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Safety Evaluation of Permanent Raised Pavement Markers (2004)

Chapter: Chapter 7 - Conclusions

« Previous: Chapter 6 - Guidelines for the Use of Snowplowable PRPMs
Page 51
Suggested Citation:"Chapter 7 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2004. Safety Evaluation of Permanent Raised Pavement Markers. Washington, DC: The National Academies Press. doi: 10.17226/13724.
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Page 51
Page 52
Suggested Citation:"Chapter 7 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2004. Safety Evaluation of Permanent Raised Pavement Markers. Washington, DC: The National Academies Press. doi: 10.17226/13724.
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Page 52

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51 CHAPTER 7 CONCLUSIONS This research study investigated the safety performance of snowplowable PRPMs on a representative sample of two-lane roadways and four-lane freeways in six states: Pennsylvania, Illinois, Missouri, Wisconsin, New Jersey, and New York. Although data were also collected for four-lane divided expressways, intrinsic data issues did not permit performing a sound safety evaluation of PRPM installations for this road- way type. The study assessed the impact of PRPMs on non- intersection-related crashes only. The PRPMs on four-lane freeways and two-lane roadways in New Jersey and Illinois were installed nonselectively (i.e., crash history was not a criterion in deciding where to imple- ment PRPMs). The PRPMs on two-lane roadways in New York and Pennsylvania were installed selectively (i.e., crash history was the basis for deciding where to implement PRPMs). Two sets of analyses were undertaken to investigate the safety impact of snowplowable PRPMs on nonintersection crashes. First, composite analyses based on the empirical Bayes before-and-after procedure were used to assess the overall impact of PRPMs on different crash types. Second, the results of the nighttime composite analysis for individual sites were used to conduct a disaggregate univariate analysis, and multivariate regression techniques were used to deter- mine the circumstances under which PRPMs are beneficial to safety. The results of these analyses were used to support the development of guidelines for the use of PRPMs. The findings that are based on the composite analysis indi- cate that the nonselective implementation of PRPMs on two- lane roadways does not significantly reduce total or nighttime crashes, nor do PRPMs significantly increase these crash types. Selective implementation policies, however, produced mixed results. Positive effects were found in New York for total, nighttime, and wet weather 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 loca- tions selected on the basis of total nighttime crash history. Improved delineation resulting from the implementation of PRPMs impacts two types of driver behavior that will affect safety at night and in poor visibility conditions: lane control and speed control. The human factors review found that drivers tend to move away from delineation measures, such as PRPMs. Thus, on two-lane roadways with centerline PRPMs, drivers will move away from the centerline toward the shoulder. While this behavior may reduce the incidence of opposing direction (e.g., head-on) crashes, it may increase run-off-road crashes, especially on roads with lower design standards (i.e., with narrow and/or gravel shoulders). The disaggregate analysis found that PRPMs are less effective on roadways with lower traffic volumes. This is likely due to the lower design standards (e.g., narrower lanes, narrower shoul- ders, etc.) associated with low-volume roads. The human fac- tors review also found some evidence that PRPMs may cause drivers to increase their speeds. Speed increases at locations where drivers already operate close to the margin of safety (e.g., sharp curves) may result in an increased number of crashes. The disaggregate safety analysis in this study con- cluded that PRPMs on sharp curves with a degree of curva- ture exceeding 3.5 may cause an increase in nighttime non- intersection crash frequency on two-lane roadways. The composite analysis of four-lane freeways concluded that PRPMs resulted in small, nonsignificant changes in total crashes in Missouri and New York and a small significant decrease in these crashes in Pennsylvania. Some statistically significant reductions were recorded for a few crash types. For example, significant decreases in wet weather crashes were found for the Missouri and Pennsylvania installations. The disaggregate analysis concluded that PRPMs are only effective in reducing nighttime crashes where the AADT exceeds 20,000 veh/day. Guidelines for the use of PRPMs have been developed using the results from the disaggregate analysis. An analytical engineering procedure using SPFs for roadways with and without PRPMs has been developed to determine the expected cost-effectiveness of installing PRPMs at a specific location. This procedure is presented as a benefit-cost tool for the agency to apply when considering PRPM installations. This would allow PRPM installation projects to be compared with other potential safety initiatives. Certain modifications to the MUTCD (3) have been pro- posed on the basis of the expected safety impact of PRPMs. An analytical engineering procedure has also been provided to establish the benefit-cost ratio of using PRPMs. The research team recommends future research studies to acquire knowledge about the safety impact of conventional PRPM installations on all roadway types, snowplowable PRPM installations on other roadway types (e.g., undivided

four-lane roadways, divided expressways, multilane facili- ties), and intersections and interchanges. It is also highly recommended that a prospective study be conducted to investigate how the presence of snowplowable PRPMs, under different roadway and PRPM design condi- tions, influence a driver’s choice of an appropriate travel speed and lane position. This type of research would provide infor- mation that would explain the seemingly counterintuitive findings that PRPMs are less effective on roadways with a higher degree of curvature and lower roadway design stan- dards. Some questions to be contemplated in the future are the following: • Does speed increase relatively more with PRPMs on small compared with larger radii curves? 52 • Does speed increase relatively more in wet as compared with dry conditions with PRPMs? • Does speed increase at night but not during the day with PRPMs? • Is centerline milling more effective than PRPMs in reducing lane encroachments without increasing speeds at night? • Are speed increases at night less pronounced on road- ways with illumination? • Do snowplowable PRPMs contribute to improved visi- bility of delineation and changes in lane placement or speed during the day in dry and wet conditions? • Do snowplowable PRPMs provide auditory feedback of centerline and lane line crossing that is noticeable to the average driver?

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 518: Safety Evaluation of Permanent Raised Pavement Markers examines the safety performance of snowplowable permanent raised pavement markers on two-lane roadways and four-lane freeways.

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