Cost Effective Safety Improvements for Two-Lane Rural Roads

Fnal Techncal Report TNW2008-04 Research Project Agreement No. 61-2394 Cost Effectve Safety Improvements for Two-Lane Rural Roads Ynha Wang Assocate Professor Ngan Ha Nguyen Graduate Research Assstant Atl Björn E. Levy Yao-Jan Wu Graduate Research Assstant Graduate Research Assstant Department of Cvl and Envronmental Engneerng Unversty of Washngton Seattle, Washngton 98195-2700 A report prepared for Transportaton Northwest (TransNow) Unversty of Washngton 135 More Hall, Box 352700 Seattle, Washngton 98195-2700 n cooperaton wth U.S. Department of Transportaton Federal Hghway Admnstraton March 2008

TECHNICAL REPORT STANDARD TITLE PAGE 1. REPORT NO. TNW2008-04 4. TITLE AND SUBTITLE Cost Effectve Safety Improvements for Two-Lane Rural Roads 2. GOVERNMENT ACCESSION NO. 3. RECIPIENT S CATALOG NO. 5.REPORT DATE February 2008 6. PERFORMING ORGANIZATION CODE 7. AUTHOR(S) Ynha Wang, Ngan Ha Nguyen, Atl Björn E. Levy and Yao-Jan Wu 9. PERFORMING ORGANIZATION NAME AND ADDRESS Transportaton Northwest Regonal Center X (TransNow) Box 352700, 129 More Hall Unversty of Washngton Seattle, WA 98195-2700 12. SPONSORING AGENCY NAME AND ADDRESS Unted States Department of Transportaton Offce of the Secretary of Transportaton 400 Seventh St. S.W. Washngton, D.C. 20590 8. PERFORMING ORGANIZATION REPORT NO. TNW2008-04 10. WORK UNIT NO. 11. CONTRACT OR GRANT NO. DTRS99-G-0010 13. TYPE OF REPORT AND PERIOD COVERED Fnal Research Report 14. SPONSORING AGENCY CODE 15. SUPPLEMENTARY NOTES Ths study was conducted n cooperaton wth the Unversty of Washngton and the Washngton State Department of Transportaton ABSTRACT Traffc accdents cause loss of lfe and property. Proper dentfcaton of accdent causal factors s essental for composng countermeasures aganst traffc accdents and reducng related costs. However, two-lane rural roads have dstnctve roadway characterstcs compared wth other types of roads. In order to fnd cost-effectve countermeasures and prortze roadway safety mprovement plans for two-lane rural roadways, a better understandng of the relatonshp between accdent rsk and respectve characterstcs s necessary. Ths study focuses on accdent analyss of two-lane rural roads n Washngton State. Sx representatve state routes (SRs), SR-2, SR-12, SR-20, SR-21, SR-97 and SR-101, are selected as study routes based on ther locaton, length, and geometrc characterstcs. Along wth the sx-year (1999~2004) accdent data from the Hghway Safety Informaton System (HSIS), roadway vdeo mage data and geographcal nformaton system data retreved from Washngton State Department of Transportaton are employed n ths study. Econometrc modelng methods are utlzed to dentfy accdent causal factors and evaluate ther mpacts on accdent rsk at roadway segments and ntersectons, respectvely. Results from the statstcal analyses and accdent rsk models not only help dentfy accdent causal factors, but also provde valuable nsghts for developng countermeasures aganst two-lane rural road traffc accdents. 17. KEY WORDS Two-lane rural roads, safety, accdent modelng 19. SECURITY CLASSIF. (of ths report) None 20. SECURITY CLASSIF. (of ths page) None 18. DISTRIBUTION STATEMENT 21. NO. OF PAGES 113 22. PRICE

Cost Effectve Safety Improvements for Two-Lane Rural Roads DISCLAIMER The contents of ths report reflect the vews of the authors, who are responsble for the facts and accuracy of the data presented heren. Ths document s dssemnated through the Transportaton Northwest (TransNow) Regonal Center under the sponsorshp of the U.S. Department of Transportaton UTC Grant Program. The U.S. Government assumes no lablty for the contents or use thereof. The contents do not necessarly reflect the vews or polces of the U.S. Department of Transportaton. Ths report does not consttute a standard, specfcaton, or regulaton.

Cost Effectve Safety Improvements for Two-Lane Rural Roads TABLE OF CONTENT DISCLAIMER... TABLE OF CONTENT... TABLE OF TABLES... v EXECUTIVE SUMMARY... v CHAPTER 1: RESEARCH BACKGROUND... 1 1.1 INTRODUCTION... 1 1.1.1 Research Background... 1 1.1.2 Research Objectve... 3 1.2 STATE OF THE ART... 4 CHAPTER 2: STUDY ROUTES AND DATA... 16 2.1 DATA COLLECTION PROCESS... 16 2.2 ROUTES SELECTION... 17 2.3 ROUTE DESCRIPTION... 19 CHAPTER 3: RESEARCH SCOPE AND METHODOLOGY... 20 3.1 RESEARCH SCOPE... 20 3.2 METHODOLOGY... 20 3.2.1 Data Management... 20 3.2.2 Data Organzaton... 20 3.2.2.1 Data for Roadway Segments... 20 3.2.2.2 Intersecton Data... 23 3.2.3 Database Desgns... 24 3.2.3.1 Roadway Segments... 24 3.2.3.2 Intersectons... 25 3.2.4 Attrbutes Explanaton... 27 3.2.5 Hypothess Test... 34 3.2.6 Accdent Rsk Modelng... 34 3.2.6.1 Statstcal Model Overvew... 35 3.2.6.2 Posson Regresson Model... 36 3.2.6.3 Negatve Bnomal (NB) Regresson Model... 38 3.2.6.4 Testng for Over-Dsperson... 39 3.2.6.5 Zero-Inflated Posson and Negatve Bnomal Regresson Models... 40 3.2.6.6 Model Estmaton... 42 3.2.6.6.1 t-statstc... 42 3.2.6.6.2 Elastcty... 45 3.2.6.7 Maxmum Lkelhood Estmaton Method... 46 3.2.6.8 Goodness of Ft Measures... 47 CHAPTER 4: DATA ANALYSIS... 50 4.1 NON-PARAMETRIC ANALYSIS... 50 4.1.1 Roadway Segments... 50 4.1.2 Intersectons... 59 4.2 STATISTICAL ANALYSIS... 68 4.2.1 Roadway Segments... 68 4.2.1.1 Tested Varables... 68 4.2.1.2 t-test... 70

Cost Effectve Safety Improvements for Two-Lane Rural Roads v 4.2.1.3 ANOVA... 71 4.2.2 Intersectons... 75 4.2.2.1 Tested Varables... 75 4.2.2.2 t-test... 77 4.2.2.3 ANOVA... 80 CHAPTER 5: ACCIDENT RISK MODELING... 83 5.1 INTRODUCTION... 83 5.2 ROADWAY SEGMENTS... 83 5.2.1 Parameter Estmaton for the All-Type Accdent Rsk Model... 83 5.2.2 Parameter Estmaton for the Rear-End Accdent Rsk Model... 86 5.3 INTERSECTIONS... 89 5.3.1 Parameter Estmaton for the All-Type Accdent Rsk Model... 89 5.3.2 Parameter Estmaton for the Strke-At-Angle Accdent Rsk Model93 CHAPTER 6: CONCLUSION AND RECOMMENDATION... 96 6.1 CONCLUSIONS... 96 6.1.1 Roadway Segments... 96 6.1.2 Intersectons... 97 6.2 RECOMMENDATIONS... 99 6.2.1 Roadway Segments... 99 6.2.2 Intersectons... 99 6.2.3 Modelng Approach... 99 REFERENCES... 100

Cost Effectve Safety Improvements for Two-Lane Rural Roads v TABLE OF FIGURE Fgure 1-1 Leadng causes of U-I deaths, U.S., 1969-2005... 1 Fgure 2-1 Map of sx Washngton State Routes used n the study... 18 Fgure 3-1 The E/R dagram for the RSA database... 24 Fgure 3-2 The E-R dagram for the SQL database... 26 Fgure 3-3 Defnton of degree of curvature... 28 Fgure 3-4 Rejecton of the null hypothess, H 0... 43 Fgure 3-5 The p-value for a two-taled test wth sgnfcance level, α=0.05... 44 Fgure 3-6 Lkelhood and log lkelhood functons for the Posson dstrbuton... 47 Fgure 4-1 Shares of accdent types on sx study routes... 51 Fgure 4-2 Shares of accdent types on SR-2... 52 Fgure 4-3 Shares of accdent types on SR-12... 52 Fgure 4-4 Shares of accdent types on SR-20... 53 Fgure 4-5 Shares of accdent types on SR-21... 53 Fgure 4-6 Shares of accdent types on SR-97... 54 Fgure 4-7 Shares of accdent types on SR-101... 54 Fgure 4-8 Average numbers of accdents per mle by route... 55 Fgure 4-9 Percentage of reported accdents by lghtng condton... 56 Fgure 4-10 Percentage of reported accdents by weather condton... 56 Fgure 4-11 Percentage of reported accdents by weekday... 57 Fgure 4-12 Percentage of reported accdents by month... 58 Fgure 4-13 Number of reported accdents by year... 58 Fgure 4-14 Shares of accdent types on sx study routes... 60 Fgure 4-15 Shares of accdent types on SR-2... 61 Fgure 4-16 Shares of accdent types on SR-12... 61 Fgure 4-17 Shares of accdent types on SR-20... 62 Fgure 4-18 Shares of accdent types on SR-21... 62 Fgure 4-19 Shares of accdent types on SR-97... 63 Fgure 4-20 Shares of accdent types on SR-101... 63 Fgure 4-21 Average number of accdents per ntersecton by route... 64 Fgure 4-22 Percentage of reported accdents by lghtng condton... 65 Fgure 4-23 Percentage of reported accdents by weather condton... 65 Fgure 4-24 Percentage of reported accdents by weekday... 66 Fgure 4-25 Percentage of reported accdents by month... 67 Fgure 4-26 Number of reported accdents by year... 67 Fgure 4-27 ANOVA test for effect of speed lmt on accdent rate... 71 Fgure 4-28 Accdent rate on segments wth dfferent curvy levels... 72 Fgure 4-29 Accdent rates on curvy segments wth dfferent speed lmts... 73 Fgure 4-30 Accdent rates on less curvy segments wth dfferent speed lmts... 73 Fgure 4-31 Accdent rates on straght segments wth dfferent speed lmts... 73 Fgure 4-32 ANOVA test for the effect of speed lmt changes on curved roadway segments on accdent rate... 74 Fgure 4-33 ANOVA test result for effect of gradaton on accdent rate... 75 Fgure 4-34 Impact of each varable on accdent rate n F-test... 82

Cost Effectve Safety Improvements for Two-Lane Rural Roads v TABLE OF TABLES Table 1-1 Average comprehensve cost by njury severty... 2 Table 4-1 Reported accdents on roadway segments of the sx study routes from 1999 to 2004... 50 Table 4-2 Reported accdents on ntersectons of the sx study routes from 1999 to 2004... 59 Table 4-3 Tested varables... 68 Table 4-4 t-test results for roadway segments... 70 Table 4-5 Tested varables... 76 Table 4-6 t-test results for ntersecton accdents... 78 Table 4-7 Informaton of the varables used n F-test... 80 Table 4-8 ANOVA results... 81 Table 5-1 Negatve bnomal estmaton results for roadway segment accdent rsk (all types)... 84 Table 5-2 Negatve bnomal estmaton results for rear-end accdent rsk... 86 Table 5-3 Negatve bnomal modelng results for ntersecton accdent rsk (all types) 89 Table 5-4 Goodness of ft value... 92 Table 5-5 Negatve bnomal modelng results for ntersecton strke-at-angle accdent rsk... 93 Table 5-6 Goodness of ft value... 95

Cost Effectve Safety Improvements for Two-Lane Rural Roads v EXECUTIVE SUMMARY Traffc accdents have been a huge fnancal burden on socety. Ther cost has not only been the pan and sufferng of the ndvduals nvolved n them but also the economc loss to socety. It s statstcally shown that the fatal accdent rate on rural hghways s more than twce as hgh as that for urban roads, even though the rate for all rural hghway accdents s barely half of that for urban hghways. Addtonally, though Washngton State s two-lane rural hghways account for only 25% of total yearly vehcle mles of travel, approxmately 56% of fatal and dsablng accdents occurred on these roads. The above statstcs clearly ndcate that traffc safety condtons on two-lane hghways need mprovement. The goal of ths study s to better understand rural roadway accdent causes n Washngton, n order to help fnd cost-effectve solutons for reducng the frequency and severty of crashes on rural two-lane roadways. To acheve such a goal, traffc accdent data, roadway geometrc data, traffc volume data, traffc control data, and related land use data from sx study routes are collected and analyzed. The sx study state routes (SRs), SR-2, SR-12, SR-20, SR-21, SR-97, and SR-101are consdered representatve to all state routes n Washngton. These sx routes are selected based on ther locaton, length, and geometrc characterstcs. A total of sx-year data from 1999 to 2004 are collected from multple sources, ncludng the Hghway Safety Informaton System (HSIS), roadway vdeo mage data (State Route Web), and geographcal nformaton systems (GIS) data retreved from Washngton State Department of Transportaton (WSDOT). Snce occurrence mechansm and casual factors are very dfferent between roadway segment and ntersecton accdents, ths project separated ntersecton accdents from roadway segment accdents for modelng and statstcal analyses. However, the methodologes used for the two groups of accdents are smlar. Statstcal analyses ncludng t-test and ANalyss Of VArance (ANOVA) are used to dentfy accdent causal factors. Statstcal models such as Posson regresson, negatve bnomal regresson, and zero-nflated Poson and negatve bnomal models are evaluated and appled to assess the

Cost Effectve Safety Improvements for Two-Lane Rural Roads v mpact of explanatory varables on accdent rsks. Results from the statstcal analyses and accdent rsk models provde valuable nsghts n developng cost-effectve solutons aganst roadway segment and ntersecton accdents on two-lane rural roads. For roadway segment accdents, we conducted regular statstcal analyses and quanttatvely evaluated the effects of explanatory varables on all-type accdent rsk (AAR) and rear-end accdent rsk (RAR). Based on the modelng and statstcal analyss results, cost-effectve measures that can be appled to reduce roadway segment accdent rsk are: Avod frequent speed lmt changes along the curvy roadway segments. Warn drvers before they enter a curved or steep roadway segment snce degree of curvature and grade have ncreasng effects on both AAR and RAR. Warnng sgns or other pavement-based warnng technques, such as pavement markers and rumble strps, can help reduce the rsk. Wden the surface wdth and add an addtonal passng lane n hgh accdent rate roadway segments. Wden shoulder wdth help reduce AAR but at the cost of ncreasng RAR. Remove roadsde curbs and walls. Smlarly, statstcal analyses and econometrc models were appled to ntersecton accdents. Based on the analyss results, cost-effectve measures that can be appled to reduce ntersecton accdent rsk are as follows: Lower speed lmt at ntersecton approaches. Put more sgns upstream of ntersecton to make drvers aware of the presence of ntersecton. Remove wall(s) at the nbounds of ntersectons. Increase shoulder wdth (greater than 6 feet) of ntersecton approaches. Keep shoulder wdths consstent along ntersecton sectons. Decrease the degree of curvature at ntersectons. Mnmze the change n slope between the nbound and outbound of an ntersecton.

Cost Effectve Safety Improvements for Two-Lane Rural Roads 1 CHAPTER 1: RESEARCH BACKGROUND 1.1 INTRODUCTION 1.1.1 Research Background Traffc accdents have been a huge fnancal burden on socety. Ther cost has not only been the pan and sufferng of the ndvduals nvolved but also the economc loss to socety. Accordng to statstcs provded by the Natonal Safety Councl (NSC, 2005), Motor-Vehcle accdents have been the leadng cause of unntentonal deaths n the Unted States from 1969 to 2005, as shown n Fgure 1-1. Fgure 1-1 Leadng causes of U-I deaths, U.S., 1969-2005 (Source: NSC, 2005) The Natonal Safety Councl estmates the average cost of motor-vehcle accdents each year, ncludng losses n wages, productvty, medcal expenses, motor-vehcle expenses, property damages, and employers unnsured costs (NSC, 2005). These costs reflect the mpact of traffc accdents on the naton s economy. They are a measure of the amount of money spent on and the loss of potental ncome caused by njury or fatal accdents (NSC, 2005). Ths measure can be used to consder how momentous traffc safety mprovement work should be. The calculable average comprehensve costs of motorvehcle accdent per njured person are estmated and shown n Table 1-1.

Cost Effectve Safety Improvements for Two-Lane Rural Roads 2 Table 1-1 Average comprehensve cost by njury severty Death $3,840,000 Incapactatng njury $ 193,800 Nonncapactatng evdent njury $ 49,500 Possble njury $ 23,600 No njury $ 2,200 Source: NSC, 2005 The above fgures cannot truthfully estmate the value of a person s natural desre to lve longer or to protect the qualty of one s lfe. However, they try to take nto account an objectve measure of the value of lost qualty of lfe based on the results from emprcal studes of people s wllngness to pay for safety mprovement. Therefore, mprovng traffc safety has been an mportant task as t not only releves the weghty mpact on socety fnancally caused by traffc accdents but also helps protect the qualty of people s lfe from beng affected or taken away by those accdents. Generally, accdent rate s defned as the number of accdent per mllon vehcle mles of travel. The fatal accdent rate for rural hghways was 1.32 and 1.43 respectvely for year 2004 and 2005 whereas that for urban hghway was 0.49 and 0.87 (WSDOT, 2004 and WSDOT, 2005). Ths mples that the average fatal accdent rate for rural hghways over the two years was more than twce as hgh as that for urban hghways. Addtonally, statstcs produced by Natonal Hghway Traffc Safety Admnstraton (NHTSA, 2004) show that 38.8% of total accdents and 74.9% of fatal accdents took place on U.S. twolane hghways. All these fgures ndcate that two-lane rural hghway accdents are much more severe than accdents on other types of roadways. In 2004, the total number of rural hghway accdent n Washngton State was 10,727. It reached to 11,215 accdents n 2005, whch s a 4.5% ncrease to that n 2004 (WSDOT, 2004 & WSDOT, 2005). Accordngly, accdent rate ncreased from 0.95 to 0.99 accdents per mllon vehcle mles of travel (WSDOT, 2004 & WSDOT, 2005). Although Washngton State s two-lane rural hghways account for only 25% of total yearly vehcle mles of travel, approxmately 56% of fatal and dsablng accdents occurred on these roads (Olson and Glad, 2004). These statstcs reflect a strong need for traffc safety mprovements on two-lane rural hghways.

Cost Effectve Safety Improvements for Two-Lane Rural Roads 3 Two-lane hghways have a unque feature of havng only one lane n each drecton; therefore, drvng behavors on these roads are dfferent from those on multple-lane roadways. It s rsky for a passng vehcle to occupy the opposng lane n order to pass a slow movng vehcle on a two-lane hghway, especally when the traffc volume n the opposng lane s hgh. It s even rsker when roadway geometrc features such as curvature, grade, etc. or roadsde objects constran the drver s lne-of-sght. Moreover, two-lane roadways have lmted space for vehcles that need to leave the road for emergency maneuvers. Roadway segments and ntersectons have ther own dstnct characterstcs; therefore, dfferent accdent rsk models should be developed for dfferent roadway locatons and also for dfferent types of accdents (Wang, 1998). Prevous studes often address safety ssues on mult-lane hghways. Ths study concentrates accdent analyss for both roadway segments and ntersectons on rural roads n Washngton State. 1.1.2 Research Objectve The goal of ths study s to better understand rural roadway accdent causes n Washngton, n order to help fnd cost-effectve solutons for reducng the frequency and severty of crashes on rural two-lane roadways. Specfcally, we have the followng objectves for ths research: Provde a better understandng of traffc accdents occurrng on rural two-lane roads; Model the relatonshps between major accdent types and causal factors quanttatvely; and Recommend dentfed controllable factors n developng cost-effectve solutons to mprove traffc safety on rural two-lane roads.

Cost Effectve Safety Improvements for Two-Lane Rural Roads 4 1.2 STATE OF THE ART The purpose of ths secton s to revew studes focused on traffc safety mprovement methods for hghways n general, not just lmted to those for two-lane rural hghways. More specfcally, ths secton covers some studes dedcated to traffc safety at ntersectons and roadway segments and a wde range of methods that have been used for accdent rsk modelng. Traffc accdents have a heavy fnancal mpact on socety, and also affect the qualty of lfe substantally. Improvng traffc safety has been an mportant task over the past decades; thus, there has been much research done tryng to fnd methods to reduce the frequency of accdents. Due to some of ther unque features, two-lane hghways are prone to fatal accdents. There have been many studes conducted to address ths problem. Most of the studes such as Polus and Mattar-Habb (2004) and Lamm et al. (2002) focused on fndng the relatonshp between geometrc features, speed, traffc condtons, envronmental characterstcs, and accdent rate. Other studes such as Persaud et al. (2004), Hckey (1997), and Washngton et al. (2002) compared data from before and after a countermeasure were mplemented to evaluate the effectveness of the countermeasure. Ftzpatrck et al (2002) performed a farly complete revew on crash treatment methods n Texas. It also dscussed low-cost safety treatments and ther effectveness. Accordng to Ftzpatrck et al. (2002), a crash study n Texas was conducted by followng the followng fve steps: dentfyng stes and crash characterstcs, gatherng exstng condtons, collectng addtonal feld data, assessng the stuaton and selectng treatments, and mplementng and evaluatng. The study also dentfed the types of treatment beng used on rural hghways ncludng rumble strps, passng mprovement, two-way left-turn lanes, lane or shoulder wdenng, pavement edge drop-off mprovements, pavement markngs, mowng, skd resstance mprovements, sde slope flattenng, recovery dstance mprovements, tree mtgaton, culvert modfcatons, advance warnng for horzontal curves, delneaton, barrer reflectors, and anmal countermeasures. Shoulder rumble strps were found effectve wth a relatvely low cost

Cost Effectve Safety Improvements for Two-Lane Rural Roads 5 that can reduce run-off-road crashes by 15 to 70 percent. Tree mtgaton was also found to reduce 22 to 71 percent of vehcle-tree crashes wth a relatvely moderate cost. Ftzpatrck et al. (2002) also dscussed some safety treatments for rural ntersectons such as advance warnng for ntersectons, approach rumble strps, left-turn bays, shoulder bypass lanes, ntersecton flashng beacons, sgnalzaton, hgh-ntensty strobe lghts, backplates on traffc sgnals, llumnaton, and sght obstructon reducton. Advance warnng for ntersectons were consdered havng relatvely low cost and effectve n reducng crashes at rural ntersectons. They mentoned that a Federal Hghway Admnstraton (FHWA) study found that addng left-turn bays, a treatment wth relatvely moderate cost, could reduce crash rate by up to 48 percent. Illumnaton was also consdered as a low-cost safety mprovement method and could reduce nghttme fatal, njury, and property-damage-only crashes by up to 65, 30, and 15 percent, respectvely. Persaud and Grffth (2001) provded a complete revew of current practce and research on statstcal methods n hghway safety analyss. They ponted out the essental types of safety analyss requred to support tradtonal hghway engneerng functons such as dentfcaton of unsafe locatons and development and assessment of countermeasures. The methodology used n ther partcular research was a survey of jursdctons wth hghway engneerng functons to assess current practces n hghway safety analyss. It also nvolved gatherng knowledge on the best avalable statstcal tools and contactng leadng researchers from twenty-seven state departments of transportaton n the Unted States and fve provncal transportaton departments n Canada to fnd the most recent research on hghway safety analyss (Persaud and Grffth, 2001). The four methods that were used among the partcpatng agences were dentfcaton of hazardous locatons (100 percent), before and after evaluatons (94 percent), cost-beneft analyss n development of countermeasures (85 percent), and analyss of collson trends (85 percent). Persaud and Grffth (2001) also dentfed current problems and ssues wth practces n hghway safety analyss. These problems are related to underreportng of

Cost Effectve Safety Improvements for Two-Lane Rural Roads 6 collsons, dentfyng comparson stes n before and after studes, nformaton on safety effectveness n developng countermeasures, approprate sklls and resources needed for safety analyss, ablty to lnk collsons and related databases. As for state of research, Persaud and Grffth (2001) affrmed that multvarate models were becomng popular n modern hghway safety analyss. To relate accdent experence to traffc and other roadway characterstcs, multvarate models were used as regresson equatons by Hauer (1997), and cted by Persaud and Grffth (2001). Persaud et al. (2004) conducted a before-and-after analyss on two-lane rural roadways n seven states before and after the nstallaton of centerlne rumble strps. Approxmately 210 mles of treated roads were analyzed n the study. The purpose of the study was to fnd an engneerng countermeasure for a major problem on the road nvolvng vehcles crossng the centerlne and ether sdeswpng or strkng the front ends of opposng vehcles (Persaud et al., 2004). Rumble strps nstalled along the centerlnes of undvded rural two-lane roads can help warn the dstracted, fatgued or speedng drvers not to cross the centerlnes and encroach on the opposng lane (Persaud et al., 2004). The study used Bayes emprcal before-after method to take nto account the regresson to the mean, n order to normalze the dfferences n traffc volume and other factors between the before-and-after perods. They concluded that the nstallaton of centerlne rumble strps helps reduce the hazard of frontal and sdewpe crashes based on the results of ther study, n whch a 14 percent reducton for all combned njury crashes and a 25 percent reducton for frontal and sdewpe njury crashes were observed. Also workng on a soluton to mprove traffc safety on two-lane rural roads, Ogden (1997) dd a study on the safety effect of pavng rural roads shoulders n Vctora, Australa. A before-and-after comparson, usng accdent data obtaned from two-lane roads that had recent shoulder-pavng projects, was carred out. The most common treatment for shoulder pavng program nvolved an nterm btumnous sealng treatment and a reseal n conjuncton wth a pavement reseal about one year later. Accordng to the result of the Ogden (1997), ths type of treatment for shoulder pavng can be statstcally-sgnfcant n reducng the frequency of njury and fatalty accdents on

Cost Effectve Safety Improvements for Two-Lane Rural Roads 7 two-lane rural hghways n Vctora. Specfcally, there was a 41 percent reducton n accdents per vehcle klometer. Ogden (1997) also specfed that the man accdent reductons were n rear end, overtakng, and off roadway to left and off roadway to rght nto fxed object accdents. The study also stated that ths safety mprovement method was a cost effectve method. The beneft to cost rato of shoulder pavng was estmated as 2.8 tmes the AADT n thousands (Ogden, 1997). For example, f the AADT at the locaton s 2000 vehcle per day, the beneft to cost rato of shoulder pavng should be estmated as 5.6. Agent and Pgman (2001) conducted a before-and-after constructon analyss on two-lane rural hghways n Kentucky to study the mpacts of constructon on the hghway safety. There were 49 roadway sectons used n the study, 25 of them were upgraded to four-lane roads whle the rest were realgned for wder lanes and shoulders. There was a dramatc ncrease n annual average daly traffc after the reconstructon on those roadway sectons. Accdent rates after the reconstructon were reduced sgnfcantly. For the sectons that were upgraded by wdenng lanes and shoulders, the crash rate was reduced by 51 percent, whereas for the sectons where lanes were added, there was a 56 percent reducton n the crash rate. Addtonally, there was also a sgnfcant reducton n njury or fatal crash rates, whch were reduced by 54 percent for realgned roads and 55 percent for upgraded roads. For both cases, the number of crashes per mle was reduced by 43 percent. Tsyganov et al. (2005) researched the safety mpacts of edge lnes on rural two-lane hghways n Texas State and performed general statstc analyss usng accdent data from Texas Department of Transportaton (TxDOT). The study nvolved the complaton of rural two-lane hghway data, examnaton of typcal characterstcs and dmensons of such roadways and used ths nformaton to carry out accdent statstcal analyss. Both roadway sectons wth and wthout edge lnes were ncluded n the research. In addton to roadway characterstc varables, other factors such as accdent type, ntersecton presence, lght condton, surface condton, severty, drver age, and drver gender were examned. The major results were that the presence of an edge lne may account for up to

Cost Effectve Safety Improvements for Two-Lane Rural Roads 8 a 26% reducton n accdent frequency and the effects are stronger on curved roadway sectons wth lane wdths of 9 to 10 ft. Also, a reducton n speed-related accdents was observed where edge-lne treatment had been gven to the road, a postve mpact from better drver paths and speed perceptons. In a study performed by Geurts et al. (2005), researchers looked at the rankng and selecton crtera of dangerous crash locatons n Flanders, Belgum. The underlyng assumpton of precedng rankng technque studes s that road accdents can be treated as random events, whch means that each accdent locaton has ts mean crash rate. Ths approach assumes that the Posson dstrbuton les behnd the occurrence of accdents, whch s wdely accepted n numerous studes. Often, rankng of crash locatons has been based on ths dstrbuton but wthout payng specal attenton to severty. In ther study, they nvestgated the dfference n results between the tradtonal rankng and an alternatve rankng crteron. The alternatve crteron gves weght to the severty of the crashes by usng herarchcal Bayesan approach. The approach takes nto account, for a specfc tme perod, the number of crashes, the number of fataltes, and the number of lght and severely-njured casualtes for each accdent ste. Results showed that the alternatve rankng crteron would change the selecton of dangerous accdent stes. It would lead to a dfferent selecton of 23.8% of a total of 800 stes. The study offers probablty plots that serve as a valuable tool for prortzng crash stes. The Bayesan rankng plots llustrate the estmated probablty for a certan roadway accdent locaton to be assocated wth the most dangerous stes. The authors recommended further research to nclude the constructon cost to mprove safety at dfferent locatons. If that were done, the rankng of locatons could have been carred out by balancng the costs and safety benefts aganst each other. Gårder (2005) analyzed head-on accdents n 2000-2002 that occurred on two-lane rural roads n Mane. The analyss, whch ncluded a total of 3136 reported head-on accdents, revealed that less than 8% of fataltes nvolved overtakng vehcles and only 14% of the accdents nvolved drvers who ntentonally crossed the centerlne. The accdent data showed that hgher speed lmts led to a hgher rsk of fatal accdents or ncapactatng

Cost Effectve Safety Improvements for Two-Lane Rural Roads 9 njures. The study concluded that, by keepng AADT and speed wthn certan lmts, the severty of head-on accdents can be mtgated through narrower shoulders. Accordng to Gårder, there are two man reasons that explan why drvers occupy the opposng traffc lane (and have head-on accdents): (a) vehcles are drven too fast for the roadway condtons and (b) vehcles occupy the opposng traffc lane unntentonally. Medan barrers on head-on accdent-prone roadway sectons were dscussed to reduce accdent rates n both categores. Because of the huge fnancal cost of the medan barrer nstallaton, rumble strps were recommended for the remanng roadway sectons, but of course rumble strps would only reduce rates n the latter category. The author also recommended a speed lmt reducton for targeted hgh-crash sectons and a more strct speed enforcement effort. The aforementoned methods are to some extent helpful n determnng approprate countermeasures; however, they cannot fully reflect the quanttatve mpact of each ndvdual causal factor on accdent frequency. Accdent rsk models must therefore be developed to evaluate countermeasures wth multple varables to consder. There have been several studes accomplshed usng lnear regresson, Posson regresson, and negatve bnomal regresson technques to model accdent rsks. Okamoto and Kosh (1989) used multnomal lnear regresson n ther study and found that the random error of ths method vared by the number of accdents and vehcleklometerage of the sectons. One major problem wth the lnear regresson model s that t may predct a negatve number of accdents, whle n real lfe accdent frequency always holds a non-negatve value. Posson regresson and negatve bnomal regresson models are the two models consdered to be more applcable for accdent modelng. Traffc accdent data are always dscrete, rare and non-negatve; thus, they ft the features of a Posson dstrbuton. Maou et al. (1992) used Posson regresson to model the truck accdent data collected from the Hghway Safety Informaton System (HSIS) of one state from 1985 to 1987. The data were assumed to be Posson dstrbuted. Unknown parameters were estmated usng the maxmum lkelhood estmator. Fnal results from

Cost Effectve Safety Improvements for Two-Lane Rural Roads 10 the modelng process showed that annual average daly traffc per lane, horzontal curvature, and vertcal grade were robustly correlated wth the truck accdent frequency. However, shoulder wdth was found to have lttle correlaton wth the truck accdent frequency. Because of the extra varaton n the truck accdent data and the lack of covarates, the model was consdered to have the potental to mprove. However, the researchers found that the mprovement would not sgnfcantly change the ntal fndngs. Maou et al. (1993) compared lnear regresson models and Posson models to determne ther sutablty for modelng vehcle accdents and hghway geometrc desgn relatonshps. The lnear regresson model was found to lack the dstrbutonal propertes to successfully descrbe random, dscrete, and non-negatve accdent data. Although, Maou et al (1993) concluded that Posson regresson models had the most approprate statstcal propertes n descrbng traffc accdent events, they also ponted out the lmtaton of Posson model. Real accdent data rarely has ts varance equal to ts means; nevertheless, the Posson regresson requres the varance of the data to be equal to the mean. If the varance of a data set s greater than the mean, the data set s consdered over-dspersed. Over-dsperson wll result n based coeffcents and flawed standard error f used for Posson regresson model. Maou et al. (1993) suggested usng negatve bnomal or double Posson dstrbutons as the soluton to overcome the problem of overdsperson. Shankar et al. (1995) later used negatve bnomal regresson to deal wth the overdsperson ssue. Both Posson model and negatve bnomal model were appled n ther study n whch the effects of roadway geometrc and envronmental factors on the frequency of freeway accdent were explored. They modeled both the overall rural freeway accdent frequency and the frequency of ndvdual accdent types such as rear end, sdeswpes, fxed objects, overturns, etc. Based on the study they concluded that usng negatve bnomal model would gve a better explanaton on the data set compared to the Posson regresson model and be more robust wth regards to over-dsperson.

Cost Effectve Safety Improvements for Two-Lane Rural Roads 11 Poch and Mannerng (1996) n dealng wth accdents at ntersectons also had the same opnon as Shankar et al. (1995). They also found that the negatve bnomal model performs better than the Posson model when dealng wth over-dspersed data. They used negatve bnomal regresson model to fnd the geometrc and traffc related factors that affect the accdent frequences at ntersectons. The data was collected from more than sxty ntersectons n Bellevue, Washngton. Four dfferent accdent-frequency models were estmated: total accdent frequency, rear-end accdent frequency, angle accdent frequency and approach-turn accdent frequency. One of ther fndngs was that the hgher accdent frequency mght be related to the ncreased left-turn traffc volume. Also, a greater number of opposng approach lanes s related to an ncrease n total accdents. One other nterestng fndng was that the ntersectons n the central busness dstrct (CBD) have a lower lkelhood of rear-end accdents. for the authors attrbute ths correlaton to the sgnal progresson n CBD areas, whch decreases the number of tmes the vehcles have to start and stop, thus decreasng the potental of rear-end accdents. Ivan et al. (2006) used negatve bnomal regresson for generalzed lnear models to evaluate the correlaton between roadway geometrc features and the ncdence of headon crashes on two-lane rural roads n Connectcut. Seven hundred and twenty roadway sectons, of the same length, were used n the analyss. Two varables based on the curvature of the road segments, one varable based on the vertcal grade of the segments, and speed lmt had sgnfcant nfluence on head-on crashes. Three models were developed nvolvng dfferent combnatons of the above-mentoned varables. The models suggested that the three geometrc varables caused an ncrease n the number of crashes but varables such as lane and shoulder wdth were not found to nfluence the occurrence of head-on-crashes. Sgnfcant correlaton was found between wet roadway surface and more severe head-on-crashes and the same apples to the latter and narrow road segments. Wang et al (2003) studed the relatonshp between the rear-end accdent frequency and the combnaton of lead-vehcle deceleraton and the neffectve response of the followng vehcle s drver to ths deceleraton. In ths paper, accdent probablty was expressed as

Cost Effectve Safety Improvements for Two-Lane Rural Roads 12 the product of the probablty of the leadng vehcle deceleratng and the probablty of the followng vehcle falng to respond n tme to avod a collson. Informaton on traffc flow, traffc regulatons, roadway geometrcs, and human factors from over one hundred four-legged sgnalzed ntersecton n Tokyo, Japan were used to model rear-end accdent probabltes. An nterestng fact about speed lmt was dscovered from the result of the modelng process. Speed lmt has a postve mpact on the probablty of encounterng an obstacle vehcle but t has negatve mpact on the probablty of a drver falure. Because dual mpacts of the explanatory varables had not been accounted for n prevous research, ths fndng was one of the major hghlghts of ths work. Applyng the mcroscopc approach developed by Wang (1998), Km et al. (2007) bult a model on the occurrence of rear-end accdents on mult-lane freeways. The probablty of encounterng an obstacle vehcle and the probablty of drver s reacton falure were estmated n ths model. The fnal model nvolved both human and non-human factors by ncorporatng the two probabltes together. They found that both the AADT and the truck percentage-mle-per-lane varables have dual mpacts on the occurrence of freeway rear-end accdents. These two varables ncrease the probablty of encounterng an obstacle vehcle but decrease the probablty of drver falure. Negatve bnomal regresson was also statstcally proven to be the rght approach for modelng freeway rear-end accdents. Ten sgnfcant varables such as area type, speed lmt, shoulder wdth etc. were found to have an effect on the accdent frequency n the modfed negatve bnomal model. Vogt and Bared (1998) conducted a study on safety analyss on segments as well as on three- and four-legged ntersectons of rural two-lane roads n assocaton wth the development of the Interactve Hghway Safety Desgn Model (IHSDM) whch s a set of tools to help hghway desgners. The data for two States, Mnnesota and Washngton, used n the study ncluded accdent data (both severty and type), traffc data, lane and shoulder wdth data, and some algnment data collected from HSIS. Data were also obtaned from photologs and constructon plans. Posson and negatve bnomal regressons were used for the three-legged and four-legged ntersecton modelng. The

Cost Effectve Safety Improvements for Two-Lane Rural Roads 13 fnal model chosen was a negatve bnomal model for Mnnesota data. The Washngton data was not used for the fnal model because of ts unrelablty. The Posson models were not chosen because of data over-dsperson ssues. Some of the fndngs from the study were that drveways seemed to decrease accdents at three-legged ntersectons; that roadsde hazards seemed to decrease accdents at four-legged ntersectons; a major road rght turn lane seemed to ncrease accdents at three-legged ntersectons, and the angle effect vares from state to state and from three-legged to four-legged ntersectons. One of the most sgnfcant ponts n ths study was that t ponted out the dfference between the ntersecton models and segment models. Intersecton models are based on fewer observatons than the segment models and thus the relatonshps between accdent frequency and ntersecton varables were not as clear-cut (Vogt and Bared, 1998). For ths reason, p-values for ntersecton models should be allowed to have a much greater range than for segment models n order to dentfy the desgn varables that nfluence accdents and can be controlled by the desgner (Vogt and Bared, 1998). The p-values used n some models have a value of 30%. An examnaton of zero-altered probablty processes, ZIP dstrbuton and ZINB dstrbuton, were ncluded n a study carred out by Shankar et al. (1997). They used a countng process n order to dstngush roadway sectons that can be evaluated as truly safe from those that can be evaluated as unsafe. The safe sectons have accdent lkelhood close to zero but the unsafe sectons can happen to have zero accdent observatons durng some pre-defned tme perod. They clamed that ths countng process works better than applcatons of Posson and negatve bnomal accdent frequency models snce they do not account for ths dstncton and can therefore produce based coeffcent estmates when zero accdent observatons preval. The authors suggested that the ZIP structure models were promsng n terms of the capablty of revealng roadway sectons wth zero accdents observatons. Lee and Mannerng (2002) studed run-off roadway accdents on a 96.6-km secton of hghway n Washngton State. The study combned a number of databases ncludng a detaled database on roadsde characterstcs. They employed zero-nflated count models

Cost Effectve Safety Improvements for Two-Lane Rural Roads 14 and nested logt models to estmate accdent frequency and severty. Both emprcal and methodologcal analyses were used to establsh the relatonshps among roadway geometrcs, roadsde characterstcs, and severty of run-off roadway accdent frequency. The purpose of the study was to dentfy the cost-effectve countermeasures that can be used to mprove hghway desgns and hghway safety. Treatments for roadsde mprovement recommended by the study nclude avodng cut sde slopes, decreasng the dstance from the outsde shoulder edge to the guardral, decreasng the number of solated trees along road-way sectons, and ncreasng the dstance from outsde shoulder edge to lght poles. The lmtaton of the study s that t was based solely on the run-offroadway accdents n the northbound drecton of SR 3 n Washngton State. Chayanan et al. (2003) explored the relatonshp between roadway and roadsde accdent rates for Washngton State hghways. They beleved that the two accdent rates for a gven roadway secton can be correlated though geometrc, traffc, and envronmental factors may have dfferent effects on roadway and roadsde accdent rates. Ths correlaton s due to unobserved effects common across the roadway and roadsde (Chayanan et al., 2003). They employed a logcal extenson of Classcal Lnear Regresson Ordnary Least Square (CLR-OLS) model called the Seemngly Unrelated Regresson Estmaton (SURE) model to systematcally approach roadway and roadsde accdent rate modelng. Accordng to Chayanan et al. (2003), the advantage of ths model s that there s nether an mposton of a pror assumptons on defnte lnkage between roadway and roadsde accdent rates nor hypothetcal support for such lnkage. Usng the SURE model makes t more effcent to estmate the parameters when dsturbances that lnk roadway and roadsde processes become sgnfcant (Chayanan et al., 2003). The study used a random sample of 500 one-mle sectons from the Washngton State hghway system for modelng. The data sets ncluded traffc data such as volumes, compostons, speeds, AADT, and so on, wth geometrc data such as lane, shoulder, medan, curve, and ntersecton nformaton. Hstorcal weather data collected from the Natonal Oceanc and Atmospherc Admnstraton database was also ncluded. The authors concluded that t would brng no sgnfcant effcency mprovements

Cost Effectve Safety Improvements for Two-Lane Rural Roads 15 compared to the current state of practce n Washngton State f the roadway and roadsde were modeled smultaneously. Also, weather varables were found to be sgnfcant n both the roadway and roadsde models. The authors, n addton, stated that data about sde slopes and lengths of guardral through-sectons are essental to mprove the explanatory capablty of the roadway and roadsde models. In concluson, the lnear regresson model was not found to be applcable to traffc accdent modelng due to ts lack of dstrbutonal propertes to accommodate traffc accdent data. Although the Posson model s frequently used n modelng traffc accdents, t cannot handle over-dspersed accdent data (Maou et al. (1993), Wang et al. (2003), Shankar et al. (1995), etc.). Ths ssue can be treated wth the negatve bnomal regresson model because t allows the varance of accdent data to be greater than the mean. For ths reason, negatve bnomal regresson models wll be used n ths study for modelng the accdent frequency at Washngton two-lane hghway ntersectons.

Cost Effectve Safety Improvements for Two-Lane Rural Roads 16 CHAPTER 2: STUDY ROUTES AND DATA 2.1 DATA COLLECTION PROCESS Data used n ths research was obtaned from three sources: HSIS, the WSDOT Offce of Informaton Technology, and the WSDOT onlne tool, State Route Web (SRweb). HSIS s a data collecton program that s operated by the Unversty of North Carolna Hghway Safety Research Center and the LENDIS Corporaton. It s supported by the Unted States Department of Transportaton (USDOT). Accordng to the Hghway Safety Informaton System Gudebook (Councl and Wllam, 2006), the Washngton database n the HSIS s mantaned by the Transportaton Data Offce (TDO) at WSDOT. HSIS receves data from the WSDOT TDO n the form of nne dfferent data fles ncludng accdent data, basc roadway nventory data, curve data, grade data, features data, roadway crossngs and roadsde facltes data, specal-use lane nformaton, ralroad grade crossng ndex, and traffc data. The data requested for ths research were extracted from those data fles. These requested data sets nclude accdent data fle, roadlog fle, curve fle, and grade fle. The accdent fle contans three subfles: the accdent subfle, the vehcle subfle, and the occupant subfle. The accdent data s collected statewde by all the Washngton State polce departments followng a standard format (Councl and Wllam, 2006). The roadlog, curve, and grade fles descrbe some basc characterstcs of each homogeneous roadway segment between begnnng and endng mleposts. Varables n the roadlog fles nclude surface wdth, lane wdth and type, shoulder wdth and type, medan nformaton, rural/urban codes, terran codes, and other roadway descrptors such as functonal class. Varables related to AADT and Legal Speed Lmt (SPD_LIMT) were extracted from other fles and merged nto the roadlog fles (Councl and Wllam, 2006). The curve fle contans varables related to angle, drecton, degree and radus, length, maxmum superelevaton, and legal speed lmt. Approxmately 70 to 80 percent of roadway sectons are straght segments and do not have ther degree of curvature and other varables lsted (Councl and Wllam, 2006). The grade fle contans nformaton on percent grade,

Cost Effectve Safety Improvements for Two-Lane Rural Roads 17 drecton, and length. The curve and grade fles nformaton was developed from constructon drawngs and straght-lne dagrams. The second data source s the WSDOT GeoData Dstrbuton Catalog, a WSDOT dstrbuton ste for roadway Geographc Informaton Systems (GIS) data. These GIS data sets nclude: ntersecton locaton, lane nformaton, Global Postonng Systems (GPS) route data, road log, etc. All data sets are mported nto ArcGIS software to be further processed. The ntersecton data requested from the WSDOT Offce of Informaton Technology were GIS data. Ths data set shows the locatons of ntersectons along Washngton state routes. The values of the data were obtaned from the Transportaton Informaton and Plannng Support (TRIPS) database. The last major data source s the SRweb, a WSDOT onlne tool. Ths web applcaton provdes a roadway snapshot every one hundredth of a mle on each state route. Some data, whch are not provded by the other two sources, can be collected manually by ths onlne resource. Moreover, SRweb can be used to examne the accuracy of the data obtaned from HSIS such as the shoulder wdth and shoulder type. 2.2 ROUTES SELECTION The data fles, as lsted below, obtaned from HSIS consst of sx separate spreadsheets for each of sx consecutve years from 1999 to 2004: Accdent Fle Occupant Fle Vehcle Fle Roadway Fle Curvature Fle Gradent Fle Ths research s meant to explore the relatonshp among accdent frequency, the roadway s geometrc features, and traffc nformaton at ntersectons and at roadway segments. Before the data s prepared for the database, t s necessary to lnk the roadway

Cost Effectve Safety Improvements for Two-Lane Rural Roads 18 fle and ntersecton fle to both the curvature and gradent fles based on the route and mlepost (MP) attrbutes. Moreover, as mentoned earler, some varables, such as the number of drveways and passng lane, have to be created by usng SRweb manually. The data collecton process was consdered too tme-consumng to explore all 141 state routes n Washngton State. Therefore, representatve routes were selected for the study. The two crtera used to select study routes were (1) route length and (2) the geographc locaton and spatal algnment of the routes. The frst factor to consder was route length. The selected routes have to be the ones that have suffcent lengths to be statstcally sgnfcant for modelng purpose and to be representatve geographcally. Geographcal locaton and spatal algnment s the other crteron and also a more substantal crteron for study route selecton. The selected routes have to cover as much area n Washngton State as possble to be geographcally representatve. After the selecton process, the sx chosen routes are SR-2, SR-12, SR-20, SR-21, SR-97, and SR-101. A GIS map showng the locaton of each route was developed and s llustrated n Fgure 2-1 below: Fgure 2-1 Map of sx Washngton State Routes used n the study