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42 crashes (by 20 percent) in two-lane roadways in New York As with two-lane roadways, the implementation of PRPMs where locations were selected for PRPM installation on the on the lane line of freeways is expected to impact two types basis of their nighttime wet weather crash history. of driver behavior: Lane control and positioning and 5.1.2.5 Slight Decreases in Daytime Speed control. Wet Weather Crashes Snowplowable PRPMs may improve daytime visibility 5.2.1.1 Lane Control and Positioning under wet weather conditions because of the profile of the Increased delineation of the lane line is likely to cause PRPM housing above the film of water covering the painted drivers to stay better centered in lanes delineated on both sides. markings. This improvement in visibility might contribute to Where the lane line but not the edgeline is delineated, drivers a decrease in daytime wet weather crashes. are likely to position themselves farther from the delineated The safety composite analysis of the two-lane roadways in New York indicated a 20-percent reduction in all wet weather line toward the edgelines demarcating the median and the crashes after selective implementation of snowplowable shoulder. The number of lane line encroachments, and there- PRPMs. The composite analysis did not separately evaluate fore the potential for sideswipe crashes, will decrease. Since daytime wet weather crashes. the possibility that a lane encroachment resulting in a crash is higher at higher traffic volumes, a measure that reduces lane line encroachments will have a proportionally greater effect at higher traffic volumes. The safety benefits of reduced- 5.1.2.6 Less Positive Effects of PRPMs lane-line encroachments are expected to be greater than the for Gentle Curves and Less Negative Effects potential negative safety impact of increased shoulder for Sharp Curves on Roads encroachments, where there are wide shoulders and shoulder with Illumination when Compared with Roads rumble strips. without Illumination The improvement in delineation visibility is expected to be 5.2.1.2 Speed Control more noticeable on roads without illumination. Illumination is expected to reduce both the positive effects of PRPMs on Improved visibility is likely to increase driver confidence visibility and the negative effects on speed, since illumina- and comfort to the extent that travel speeds will increase. tion assists drivers in determining lane position and control. Freeways have high design standards (e.g., high standards for The presence of illumination on sharp curves is hypothesized degree of curvature, lane widths, and shoulder widths); there- to reduce the potential negative effect of PRPMs due to fore, it is unlikely that small speed increases will cause drivers increased speeds. Because of limited sample sizes of two- to operate at or close to the margin of safety with respect to lane curves with illumination, it was not possible to deter- these parameters. Speed increases, however, may result in mine the net effect of illumination and PRPMs. increased crash occurrence due to increased stopping, decel- The presence of illumination on gentle curves and tan- eration, and weaving distances required, especially during gents could reduce the positive effects of PRPMs on forward conditions of reduced visibility. visibility and could cause the results in Table 4-3 to be an overestimation of the effectiveness of PRPMs on illuminated roadways with gentle curvature. On these roadways, as on 5.2.2 Expected PRPM Impacts roadways with sharp curves, it was not possible to determine on Four-Lane Freeways the net effect of illumination and PRPMs because of limited sample sizes of two-lane curves with illumination. To summarize, the substantial improvements in visibility of delineation at night and during poor weather conditions, and the associated increase in driver comfort after the imple- mentation of PRPMs, could have the following impacts on 5.2 FOUR-LANE FREEWAYS driver behavior at night and poor daytime weather conditions: 5.2.1 Overview of Human Factors Issues Reduced encroachments over the lane line, The common practice on four-lane freeways is to imple- Increased shoulder encroachments, and ment PRPMs nonselectively with the aim of providing a Small increases in speed at night. comfortable driving environment and improving safety in con- ditions of decreased visibility (i.e., nighttime and wet weather These changes in driver behavior are hypothesized to have conditions). the following impacts on crashes in turn:

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43 Decreases in nighttime crashes, with increasing benefits guidance-related crashes after the implementation of snow- at higher traffic volumes; plowable PRPMs. A similar statistically significant result was Decreases in guidance-related crashes (e.g., sideswipes); not observed for Pennsylvania four-lane freeways. This differ- and ence between the two states may be explained by two design Decreases in wet weather crashes. attributes: rumble strips and shoulder width. On average, Missouri freeways have wider shoulders and a higher pro- 5.2.2.1 Decreases in Nighttime Crashes, portion of freeways with shoulder rumble strips than Penn- with Increasing Benefits sylvania freeways have. Table 3-15 shows that the average at Higher Traffic Volumes shoulder width on Pennsylvania freeways is 6.3 ft (1.9 m) compared with the 9.3 ft (2.8 m) in Missouri. Table 3-25 The results of the composite analysis in Table 4-5 show shows that 53 percent of four-lane freeways in Missouri have that PRPMs had no overall effect on nighttime crashes. How- shoulder rumble strips compared with 38 percent of Penn- ever, the results of the disaggregate analysis, presented in sylvania freeways. Table 4-7, show that snowplowable PRPMs may only be effective in reducing nighttime crashes on four-lane freeways with AADTs exceeding 20,000 veh/day. 5.2.2.3 Decreases in Wet Weather Crashes 5.2.2.2 Decreases in Guidance-Related Crashes The results of the composite analysis indicated that snow- plowable PRPMs were effective in reducing wet weather The results for four-lane freeways in Missouri (Table 4-5) crashes in four-lane freeways in Missouri (12.8 percent) and show a statistically significant 10.3-percent reduction in Pennsylvania (5.4 percent).