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

OCR for page 51
52 four-lane roadways, divided expressways, multilane facili- Does speed increase relatively more in wet as compared ties), and intersections and interchanges. with dry conditions with PRPMs? It is also highly recommended that a prospective study be Does speed increase at night but not during the day with conducted to investigate how the presence of snowplowable PRPMs? PRPMs, under different roadway and PRPM design condi- Is centerline milling more effective than PRPMs in tions, influence a driver's choice of an appropriate travel speed reducing lane encroachments without increasing speeds and lane position. This type of research would provide infor- at night? mation that would explain the seemingly counterintuitive Are speed increases at night less pronounced on road- findings that PRPMs are less effective on roadways with a ways with illumination? higher degree of curvature and lower roadway design stan- Do snowplowable PRPMs contribute to improved visi- dards. Some questions to be contemplated in the future are bility of delineation and changes in lane placement or the following: speed during the day in dry and wet conditions? Do snowplowable PRPMs provide auditory feedback of Does speed increase relatively more with PRPMs on centerline and lane line crossing that is noticeable to the small compared with larger radii curves? average driver?