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23 resulted in a significant reduction in the nighttime 85th per- With respect to speed, findings were very mixed, with two centile speeds. There was no significant difference in day- studies finding smoother speed profiles through curves (29, time speeds. Encroachments over the centerline were also 30 night only at one sight), one study finding one site with no reduced significantly for both the daytime and nighttime. significant difference in speed after PRPM application (35), The report recommended that PRPMs be placed on both the one study finding a significant reduction in 85th percentile edgeline and centerline to delineate the decrease in pave- speed at one site (31), three studies finding significant ment width. increases in speed (27, 31 at one approach only, 34), and one study finding a reduction in speeds both day and night (33). One study that examined the effects 11 months after instal- 2.3.4 Summary lation of PRPMs found that speeds were lower, possibly Five studies found that PRPMs were associated with because the lower reflectivity reduces preview distance (34). fewer encroachments into the adjacent lane on horizontal At night, better delineation may induce higher speeds, par- curves (30, 31, 33, 34, 35). One of these studies examined ticularly on tangents and large radius curves, and possibly on sites with and without lighting and found that while smaller radius curves where only centerline (not edgeline) encroachments were significantly reduced at both sites, they PRPMs are implemented. This possibility has not been ade- were reduced more at the site with lighting (30). This find- quately studied, but is likely given the results of studies ing confirms the findings of other studies showing that showing that improved delineation (i.e., higher contrast lane encroachments were reduced during the day as well as at striping) was associated with higher speeds. night after the placement of PRPMs on curves. Two studies One study found that applying PRPMs in deceleration lanes found that drivers moved away from the PRPMs (27, 34). resulted in drivers entering the deceleration lane earlier (30). One study found that, at one site, lane position variability This study also found that using PRPMs in gore areas reduced decreased significantly (31). the frequency of encroachments to the gore areas.