*Manuscrpt Clck here to vew lnked References Ye, Venezano, and Lord 1 Safety Impact of Gateway Monuments Zhru Ye a,*, Davd Venezano a, Domnque Lord b a Western Transportaton Insttute, Montana State Unversty, Bozeman, MT, USA b Zachry Department of Cvl Engneerng, Texas A&M Unvserty, College Staton, TX, USA Abstract Gateway Monuments are free standng roadsde structures or sgnage that communcate the name of a cty, country or townshp to motorsts. The placement of such monuments wthn statecontrolled rght-of-way s a relatvely recent occurrence n Calforna. As a result, the Calforna Department of Transportaton (Caltrans) ntated research to quantfy the mpacts that ths type of sgnage may or may not have on crashes n ther vcnty. To date, no specfc research has examned the mpact such features have on crashes. To determne whether these features mpacted safety, the before-after study method usng the Emprcal Bayes technque was used, wth reference groups and Safety Performance Functons adapted from exstng studes, elmnatng the need to calbrate new models. Results ndcated that, on an ndvdual bass, no deteroraton n safety was observed at any monument ste. When all stes were examned collectvely (usng two dfferent scenaros), the calculated ndex of effectveness values were 0.978 and 0.680, respectvely, correspondng to. percent and 3.0 percent reductons n crashes. In addton to the EB method, naïve study methods (wth and wthout AADT taken nto account) were appled to the study data. Results (crash reductons) from these methods also showed that the presence of Gateway Monuments dd not have negatve mpact on traffc safety. However, the use of EB technque should be very careful employed when adoptng reference groups from dfferent jursdctons, as these may affect the valdty of EB results. In lght of these results, Caltrans may contnue to partcpate n the Gateway Monument Program at ts dscreton wth the knowledge that roadway safety s not mpacted by monuments. Key Words: Traffc Safety; Gateway Monuments; Emprcal Bayes; Before-after Study * Correspondng Author: Tel: + 1 406 994 7909; fax: +1 406 994 1697; E-mal address: jared.ye@coe.montana.edu (Z. Ye)
Ye, Venezano, and Lord 1. Introducton Gateway Monuments are free standng roadsde structures or sgnage that communcate the name of a cty, country or townshp to motorsts. They provde communtes wth a means of dentfyng themselves whle creatng a favorable mage. The monuments typcally nclude the offcal seal or slogan of the local entty and are consdered non-ntegral and/or non-requred hghway features. The concept of Gateway Monuments s not new. For years local enttes have erected sgnage welcomng motorsts to ther communty. Gateway Monuments, whle smlar n appearance to the marker monuments constructed at the entrances to communtes, natonal parks, etc., dffer n a sgnfcant way. Most marker monuments are constructed n close proxmty to the roadway edge. Ths s the result of the nature of roadway they are constructed along, whch s typcally lower speed (approxmately 35 mles per hour or less), wth a well defned travel way (curb and gutter, etc.). Secondly, marker monuments ncorporate dfferent desgn standards than those employed by the Gateway Monuments dscussed here, partcularly n terms of scale and dmensons. On the other hand, Gateway Monuments are constructed along hgher speed state routes, wth the result beng a need for clear zone consderatons to be made. The end result s that Gateway Monuments, unlke marker monuments, are set back approxmately 30 feet or more from the roadway edge. The placement of such monuments wthn state-controlled rght-of-way s a relatvely recent occurrence n Calforna. Indeed, the ncluson of such monuments s part of a demonstraton project (Gateway Monument Demonstraton Program, GMDP) spannng a four year perod, commencng on January 1, 005 and concludng December 31, 008. Durng the course of ths project, four communtes receved constructon approval, wth nstallatons beng completed at fve stes. Two addtonal monuments were constructed pror to the start of the program, resultng n a total of seven Gateway Monuments beng nstalled n fve Calforna communtes. The locatons of these stes and mages of each monument are presented n Fgure 1 and Fgure. Monuments were constructed at the followng stes: Wllow Creek (one monument); Rockln (two monuments, each located at the same nterchange); Nevada County (one monument); Paso Robles (two monuments, constructed pror to start of program); Tehachap (one monument). In general, the monuments were placed along the hghest traffc faclty n the communty, whch, as s the case n most communtes, happened to be a state-controlled route. The number of Gateway Monuments present n a communty was not the result of any specfc factor, asde from perhaps avalable fnances. In the case of Rockln, two monuments were constructed n conjuncton wth one another as part of an nterchange beautfcaton. In the case of Paso Robles, the monuments were each constructed n dfferent locatons at approxmately the same tme for landscapng purposes. Whle all of the monuments were bult to welcome vstors to the communty, estmates of the percentages of tourst versus local traffc passng each ste would be dffcult to measure and therefore were not avalable for ths research.
Ye, Venezano, and Lord 3 Fgure 1: Gateway Monument stes
Ye, Venezano, and Lord 4 Fgure : Gateway monuments examned n ths research Approval of monuments s based on consderatons related to safety (e.g. locaton beyond the clear zone, provson of adequate sght dstance, drver dstracton), approprateness, aesthetcs, mantenance access, and the message beng communcated. Before applyng to the Gateway Monuments program, local enttes were expected to explore other feasble alternatves to placement wthn the state s rght-of-way. These optons nclude locaton of the monument outsde the operatonal rght-of-way, communty dentfcaton on exstng or proposed hghway features, aesthetc treatment for an exstng or proposed faclty, or utlzaton of exstng or natural topographc features n the placement of the monument (Calforna Department of Transportaton, 005). Applcants were allowed to petton for only one monument per State Hghway or Interstate per drecton. Whle the GMDP seeks to foster partnershps wth local enttes n the placement and constructon of sgnage, there s a knowledge gap concernng how such sgns affect hghway safety wthn ther vcnty. Therefore, research was needed to quantfy the mpacts that ths type
Ye, Venezano, and Lord 5 of sgnage may or may not have. Such quantfcaton would provde Calforna Department of Transportaton (Caltrans) wth the necessary nformaton to decde whether the Gateway Monuments program should contnue. The research results presented here begn to address the exstng knowledge gap by examnng the safety performance of Gateway Monuments.. Smlar Research Despte the prevalence of monument sgnage along roadways throughout the naton (and nternatonally) announcng to motorsts that they are enterng a communty, no research has been performed to date specfc to the potental safety mpacts of such sgnage. Due to ths lack of monument-specfc safety evaluatons, lterature related to the safety mpacts of smlar roadsde features, general landscapng and addtonal fxed features was revewed..1.gateway Monuments Whle no safety or operatonal analyses were performed, Hallmark et.al. (007) dscussed several low-cost traffc calmng strateges that communtes may employ to address speedng ssues, ncludng elements dentfed as communty gateways. Ths work acknowledged that such monuments may be effectve n communcatng to motorsts that they are makng a transton from a rural roadway to a cty street, havng dstnct safety mplcatons. In a report publshed by the Unted Kngdom s Department of Transport, a seres of traffc calmng treatments along major roads were evaluated, ncludng communty gateways. The report ndcated that mean speed reductons of between 3 mph and 13 mph (wth an average of 5 mph) had been observed at such gateways (Sustrans, 008). These results ndcate that there are tangble speed-reducton benefts provded by monument sgnage. Addtonal research from the Unted Kngdom found up to a 15 mph speed reducton for the 85th percentle of vehcles when usng gateways n combnaton wth other treatments (Department of the Envronment, Transport and the Regons, 005). Ths further ndcates that monuments have an mpact on vehcle speeds that may translate nto safety benefts...landscape Features Nader (003) nvestgated the effects that varous roadsde landscape varables wthn the clear zone had on drver safety. Whle Gateway Monuments were not among the varables, rased concrete planters, sculptures and decoratve nose barrers were examned. The research found accdents decreased between 5 and 0 percent wth such features n place. Furthermore, a postve effect exsted from havng a well-defned roadway edge, resultng n decreased run-offthe-road crashes wth obstacles. Mok and Landphar (003) lnked safety performance to corrdor landscape types for four states usng Fatalty Analyss Reportng System (FARS) data between 1994 and 000. The research ndcated that parkways were consdered to be routes whch had hgher aesthetc values, whle freeways had paved shoulders, concrete medans and lmted landscape. Comparsons of the safety performance of parallel sectons of freeways and parkways by fatal crash rates and accdent costs were made, wth results ndcatng that parkway sectons were sgnfcantly safer n terms of fatal crash rates per one hundred mllon vehcle mles travelled. Accdent costs per one mllon vehcle mles traveled were also sgnfcantly lower compared to freeway sectons.
Ye, Venezano, and Lord 6 The researchers stressed that the study dd not allow for conclusons regardng whch specfc landscape elements contrbuted to the observed dfferences (Mok and Landphar, 003). Mok et.al. (006) looked at the mpacts of roadsde landscape mprovements on safety n Texas usng a before and after study of crashes as a quanttatve measure of roadsde greenng. Results showed a sgnfcant decrease n crash rates after landscape mprovements were mplemented. However, the researchers do not explctly lst the landscape features that were ncluded n the mprovements, asde from a menton of trees. As a result, t s mpossble to determne f any of the study stes possessed monuments or smlar desgn features. Fnally, Tay (009) examned the reactons of drvers to the presence of roadsde memorals. These features typcally consst of a cross and personal tems placed on a roadsde to mark the locaton of a fatal crash. Whle the author noted that such features could result n rear-end collsons nvolvng drvers stoppng at a memoral, typcally features such a bllboards held a greater potental for servng as a dstracton. In examnng the mpacts of mock monuments (.e. monuments set up at sgnalzed ntersectons to elct a response but where no fatal crash had occurred), t was found that red lght runnng volatons fell by 16.7 percent, whle volatons at comparson stes wthout monument ncreased by 16.8 percent..3.addtonal Roadsde Features Work by Holdrdge, et.al. (005) nvestgated the hazards presented by fxed urban roadsde objects. Models developed to estmate njury severty ndcated that several roadsde features had a sgnfcant mpact on severtes. Such features ncluded sgn posts, whch ncreased the propensty toward non-njury crashes, and trees or poles, whch ncreased the propensty toward fatal crashes/njures. Consequently, the researchers stress that protectng traffc from fxed obstacles s essental for features that have shown a hgh propensty for resultng n hgher severtes. Lee and Mannerng (00) developed statstcal models of run-off-roadway crashes to solate the factors that sgnfcantly nfluenced crash frequency and severty. Fxed roadsde elements n the research ncluded trees, barrers, utlty and lght poles, wooden sgn supports, boulders, and malboxes. Results ndcated that ncreasng the dstance from the edge of the traveled way to lght poles and removng solated trees along the roadway would reduce crash frequences. Ths was not surprsng, wth the mplcatons on monuments beng that ther locaton should be as far as possble from the edge of the roadway. Troxel et. al. (1991) used FARS data from 1980-1985 and Natonal Accdent Samplng System (NASS) data from 198-1985 to examne sde-mpact collsons wth roadsde obstacles. One of the classes of obstacles examned was dentfed as beng Broad. These were most smlar to monuments (although such features were not explctly lsted among the examned features), wth crashes nto such features totalng 18 percent of all sde mpact collsons (Troxel et al., 1991). These results would seem to ndcate that crashes wth such features, at least durng the perod of analyss, were less prevalent. Ths was lkely due to the lmted number of large features n the roadway envronment. Note that the results presented n ths secton seem to contradct those presented n Sectons.1 and.. The reason for ths stems from the nature of the features dscussed n the prevous sectons. These were tems that were lmted n frequency (ex. monuments, planters, etc.) and whch had been constructed wth safety n mnd (ex. clear zone areas ncorporated). Conversely, the features dscussed n ths secton are more prevalent (sgns, trees, etc), and often located n close proxmty to the roadway edge.
Ye, Venezano, and Lord 7 A second explanaton for these dfferences could be consdered from a human factors standpont. The tems dscussed n Sectons.1 and. represent landscapng features that are often found n locatons where speed or envronment (rural to urban) transtons occur. The presence of such features n a potental transton area may elct a drver to slow down and be more cautous, producng potental safety benefts. On the other hand, t s possble that these monuments may ncrease the rsk for drvers to be dstracted by the monument, hence the request by CALTRANS to examne whether safety decreased at the locaton where these monuments were erected. Fnally, the work of Tay (009), whle t ndcates safety benefts from roadsde monuments, s not drectly applcable to Gateway Monuments. Ths stems from the features of Gateway Monuments, whch are ntended solely to convey a welcomng sense of place and are located a sgnfcant dstance from the roadsde tself. In no way are they ntended to produce a safety beneft, although such benefts may ndrectly result from drvers reducng speed, etc. Conversely, roadsde memoral monuments, whle ntended smply to serve n remembrance of a loved one, may also produce an added safety beneft of makng drvers aware of locatons where potental hazards may be present. As a result, whle each feature represents a monument, ther ntenton and desgn are lkely to produce dfferent behavoral reactons. 3. Hypotheszed Impact The hypotheszed mpact of Gateway Monuments on traffc safety, specfcally the number of crashes occurrng n ther vcnty, was expected to be mnmal. Of course, whle no change was expected, t was also possble that an ncrease or decrease n crashes mght occur at any of the study stes for varous reasons. A decrease mght be possble f the sgnage nfluences the speed of vehcles (.e. sgnage serves as an nformal speed transton area ndcator). Conversely, from a human factors standpont, the sgnage may be a dstracton to motorsts, resultng n an ncrease n crashes. Whle Gateway Monuments are ntended to provde motorsts wth a bref sense of place whle requrng a mnmum of tme to nterpret the message, t s stll possble that they may serve as a source of dstracton, leadng to a crash. 4. Study Data To analyze the safety mpact of gateway monuments, traffc flow, crashes, and other data (geometrcs) were collected for the before and after monument constructon perods. These data are summarzed n Table 1. Fve of the seven monuments were constructed along roadway segments and two were constructed at sgnalzed ntersectons. The segments at Nevada County and Tehachap were rural four-lane two-way undvded roadways and the Paso Robles Route 46 and Wllow Creek stes were along rural two-lane two-way undvded roadways. The sgnalzed ntersectons at the Rockln stes were four-legged and located n urban areas. The Paso Robles U.S. 101 ste was on an nterchange ramp located on the perphery of town. The constructon perods for monuments vared from one to sx months. Snce ther constructon may affect motorsts, crashes from the constructon perod were ncluded n the after perod. The constructon perod was ncluded n the after perod because of the nature of the constructon actvty tself. The monuments were constructed at a dstance of 30 feet or greater from the roadway edge, and, unlke typcally road constructon work, dd not consttute a work zone (.e. no lane closures). On the other hand, once constructon of the monument had begun, even n an uncompleted manner, t conssted of features that would brefly draw motorsts
Ye, Venezano, and Lord 8 attenton n the same fashon that a completed monument would. For example, stes that ncorporated rockwork often had these features present from the very early stages of constructon. Ths rockwork represented a partally completed monument whch was lkely to attract motorst attenton and possbly nfluence ste safety. That sad, the senstvty of the results wth or wthout the constructon perod ncluded n the after data were neglgble. Ths stems from the fact that only two crashes occurred durng the constructon of all monuments, one at the Rockln Westbound ramp ste, and one at the Rockln Eastbound ramp ste. In addton to the constructon perod of the monuments, Caltrans records were examned to determne what, f any, addtonal constructon actvtes occurred over the course of the years for whch crash data was to be analyzed. Asde from ordnary mantenance actvtes (repantng pavement markngs) no constructon actvtes occurred at any ste durng the study perod. Whle the monuments are vsble from each sde of the road n some cases (Tehachap), n others they are not (Wllow Creek). These factors were taken nto account when dentfyng the crashes of nterest. The crash counts n the table nclude all accdents (njury and Property Damage Only PDO; no fatal crashes occurred durng the study perod). The total before and after crashes were 13 and 13 for segments and 3 and for sgnalzed ntersectons. The measured Average Annual Daly Traffc (AADT) data durng after perods was generally hgher than before perods except at the Wllow Creek ste, whch saw a slght decrease. The Paso Robles U.S. 101 ramp segment experenced a moderate ncrease of AADT over the course of three years.
Ye, Venezano, and Lord 9 Table 1: Summary of ste data. Segments Before Perod After Perod Total Crashes AADT ** Years of Data Locaton Length (m) No. of Lanes Lane Wdth (ft) Shoulder Wdth* (ft) AADT ** Years of Data Wllow Creek 0.7 1 11 3400 Aug 03 Jul 06 3300 Aug 06 Apr 08 1 Paso Robles Rt 46*** 0.6 1 8 17000 Aug 98- Jul 01 0 0000 Aug 01 Jul 04 4 Nevada County 0.4 4 1 10 7500 Sep 03 Aug 06 5 34000 Sep 06 Apr 08 1 Tehachap 0.4 4 1 8 10100 Nov 0 Oct 05 3 10900 Nov 05 Apr 08 1 Paso Robles US 101*** 0.05 1 1 9 9600 Aug 98 - Jul 01 3 1900 Aug 01 -Jul 04 6 Before Perod After Perod Locaton Mnor AADT Major AADT Years of Data Total Crashes Mnor AADT Major AADT Years of Data Total Crashes Rockln WB nt**** 770 5000 May 0 Apr 05 1 3400 9500 May 05 Apr 08 7 Rockln EB nt**** 11050 4000 May 0 Apr 05 11 11350 800 May 05 Apr 08 15 * Average wdth for two sdes of roadway ** AADT was obtaned for the mddle year of each before and after perod *** The Paso Robles stes were located on the east and south sdes of town, respectvely **** Each of the Rockln stes were located on opposte sdes of the same nterchange Total Crashes
Ye, Venezano, and Lord 10 5. Methodology The purpose of ths research was to nvestgate crash hstory before and after the constructon of monuments and determne whether ther presence negatvely/postvely affected traffc safety. Based on the avalable data, the effect of Gateway Monuments on traffc safety can be evaluated through an observatonal before-after study (Hauer, 1997). To do ths, t was mportant to know about the change n safety n terms of crash counts. Ths change may be determned through the equatons: or / (1) Where: = crash reducton (or ncrease); = ndex of safety effectveness (Persaud et al., 001); = the predcted number of crashes n the after perod wthout the presence of Gateway Monuments; and = the number of reported crashes n the after perod (wth a monument present). Before-after studes can be grouped nto three types: the smple (naïve) before-after study, the before-after study wth control groups, and the before-after study usng the Emprcal Bayes (EB) technque. The selecton of the study type s usually governed by the avalablty of the data, such as crashes and traffc flow, and whether the transportaton safety analyst has access to enttes that are part of the reference group. The selecton can also be nfluenced by the amount of avalable data (or sample sze). It s worth notng that n observatonal studes, the term comparson group s used nstead of control group snce the enttes (stes for treatment) are not selected at random (Hauer, 1997). In ths study, t was easy to estmate, as crash counts durng the after perod are known. Thus, t was essental to estmate by usng an approprate technque to acheve maxmum accuracy. The problem wth the naïve approach s that t cannot dstngush the effects of causal factors. For example, ths method does not account for large or regonal changes, such as economc downturn, weather pattern or composton of vehcle fleet wthn the study regon (whch may nfluence the severty of crash for nstance). In addton, there are causal factors that cannot be measured or are unknown but may affect safety. The Comparson-Group (C-G) method can be used to mprove upon the naïve approach, but a comparson group s not avalable for ths study because of the dffcultes nherent n dentfyng stes that match those of Gateway Monument locatons. For these reasons, the EB method was employed as t has been shown to have better performance than both the naïve and the C-G methods (Hauer, 1997), addressng problems assocated wth the naïve and the C-G methods (e.g., regresson-to-mean (RTM), whch s the potental for a hgh or low number of crashes to occur durng any gven year, but over tme, for such crashes to hover around a mean annual fgure), and approprate selecton of a before perod. Ths technque has been effectvely used n numerous traffc safety evaluatons (Persaud et al., 001; Persaud et al., 003; Bauer et al., 004; Persaud et al., 004; Mller et al., 006; Hadayeqh et al., 007; Patel et al., 007; Srnvasan et al., 008 ; Gross et al., 008 ; Alfonso, 009). The dfference between ths study and prevous studes s that Gateway Monuments are not lkely to be nstalled at locatons wth hgh crashes. Thus, the EB technque s not used to account for the effect of unusual hgh crashes n ths study. On the contrary, t could be used to account for the RTM effect assocated wth unusually low crashes. As dscussed by Park and Lord (010), RTM s a ubqutus phenomenon that occurs whenever ndvdual enttes or groups of enttes are measured at dfferent tme ponts n tme unless the successve
Ye, Venezano, and Lord 11 measurements at the dfferent tme ponts show a perfect correlaton wth each other. In other words, unless there s a perfect correlaton, whch s very rarely observed n practce, there wll always exst RTM n before-after studes. The curous reader s referred to Stgler (1997) and Chuang-Sten and Tong (1997) for further dscussons about the RTM phenomena. 5.1.Emprcal Bayes Approach Lke the naïve and the C-G study, the EB before-after procedure ncludes four steps for estmatng safety effects of a treatment (Hauer, 1997). Among the steps, an mportant task s to estmate the number of crashes n the after perod wthout the safety treatment ( ), or n ths case, had a monument not been bult. Ths secton manly ntroduces the EB method for the estmaton of. For more nformaton about the other steps, please refer to Hauer (1997). Two peces of nformaton are essental to the EB approach: the accdent hstory of an entty and knowledge about the safety of a group of untreated enttes wth smlar trats. The group of enttes wth smlar trats s referred to as the reference group. The purpose of a reference group s to capture the relatonshp between safety, traffc volume, and ste characterstcs (e.g., lane wdth, shoulder wdth). The model that depcts ths relatonshp can then be appled to the entty of nterest to estmate the assocated expected crash rate. To evaluate safety before and after the treatment for n enttes wth smlar trats, the expected number of crashes per year for those enttes ( u u B,..., u }) must be calculated usng a { 1 Bn Safety Performance Functon (SPF). These expected numbers are further combned wth observed crash counts ( y y,..., y }) n the before perod to obtan the expected number of { 1 n crashes per year ( z z,..., z } ) before the treatment (Gateway Monuments n ths case). In ths { 1 n study, the observed crashes for each entty and each year were avalable. However, AADT nformaton was only avalable for the mddle year of the before perod. Thus, the observed crashes durng the before perod for each entty ( y ) were used and the AADT was assumed to be the same for each year. Wth ths nformaton, z could be estmated by the followng equaton usng the EB technque (Hauer, 1997): z w1 * y (1 w1 ) * ub, 1,..., n () where: w1 ub /( ub k) ; k = nverse dsperson parameter (Posson-gamma model); and y = the average number of crashes per year durng the before perod. In ths case, y y / mb, where m B s the number of years durng the before perod. w 1 and ( 1 w1 ) determne the weghts for observed crash data ( y ) and the expected crash rate ( u B ). Ths equaton can be further denoted by: y k y / mb k z, 1,..., n (3) k / u B 1 k / u B 1 To calculate the expected number of crashes durng the after perod for a group of enttes (,..., }), a seres of factors ( r r,..., r }) are used to account for the dfferences n traffc { 1 n { 1 n volumes and other explanatory varables between the before and after perods. Factors are estmated by:
Ye, Venezano, and Lord 1 where: u B r, 1,..., n u A u A = The expected number of crashes durng the after perod for entty. The SPF s used to calculate u A usng data collected durng the after perod. Wth these, s computed by: r * ma * z, 1,..., n (5) where: m A = the number of years n the after perod. The varance of s calculated by: Var( ) r * m *( k y / m ) /( k / u 1) (6) Once A B n 1 n are obtaned, n Equaton 1 can be estmated ( 1,..., B (4) ) to evaluate safety before and after the treatment. Instead of calculatng the ndex of effectveness ( ) presented n Equaton 1, an approxmate, unbased estmate of was determned by the followng equaton (Hauer, 1997): ( / ) /(1 Var( ) / ) (7) The varance of was calculated by: [( Var( ) / ) ( Var( ) / )] Var( ) (8) [1 ( Var( ) / )] 5..Selecton of Reference Groups and SPFs As mentoned above, a reference group of enttes wth smlar trats are requred to use the EB technque for an entty (or a group of enttes). In the case of ths study, such reference groups were not avalable. Gven that numerous studes have been carred out to develop SPFs for road segments and ntersectons, exstng reference groups were chosen from the lterature, wth assocated SPFs appled to ths research. Based on the features present at gateway monument stes, SPFs for rural two-lane undvded hghway segments, rural four-lane undvded hghway segments, rural one-lane ramp segments, and urban four-legged sgnalzed ntersectons were selected. A lterature revew was conducted wth a focus on those four types of roadway confguratons. Over ten prevous studes conducted n North Amerca were dentfed for urban four-legged sgnalzed ntersectons SPFs (Bauer and Harwood, 000; Lord, 000; Lyon et al., 005; Persaud et al., 005; Harwood et al., 007; Sayed and Leur, 008) and rural two-lane and multple-lane undvded road segment SPFs (Vogt and Bared, 1998; Councl and Stewart, 1999; Persaud et al., 004; Lord et al., 008), and ramp segments SPF (Bauer and Harwood, 1997). Conceptually, the role of the monuments s foremost to convey a sense of place. As a result, they were evaluated n the context of the roadway envronment n whch they resde. Due to the varyng geometrc and traffc control envronments n whch the monuments resde, lumpng them together nto one category for analyss whle neglectng the contrbuton of the roadway envronment tself, would potentally muddle the pcture of the overall safety performance of each ste followng the addton of a monument.
Ye, Venezano, and Lord 13 In choosng approprate reference groups and SPFs, several factors need to be consdered. Frst, a study must provde SPFs and nverse dsperson parameters ( k ) to apply the methodology prevously dscussed. Second, gven the study locatons of ths research, reference groups n Calforna were preferred. Applyng SPFs to geographcal areas other than those used n developng the models requres Accdent Modfcaton Factors (AMFs) to convert predcted accdent frequency to local condtons. Thrd, studes usng more recent years of data are preferred because the expected accdent rate may be changed over a longer perod due to mprovements n traffc sgnal tmng, presence of new sgns and markngs, and other unobserved factors. As a result, the followng SPFs, developed for predctng total crashes (wthout crash severty), were selected for ths study and are summarzed n tables and 3: For the two rural two-lane undvded hghway segments, the study by Persaud et al. (004) was chosen. Persaud s study used data collected from 9 two-lane road segments n Calforna and other states from 003. The SPF developed from that work for total accdents s presented n Table 3. The model from the NCHRP (Natonal Cooperatve Hghway Research Program) study (Lord et al., 008) was appled to the two four-lane road segments n Nevada County and Tehachap. Ths model s presented n Table 3, wth data collected from 356 rural multple-lane hghway segments n Calforna. As some of the results from ths study wll be ncorporated nto Chapter 11 of the forthcomng Hghway Safety Manual (HSM), t s an deal canddate for applcaton to ths research on account of the HSM s ntenton to be appled across multple jursdctons natonally (Lord et al., 008; Nessner, 008). For the ramp segment, the SPF developed by the FHWA study was utlzed (Bauer and Harwood, 1997). As shown n Table 3, the data were collected from 737 ramps for 3 years n the State of Washngton. Gven the large sample sze and the number of varables (e.g., ramp type, number of lanes, ramp confguraton) ncorporated nto the model, applyng t to ths study was consdered reasonable. For the two urban four-legged sgnalzed ntersectons, two SPFs were selected from the studes of Persaud et al. (005) and Harwood et al. (007). The frst study used data from three ctes (San Francsco, El Cajon, and San Dego) n Calforna to develop models. The SPF that was developed by usng the San Francsco data set was chosen for ths study, as San Francsco s closer to study locatons than the other two ctes. The authors stated that the data collected from the three ctes n Calforna dd not have full reportng of PDO crashes. For ths reason, the NCHRP study by Harwood et al. (007) was also selected. Ths study used data collected from 111 stes n MN and NC to develop base models, whch were used to predct the safety performance of an ntersecton wth base values of varables (e.g., 1-feet lanes, 6-feet shoulders). Ths study also provded AMFs for geometrc desgn and traffc control varables to convert safety performance to local condtons. The AMFs for traffc control nclude left-turn lanes, left-turn sgnal phasng, rght-turn lanes, rght turn on red, and lghtng. The results from ths NCHRP study wll be ncorporated nto Chapter 1 of the forthcomng HSM (Nessner, 008). The SPFs from both studes were appled to ths research and results are compared as descrbed n the next secton.
Ye, Venezano, and Lord 14 Treated Locatons Rockln EB; Rockln WB Authors Persaud et al. (005) (31) * Harwood et al. (007) (3) Table : Reference groups and SPFs for urban four-legged sgnalzed ntersectons. Geographcal Areas El Cajon, CA San Dego, CA San Francsco, CA Mnnesota (MN) and North Carolna (NC) Perod or Duraton of Data Reference Group Sze 786 ste-months 53 1,896 stemonths,98 stemonths 1998-00 (MN) 1997-003 (NC) 54 5 111 Regresson Model Negatve Bnomal regresson Negatve Bnomal regresson e 3.95 ( F 1 SPF El Cajon: 0.53 F ) e *: PDO crashes underreported n the study data; may result n underestmaton of total crashes. **: Mnlane represents the number of left-turn lanes on the mnor road. ***: Base models for total accdents. AMFs were also provded to convert predcted accdent frequency to local condtons. (0.79*mnlane) ** Inverse Dsperson Parameter 5.63 San Dego: 4.64 0.53 (0.79*mnlane) 4.167 e ( F F ) e 1 San Francsco 4.477 0.53 (0.79*mnlane) 4.167 e ( F F ) e 1 Total sngle-vehcle***: 9.85 0.68 0.7 e ( F1 ) ( F ) Total multple-vehcle***: 10.63 1.07 0.3 e ( F ) ( F ) 1.778
Ye, Venezano, and Lord 15 Treated Locatons Wllow Creek; Paso Robles Rt 46 Nevada County; Tehachap Paso Robles US 101 No. of Lanes Two lanes Multple lanes One lane (ramp) Authors Persaud et al. (004) (36) Lord et al. (008) (37) Bauer and Harwood (1997) (38) Table 3: Reference groups and SPFs for rural undvded road segments. Geographcal Areas Perod of Data Collecton Reference Group Sze CA* 003 9 CA* N/A 357 Washngton 1993-95 737 Regresson Model Negatve Bnomal regresson Negatve Bnomal regresson Negatve Bnomal regresson e e L F SPF e 7.43 0.834 0.933 ( 0.036 shldwd terran* a) LF e 6.7469 1.198 ( 0.1905 LW 0.037 shldwd 0.1005INT ) e 9.81 ( F) 0.93 e (0.78* L 0.78( off _ ramp)) *: Data also collected from other states. **: a 0 for flat terran; a 0. 354 for rollng terran; a 0. 69 for mountanous terran ***: shldwd = average shoulder wdth; sfwd = surface wdth; LW = lane wdth; INT = number of ntersectons located on the segment Inverse Dsperson Parameter **.70 ***.107 1.47
Ye, Venezano, and Lord 16 6. Analyss and Results The EB before-after approach was appled to ths research wth those selected SPFs. To llustrate the calculaton of, the followng takes the rural four-lane undvded hghway segment at Nevada County (assumed entty no. 1) as an example. Usng the SPF from the NCHRP study (Lord et al., 008), the estmated numbers of crashes each year durng the before and after perod were: 6.7469 1.198 ( 0.1905LW 0.037shldwd 0.1005INT ) u e L( F ) e u B1 B1 0.001175* 0.4*(7500).36( crashes / year) A1 e 6.7469 L( F A1 0.001175* 0.4*(34000) ) 1.198 e 1.198 1.198 * e * e ( 0.1905*1 0.037*1 0) ( 0.1905LW 0.037shldwd 0.1005INT ) ( 0.1905*1 0.037*1 0) 3.00( crashes / year) Usng the EB approach, the expected number of crashes per year durng the before perod was: y1 k (5/ 3).107 z1 1.99( crashes / year) k / u B1 1.107 /.36 1 The expected total number of crashes that would occur durng the after perod had the Gateway Monument not been constructed was: 1 r1 * ma 1 * z1 (3/.36)*1.67 *1.99 4.3( crashes) The varance of s calculated by: Var( 1) r1 * ma 1 *( k y / mb ) /( k / u B 1) 3.35( crashes ) The same approach was appled to other enttes usng the approprate SPFs. The results of that analyss are presented n Table 4. EB estmates for ntersectons usng the NCHRP equatons (Harwood et al., 007) ncluded two SPFs, one for sngle-vehcle collsons and the other for multple-vehcle collsons. Correspondngly, the estmated crashes durng the after perod had the Gateway Monuments not been constructed were the sum of sngle- and multple-vehcle collsons. Four AMFs (left-turn lanes, rght-turn lanes, rght turn on red, and lghtng) were used to convert predcted crashes to local condtons (Harwood et al., 007). Unfortunately, nformaton about leftturn sgnal phasng was not avalable from any study ste. Instead, a conservatve mnmum AMF for left-turn sgnal phasng (0.937) was employed. For the two ntersectons, the results from usng the two dfferent reference groups show that the EB estmated crashes ( ) are sgnfcantly dfferent, wth 17. crashes (reference group from San Francsco, Calforna) and 3.9 crashes (reference group from Mnnesota and North Carolna) respectvely. Ths s due n part to the fact that the San Francsco study dd not fully nclude all PDO crashes, whch affected the calbrated SPF and further mpacted the estmates of and k. Wth fewer crashes used for the negatve bnomal regresson, could have been underestmated, whch could further underestmate z gven a fxed k. Consequently, n examnng the results, estmated crashes produced usng the SPF of Harwood et al. should be vewed as more accurate. In examnng estmated and observed crashes followng monument constructon, t s evdent that no deteroraton n safety occurred at any ste (wth the excepton of Rockln eastbound usng Persaud, et al. s SPF and Paso Robles U.S. 101). For rural two-lane stes, Wllow Creek experenced roughly the number of crashes that could be expected, as dd the Paso Robles Route 46
Ye, Venezano, and Lord 17 monument ste. For rural four-lane stes, Nevada County and Tehachap experenced fewer crashes than expected, wth dfferences of 3.3 and.0 crashes respectvely. The Paso Robles U.S. 101 experenced slghtly more crashes than the SPF predcted. Examnng the crashes at sgnalzed ntersectons, the lack of ncluson of PDO crashes by Persaud et al. s SPF becomes evdent n the Rockln eastbound results. Here, 15 crashes occurred, whle the SPF estmated 8.67. Snce the ste s an ext ramp from I-80 wth a hgh AADT (11,050), the majorty of crashes are PDO (rear-end) and unaccounted for by that SPF. The results generated by the comprehensve Harwood et al. model are consdered more accurate. Ths SPF estmated 16.63 crashes compared to seven observed at the Rockln westbound ste. For the Rockln eastbound ste, 16.9 crashes were estmated, whle 15 were observed. Based on these results, the observed crash hstory at the sgnalzed stes followng monument constructon was lower than what would be expected at comparable stes. Wth the estmates of, the before-after study usng the EB technque was conducted by followng the steps presented n Hauer (1997). Two scenaros were used n the before-after analyss. Each used the same EB estmated crashes for the rural two-lane and rural four-lane undvded hghway segments, but used dfferent results for the urban four-legged sgnalzed ntersectons, wth one usng the SPF from Persaud et al. (005) and the other from Harwood et al. (007). These results are shown n Table 5, where and are the summed crashes for all seven stes. From the table, nether scenaro shows an ncrease n crashes followng the constructon of gateway monuments. On the contrary, reductons n the number of crashes were dentfed. The values were 0.978 and 0.680, respectvely, whch correspond to. percent and 3.0 percent reductons n traffc crashes. Overall, the before after study wth the EB technque showed that the gateway monument stes dd not deterorate traffc safety.
Ye, Venezano, and Lord 18 Type of Entty Locaton Resource of SPF Rural two-lane undvded segment Wllow Creek Paso Robles Rt 46 Rural four-lane undvded Nevada County segment Tehachap One-lane offramp segment Paso Robles US 101 Urban fourlegged sgnalzed ntersecton Persaud et al. (004) Lord et al. (008) Table 4 Emprcal Bayes analyss results. Bauer and Harwood (1997) Observed Crashes durng the Before Perod ( y ) EB Estmated Crash Rate durng the Before Perod ( z ) Observed Crashes durng the After Perod ( ) EB Estmated Crashes durng the After Perod ( ) Varance of 0.74 1 1.6 0.600 0 1.55 4 5.4 13.57 5 1.99 1 4.3 3.350 3 0.9 1 3.00.76 3 0.85 6 3.36 3.64 Rockln WB Persaud et al. 1.60 7 8.53 10.379 Rockln EB (005)* 11.71 15 8.67 11.106 Rockln WB Harwood et al. 1 4.50 7 16.6 0.337 Rockln EB (007) 11 4.63 15 16.30 0.697 Table 5 Before-after study results. Statstcs Scenaro 1* Scenaro ** EB estmated crashes 34.5 50.0 Varance ( ) of estmated crashes 44.605 64.154 Observed crashes 35 35 Varance { } of observed crashes 35 35 Crash Reducton 5.5 15.0 Varance ( ) of crash reducton 79.605 99.154 Index of Safety Effectveness 0.978 0.680 Varance ( ) of Index of Safety Effectveness 0.059 0.04 *: The SPF from the study by Persaud et al. (005) was used for ntersectons. **: The SPFs from the study by Harwood et al. (007) were used for ntersectons.
Ye, Venezano, and Lord 19 7. Dscusson So far, the safety evaluaton of Gateway Monuments has employed the EB technque for ts advantages n accountng for the RTM among others. However, the reader should be aware that there are ssues that could affect the valdty of EB results. The most mportant ssue les n the use of reference groups and assocated SPFs. Ths study dentfed several SPFs from exstng studes and chose the best ones for crash predcton. Reference groups from the same jursdcton are preferred snce they are more representatve of the study area. In the case that a reference group s not avalable from the same jursdcton, the HSM methodology s recommended through the use of AMFs to convert predcted safety to local condtons. To valdate the performance of selected SPFs, further nvestgaton was conducted by usng the naïve before-after studes method (wth and wthout takng AADT nto account) and the results are presented n Table 6. In the naïve (wth AADT) method, the proportonal effect on the expected crash frequency n the after perod s assumed to be the same as the proportonal ncrease n AADT. It s generally perceved that crash frequency and traffc volume have a nonlnear decreasng slope relatonshp. Thus, the crashes n Column 8 could be overestmated gven a lnear relatonshp between crash and traffc volume, expect at Ste wth 0 crashes. For Stes 1, 3, 4, and 5, the EB crashes n Column 10 are reasonable snce they are smaller than those values n Column 8. Thus, usng reference groups from the same state (Calforna n ths case) produced antcpated results; usng the large reference group from Washngton state (Bauer and Harwood, 1997) to predct crashes at the ramp ste also generated reasonable results. For ste, the EB estmate s 5.4 crashes although there were 0 crashes n the before perod, whch could have been due to the RTM phenomenon (unusually low crashes n the before perod). For stes 6 and 7, the reference groups were from dfferent states and the EB crashes n Column 9 are hgher than those n Column 8. Assumng no RTM effect on these two stes, the EB method could have overestmated the crashes n the after perod. Fnally, by comparng columns 6 and 9, t s found that n all cases wth the expecton of stes 4 and 5, the number of crashes regressed upwards, confrmng that some stes experenced less crashes than ther long-term mean, as estmated by the EB method n the before perod. In summary, all of the results (n columns 11, 1, and 13) from the three methods (naïve method wthout AADT, naïve method wth AADT, and EB method) ndcate that Gateway Monuments dd not create negatve mpact on traffc safety. However, the use of reference groups from other jursdctons should be very carefully consdered when the EB method s appled for safety evaluatons smlar to that presented here.
Ye, Venezano, and Lord 0 Table 6 Comparson of results from dfferent methods Column 1 Col. Col. 3 Col. 4 Col. 5 Col. 6 Col. 7 Col. 8 Col. 9 Col. 10 Col. 11 Col. 1 Col. 13 Ste No Ste Name AADT (Bef.) AADT (Aft.) % AADT Change Before Crash After Crash After Crash (wth AADT) EB Crash (Bef.) EB Crash (Aft.) 1 Wllow Creek 3400 3300 -.94 1 1.94. 1.6 Paso Robles Rt 46 17000 0000 17.65 0 4 0.00 4.66 5.4 3 Nevada County 7500 34000 3.64 5 1 6.18 5.98 4.3 4 Tehachap 10100 10900 7.9 3 1 3.4.75 3 5 Paso Robles US 101 9600 1900 34.38 3 6 4.03.56 3.36 6 Rockln WB 7770 3900 18.47 1 7 14.1 13.5 16.6 7 Rockln EB 35050 39550 1.84 11 15 1.41 13.88 16.3 Study Results Naïve (wthout AADT) Crash Reducton Naïve (wth AADT) Total 36.00 35.00 4.01 45.55 50.19 1.00 7.01 15.19 Segments Only 13.00 13.00 15.39 18.17 17.7 0.00.39 4.7 Intersectons Only 3.00.00 6.6 7.38 3.9 1.00 4.6 10.9 EB Note: For stes 6 and 7, the AADT s the sum of major- and mnor-street AADTs. Segments nclude stes 1,, 3, 4, and 5; ntersectons nclude stes 6 and 7.
Ye, Venezano, and Lord 1 8. Concluson Ths paper has presented the results of research that nvestgated whether the presence of Gateway Monuments had a detrmental mpact on safety followng ther constructon. These features are free standng roadsde structures or sgnage that communcates the name of a cty, country or townshp to motorsts. To date, no specfc research has examned the mpact (f any) such features have on crashes. To determne whether these features mpacted safety, the before-after study method wth Emprcal Bayes technque was used, wth reference groups and SPFs adapted from exstng studes elmnatng the need to calbrate new models for the lmted sample avalable. Results ndcated that, on an ndvdual bass (usng Harwood et.al. s comprehensve ntersecton SPF), no deteroraton n safety was observed at any monument ste. When all stes were examned collectvely (usng ether sgnalzed ntersecton SPF), reductons n the total number of crashes were observed. The calculated ndex of effectveness ( ) values for ether scenaro were 0.978 and 0.680 respectvely, correspondng to. percent and 3.0 percent reductons n crashes. In addton to the EB method, naïve study methods (wth and wthout AADT taken nto account) were appled to the study data. Results (crash reductons) from these methods also showed that the presence of Gateway Monuments dd not have negatve mpact on traffc safety. However, the use of EB technque should be carefully consdered when adoptng reference groups from dfferent jursdctons, as these may affect the valdty of EB results. These results may not mean that monuments are a safety treatment; rather, they ndcate that ther presence was not detrmental on safety. In lght of these results, Caltrans may contnue at ts dscreton to partcpate n the Gateway Monument Program wth the knowledge that, for the stes examned n ths analyss, roadway safety was not mpacted. Whle ths research focused on monuments present along statecontrolled routes, smlar stes are located along local roads. The crtera by whch these stes were constructed may not be as rgorous as that employed by Caltrans, and so ther safety performance may dffer. In lght of ths, t s suggested that further research be undertaken to examne the safety performance of these stes n the future both n Calforna and n other states. Addtonally, t may be of nterest, partcularly wth respect to urban areas, to examne drver behavor versus monument placement n future efforts. Ths would lkely be facltated through a drver smulator study. Acknowledgements The authors wsh to thank the Calforna Department of Transportaton (Caltrans), ts Landscape Archtecture Program, and the Dvson of Research and Innovaton for the fundng of ths research. They also wsh to thank the project steerng commttee, Doug Brown, Lor Butler, Hanel Chung, Len Nelson, Don Howe, and Phlp Phan for ther nput to ths work. Addtonally, the authors thank Brad Boehm and Debbe Slva for ther assstance n the provson of crash data for each of the study stes. Fnally, the authors thank one anonymous revewer for hs or her very helpful feedback and nsghts.
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