1 k m ==.62 mle. e (X 1) accdt rate, and the rate of njry accdts s approxmately three tmes lower than the total accdt rate. The change n slope for the three crves n Fgre 6 ndcates ther sstvty to traffc volme. The most sstve (steepest-sloped) crve s for total accdts, the next most ss tve s for njry accdts, and the least sstve s for fatal accdts. Ths renforces the prevos observaton that addtonal accdts at hgh traffc volmes are not hgh-severty accdts. R n-off- Road Acc dts The lack of a stable recovery area for vehcles that leave the travel lanes shold case an ncrease n the r n-off-road type of acc dt. Ths premse s spported by Fgre 7, wh ch ndcates that two-lane roads wth sholders have the lowest level of occrrce of sch accdts and a nform level of ncdce over a wde range n traffc volmes. On the other hand, both types of roadways wthot sholders exhbt hgher and more erratc rates of rn-off-road accdts. The flctaton of these crves wold seem to conform to the level of drver workload. At low traffc volmes, drvng s an easy task that reqres few decsons and drvers are lkely to become natttve. As volme ncreases, drvers a re forced to make more freqt decsons as they meet addtonal oncomng traffc and ha ndl e varos drvng sta t ons. Ths, t hey are forc ed to become more atttve. At hgh traff c volmes, the drver workload becomes very heavy, r eqrng almost constant eval a ton of vehcle poston, speed, and smlar tems. The drver s lkely to become fatged or to mss mportant stml that mght warn of approachng danger. Drver natttvess at low traffc volmes cold accont for the hgh rn-off-road accdt rate shown n Fgre 7. As volme and drver atttvess ncrease, the rate drops for both types of hghways wthot sholders. For two-lane roads, ths drop occrs n the 1- to 3-vehcles/ day range. For for-lane roads, the addtonal paved lane allows drvers add-ftonal lateral freedom and decreases ther workload. Ths, the ncrease n atttvess does not occr ntl the volme level reaches 4-5 vehcles/day. Drng the per od of ncreased a tttvess, both t y pes of roads have lower ncdce rates (for mdvolme ranges). As the two-lane road reaches relatvely hgh traffc volmes, the rate of rn-off-road accdts begns to ncrease, probably de to drver work overload and hgh-volme accdt statons that force the drver to make e mergcy manevers away from the normal travel lane. Ths s ndcated by the rsng accdt rate arond 5 vehcles/day. For forlane roads, ths pont has not be reached at the traffc volmes shown n Fgre 7; however, the poor-boy crve s begnnng to clmb (at 8 vehcles/day), whch ndcates that the rate of rnoff-road accdts s begnnng to ncrease wth ncreasng volme. It cold well be that the forlane crve peaks at hgh volme n the same manner that the crve for two-lane, no-sholder roads peaked at 5 vehcles/ day. Types o f Collsons Another meanngfl accdt characterstc s the frst harmfl evt n a collson seqce. Fgre 8 shows how many tmes httng another car was the frst harmfl evt. A nmber of conclsons can be drawn from ths fgre. Frst, the ncreasng slopes of the lnes ndcate that the chances of httng another vehcle ncrease as traffc volmes ncrease. Sch s to be expected, snce there are more vehcles on the hghway as the volme ncreases. At low volmes, more vehcles are rnnng off the road than are httng other vehcles. A comparson of the crves for for-lane accdt rate n Fgres 7 and 8 shows that approxmately twce as many vehcles rn off the road as ht other cars at a volme level of 2 vehcles/day. At volmes less than 4 vehcles/day, the rn-off-road ace dt s domnant. Perhaps the most mportant fndng that can be drawn from Fgre 8 s the almost dtcal crves for two-lane roads wth s holders and the poor-boy roads. The crves are parallej. and le a l most on top of one a nother. Ths ndcat e s that both types of roads exhbt the s ame behavor for ht-other-car accdts. It s hghly probable that the paved sholders and extra paved travel lanes both serve the same fncton by provdng addtonal maneverng room for ve h cles drng the collson seqce. Fgre 9 emphases the ncrease n ht-other-car

7 36 Transportaton Research Record 819 Fgre 8. Rates of ht-other-car accdts for dffert hghway classes (nonntersecton accdtsl ! : %.8 > ::J _, / //.4 Note. 1 km=.62 mle. CL 2-L.ANE NO SHOUL.OER.. 4-L.ANE NO SHOUL.DElt t, 2-L.ANE WITH SHOUL.DER. <l 6 8 AVERAGE DAILY TRAFFIC (X 1) Fgre 9. Increase n ht-other-car accdts wth ncreasng traffc volme for dffert hghway classes... <l 4.. "-.. 2 CL 6 8 AVERAGE DAILY TRAFFIC l IOOO) accdts as volme ncreases. The perctage of accdts that nvolved httng another vehcle clmbs steadly as traffc volme ncreases. As Fgre 9 shows, the same trd exsts for all three types of roads n ths stdy. Both the perctage and the rate of ht-other-car accdts ncreased smlarly for all three types of roads. Fxed-Object Accdts A fxed-object ncdt s oft fond n the chan of harmfl evts n a collson seqce. These types of accdts were examned and fond to closely resemble the rn-off-road accdt pattern, especally for for-lane hghways wthot sholders and two-lane hghways wth sholders. At low tra f c volmes, the for-lane road exhbted a hgh rate of fxed-object collsons. The rate began to decrease as the volme rose above 4 vehcles/day n the same manner noted earler for rn-off-road accdts. At 8 vehcles/day, the rate of fxed-object accdts had dropped by 5 perct. Tps echoes the prevos premse that accdts on low-volme, for-lane roadways are sally not the mltple-vehcle type bt rather some fncton of drver nattt vess anc;j lack of paved recovery area. Nondaylght Accdts Fgre 1 shows the rate for the nmber of nondaylght accdts compared wth total vehcle travel Fgre 1. Rate of nondaylght accdts for dffert hghway classes (nonntersecton accdts)... :::: 1.2 x >.1 :J CL.4-1- <l I 2-LANE NO SHOULDER 4-L.ANE NO SHOUL.DER t,.2-l.ane WITH SHOUL.OER Note : 1 km = O.fl2 mle. ' I I AVERAGE DAILY TRAFFIC l x 1) (both daylght and nondaylght). The three crves hold ther famlar poston. The most dangeros type of road s the two-lane hghway wthot sholders, and the least dangeros s the two-lane hghway wth sholders. ';l'he crves are farly flat, whch ndcates nform accdt rates for all roadway types regardless of traffc volme. It s noteworthy that a dsproport onate nmber of the accdts occrred at n ght. Ths s especally tre for for-lane roads. For example, by comparng accdt rates n Fgres 4 and 1, t may be noted that approxmately 6 perct of all accdts on for-lane roads occrred drng nondaylght hors. The vast majorty of rral traffc occrs drng t he day, whch means that 6 perct of the accdts reslt from the small perctage of total traffc that occrs at nght and that for-lane hghways wthot sholders are sgnfcantly more dangeros at nght than drng the day. CONCLUSIONS Based on the fndngs from ths stdy, several conclsons can be drawn. These conclsons nvolve both accdt rates and accdt characterstcs for three types of rral Texas hghways and are dscssed below. Accdt Rate Wh all accdts that occrred at the stdy stes were consdered, the accdt rate for each roadway type ncreased as the traffc volme ncreased. Two-lane hghways wthot paved sholders have the hghest accdt rates and are the most sstve to changes n traffc volme. Two-lane hghways wth paved sholders had the lowest accdt rates for the traffc volmes nvestgated n ths stdy (1-7 vehcles/day). The thrd type of road nvestgated, for-lane roads wthot sholders, was the least sstve to traffc volme and bad an accdt rate that fell n betwe the rates for the other two types of roads. The presce of paved sholders had a notceable effect n redcng the accdt rate. Wh ntersecton accdts were removed from the data set, the road types retaned the same rank: Two-lane roads wthot sholders were the most

8 Transportaton Research Record dangeros, and two-lane roads wth sholders were the least dangeros. For-lane roads were fond to have a farly nform accdt rate regardless of traffc volme, whereas the crves for the other two roads were almost parallel bt notceably dffert. Fll-wdth paved sholders were agan shown to have postve effects n redcng accdt rates. An extson of the data ndcated that two-lane hghways wth paved sholders had lower accdt rates than poor-boy hghways at traffc volmes of less than 75 vehcles/day. A comparson of the non-ntersecton-accdt data grop wth the all-accdt data grop ndcated that two-lane hghways wthot paved sholders are very sstve to ntersecton accdts, especally at hgh traffc volmes. For-lane roads are somewhat sstve and two-lane roads wth paved sholders are relatvely nsstve to sch accdts. Ths, t appears that the constrcton of fll-wdth paved sholders at rral ntersectons may be effectve n redcng the nmber of accdts on hgh-volme roads. Accdt Characterstcs The severty analyss revealed that crves for njry-accdt rate are approxmately parallel to crves for the total accdt rate whereas crves for fatal-accdt rate are very low and farly nform for all three types of roads over the range of traffc volmes nclded n the stdy. The lowest fatalty rate was o. 4 fatal accdts per mllon vehcle klometers of travel (.6 accdts per mllon vehcle mles) for two-lane hghways wth paved sholders. The absce of fll-wdth paved sholders ncreased the rate of rn-off-road accdts, especally at low traffc volmes. The two-lane roads wth paved sholders had a farly nform rate of rn-off-road accdts, whereas the rates for the other two roads were of a hgher level and vared consderably wth volme. The most probable reason for the varatons and the hgh rn-off-road rates at low volmes s drver natttvess and lack of a paved recovery one for vehcles that accdtally ext the travel lane. Ht-other-car accdts td to ncrease drastcally wth ncreasng traffc volmes two-lane, no-sholder hghways have a greater ncdce of these accdts than the other two hghway types. The forand two-lane hghways wth sholders have vrtally dtcal crves for ht-other-car accdts. At lower volmes, other types of collsons were more freqt. For example, on for-lane hghways, the rn-off-road accdt s the most freqt type of accdt at all volmes less than 4 vehcles/day. On roads wthot sholders, rates for fxed-object accdts were fond to closely resemble rates for rn-off-road accdts. Agan, ths reflects the lack of a paved recovery area and the presce of fxed objects n the clear one adjact to the roadway. ACKNOWLEDGMENT Ths paper dscsses one phase of a research project ttled, "Operatonal Effects of Drvng on Paved Sholders". Ths stdy was condcted by the Texas Transportaton Insttte (TTI) and sponsored by TSDHPT n cooperaton wth the Federal Hghway Admnstraton (FHWA),. S. Departmt of Transportaton. We wsh to thank Ernest w. Kanak, John F. Nxon, and Harold D. Cooner of TSDHPT for ther techncal npt and constrctve sggestons throghot ths project. The assstance of Carroll J. Messer, Thomas E. Kessler, and Stev R. Bartlett of TT! s also apprecated. The contts of ths paper reflect or vews, and we are responsble for the facts and accracy of the data prested. The contts do not necessarly reflect the offcal vews or polces of FHWA. Ths paper does not consttte a standard, specfcaton, or reglaton. REFERENCES 1. D.M. Belmont. Effects of Sholder Wdth on Accdts on Two-Lane Tangts. HRB, Bll. 91, D.M. Belmont. Accdts Verss Wdth of Paved Sholders on Calforna Two-Lane Tangts: 1951 and HRB, Bll. 117, 1956, pp R.C. Blsly and J.A. Head. Statstcal Determnaton of Effect of Paved Sholder Wdth on Traffc Accdt Freqcy. HRB, Bll. 24, E.T. Perkns. Relatonshp of Accdt Rate to Hghway Sholder Wdth. HRB, Bll. 151, A. Taragn and H.G. Eckhardt. Role of Hghway Sholders n Traffc Operaton. HRB, Bll. 151, M.S. Raff. Interstate Hghway Accdt Stdy. HRB, Bll. 74, T.J. Foody and M.D. Long. The Idtfcaton of Relatonshps Betwe Safety and Roadsde Obstrctons. Oho Departmt of Transportaton, Colmbs, W.R. Stohner. Relaton of Hghway Accdts to Sholder Wdth on Two-Lane Rral Hghways n New York State. Proc., HRB, Vol. 35, 1965, pp Evalaton of Crtera for Safety Improvemts on the Hghway. Roy Jorgs and Assocates, Inc., Gathersbrg, MD, c.v. Zegeer and J.G. Mayes. Cost Effectvess of Lane and Sholder Wdng on Rral, Two-Lane Roads n Ktcky. Ktcky Departmt of Transportaton, Frankfort, Res. Rept. 999, J.M. Portgo. State-of-the-Art Revew of Paved Sholders. TRB, Transportaton Research Record 594, 1976, pp C.L. Hembach, W.W. Hnter, and G.C. Chao. Paved Hghway Sholders and Accdt Experce. Transportaton Engneerng Jornal, Proc., ASCE, Vol. 1, No. TE4, 1974, pp D.B. Fambro, D.S. Trner, and R.O. Rogness. Operatonal and Safety Effects of Drvng on Paved Sholders n Texas. Texas Transportaton Insttte, Texas A&M Unv., College Staton, Res. Rept, 265-2F, Sept E.A. Rnde. Accdt Rates Verss Sholder Wdth. Calforna Departmt of Transportaton, Sacramto, Rept. CA-DOT-TR , D. I. Davs. Sholder Improvemts on Two-Lane Roads. TRB, Transportaton Research Record 737, 1979, pp c.v. Zegeer and D.D. Perkns. Effect of Sholder Wdth and Condton on Safety: A Crtqe of Crrt State of the Art. TRB, Transportaton Research Record 757, 198, pp L.J. Pgnataro. Traffc Engneerng: Theory and Practce. Prtce-Hall, Inc., Englewood Clffs, NJ, D.W. Gwynn. Accdt Rates and Control of Access. Traffc Engneerng, Nov