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SAFETY EVALUATION OF PERMANENT RAISED PAVEMENT MARKERS SUMMARY S.1 BACKGROUND AND OBJECTIVE Permanent raised pavement markers (PRPMs) are delineation devices that are often used to improve preview distances and guidance for drivers in inclement weather and low-light conditions. Recent studies in New York, Texas, and Pennsylvania have raised concerns about the safety effects of PRPMs after potential negative side effects were reported. These studies pertained to single jurisdictions only, and their results were questioned because of some identified data and methodological difficulties. NCHRP Project 5-17 responds to the need to use state-of-the-art analytical methods and exten- sive data to comprehensively assess the safety effects of PRPMs and to identify criti- cal design parameters. The primary objectives of NCHRP Project 5-17, as presented in the project statement, are "to assess the safety effects of permanent raised pavement markers (PRPMs) and to develop guidelines for their use." An empirical Bayesian before-and-after safety evaluation methodology was selected to address methodological issues and challenges associated with previous efforts under- taken to evaluate PRPMs. The methodological issues and challenges (e.g., regression to the mean and traffic volume changes) were identified during a comprehensive liter- ature review of previous PRPM evaluation studies. In total, 29 states were surveyed by iTRANS and assessed for possible inclusion in the study. Six states were selected on the basis of their ability to provide the necessary crash, traffic volume, roadway attribute, and PRPM installation data required to perform the evaluation study. This study gath- ered data to evaluate the effects of snowplowable PRPMs on nonintersection crashes on a representative sample of two-lane roadways, four-lane expressways, and four-lane freeways in the states of Illinois, Missouri, Pennsylvania, New York, Wisconsin, and New Jersey. Because of data-intrinsic constraints that were proved impossible to over- come despite all efforts by the research team, it was not viable to perform a sound safety assessment of the effect of PRPMs on four-lane expressways. S.2 STUDY METHODOLOGY Safety performance functions (or crash prediction models) were developed for var- ious crash types: total, fatal and injury, nighttime, nighttime fatal and injury, daytime,
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2 daytime fatal and injury, wet weather, dry weather, and guidance-related. These safety performance functions (SPFs) served as a statistical tool to determine the overall effec- tiveness of PRPMs for particular crash types at the PRPM treatment locations. Further disaggregate analysis, using regression techniques, investigated the rela- tionship between the effect of PRPMs on nighttime crashes and various roadway, traf- fic, and PRPM design factors. The purpose of this disaggregate analysis was to deter- mine some of the specific conditions under which PRPMs are effective in reducing crashes or vice versa. S.3 DISCUSSION OF RESULTS The analysis showed that the nonselective implementation of PRPMs on two-lane roadways, overall, does not significantly reduce total or nighttime crashes, nor does it significantly increase these crash types. On the other hand, for those locations where PRPMs were implemented on the basis of selective policies (i.e., poor crash history, among other criteria), the analyses produced mixed results. Positive effects were found in New York for total and nighttime crashes where PRPMs were installed at locations selected on the basis of the wet weather nighttime crash history. Similar safety effects were not found in Pennsylvania, where PRPMs were implemented at locations selected on the basis of total nighttime crash history. The analysis results have also revealed that selective implementation of PRPMs requires a careful consideration of traffic volumes and roadway geometry (degree of curvature). At low volumes (where the annual average daily traffic [AADT] is less than 5,000 vehicles per day [veh/day]), PRPMs can in fact be associated with a negative effect, which is magnified by the presence of sharp curva- ture. For example, for PRPMs installed on roadways with AADTs ranging between 5,000 and 15,000 veh/day and with a degree of curvature greater than 3.5, an increase of nighttime crashes of 26 percent can be estimated from the model. Overall, the installation of PRPMs at noninterchange locations on four-lane freeways showed neither a positive nor a negative overall safety effect on total and nighttime crashes. However, some significant reductions were recorded for wet weather crashes at locations on four-lane freeways, and there are indications that PRPMs are only effec- tive in reducing nighttime crashes where the AADT exceeds 20,000 veh/day. The results from the disaggregate analyses were used to develop guidelines for the use of snowplowable PRPMs for two-lane roadways and four-lane freeways. The guidelines are based on a two-step procedure. First, the expected safety benefit after the installation of PRPMs is determined in relation to the expected reduction in future nighttime crashes. Second, a positive expected safety effect is followed by an analyti- cal engineering procedure relying on safety performance functions or crash prediction models for roads with and without PRPMs to determine the potential cost-effectiveness of implementing PRPMs at a location. The guidelines were discussed in the context of the present "Manual on Uniform Traffic Control Devices" (MUTCD), and modifica- tions are proposed for future editions.