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41 which is an increase in crash frequency on roads with low There were statistically nonsignificant decreases in head- design standards. on crashes and a statistically significant increase in total Wet weather is another situation in which drivers are likely crashes in two-lane roadway Illinois data (nonselective operating closer to the safety margin by not slowing suffi- implementation). ciently to compensate for increased braking distance. Thus, Although available sample sizes did not permit a com- the negative impacts of any speed increases on tight curves posite or disaggregate analysis of nighttime head-on may be exacerbated in wet weather. crashes, the results in Table 4-3 (AMFs for two-lane roadways) show statistically significant improvements in the safety performance of PRPMs at night as traffic 5.1.2 Expected PRPM Impacts on Two-Lane Roadways volumes increase. To summarize, the substantial improvements in nighttime centerline visibility and the associated increase in driver com- Decreases in Safety Benefits fort after the implementation of PRPMs are expected to have as the Degree of Curvature Increases the following impacts on driver behavior: On sharper curves (i.e., with a higher degree of curvature), Reduced oncoming and left-lane encroachments at night, it is possible that the negative safety impact of speed increases Increases in shoulder encroachments at night, and is not offset by the positive safety impact of improved visibil- Small increases in speeds at night. ity; failure to offset the negative safety impact would result in an increase of nighttime crashes. This proposition is sup- These changes in driver behavior are expected to have the ported by the univariate analysis of two-lane roadways and following impacts in turn: by the results of the disaggregate analysis in Table 4-3, which show that PRPMs will have negative safety effects on road- Decreases in nighttime head-on crashes, with increasing ways with a degree of curvature exceeding 3.5. The negative benefits as traffic volumes increase; safety effect holds true for all ranges of traffic volumes avail- Decreases in safety benefits as the degree of curvature able in this research study. increases; Decreases in safety benefits as the vehicle moves closer to the edgeline; Decreases in Safety Benefits Decreases in wet weather nighttime crashes; as the Vehicle Moves Closer to the Edgeline Slight decreases in daytime wet weather crashes; and Less positive effects of PRPMs on gentle curves and less The risk of run-off-road crashes on two-lane roadways is negative effects on sharp curves on roads with illumina- hypothesized (as described in Section 5.1.1) to be higher tion when compared with roads without illumination. on roadways with lower design standards (e.g., with higher degrees of curvature and narrower pavements widths) because vehicles move away from the centerline to the edgeline to avoid Decreases in Nighttime Head-On Crashes, the PRPMs. For example, narrower shoulder widths reduce the with Increasing Benefits recovery area for vehicles that leave the travel lane. The uni- as Traffic Volumes Increase variate analysis indicated a positive correlation between traf- fic volumes and pavement widths, meaning that higher-traffic- The majority of head-on crashes are due to inadvertent volume roadways are normally associated with higher roadway excursions into the oncoming lane (only 4 percent of head- design standards. This may in part explain why the AMFs in on fatalities are associated with overtaking) (46). The prob- Table 4-3 show decreases in safety benefits with decreased ability that inadvertent excursions result in head-on crashes traffic volumes, which are in turn associated with roads with increases as traffic volumes increase. Given driver behavior, narrower pavement widths. it is expected that (1) improved delineation of the centerline by PRPMs at night and the consequent movement away from the centerline will reduce head-on crashes at night and (2) the Decreases in Wet Weather benefit of PRPMs will increase as traffic volumes increase. Nighttime Crashes The safety impacts expected, as described above, are sup- ported by the results of the composite analyses (see Chapter 4) The significant improvement in visibility in wet weather at undertaken in this research study. These results are as follows: night would be expected to reduce run-off-road crashes and head-on crashes on gentle curves where small increases in There were statistically significant decreases in head-on speed would not significantly increase crash risk. The results crashes on two-lane roadways in New Jersey (nonselec- of the safety composite analysis shown in Table 4-1 indicated tive implementation). a statistically significant decrease in wet weather nighttime