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26 It became evident that there are very few differences in TABLE 3-3 Percentage of two-lane the PRPM implementation practices among states. On two- treatment sites with 40-ft and 80-ft lane roadways, the general practice is to implement two-way PRPM spacings yellow PRPMs only on the centerline at a spacing of 80 ft State % that had 40-ft % that had 80-ft (24 m). On most curves, the spacing of PRPMs is reduced to (12-m) PRPM (24-m) PRPM spacing spacing 40 ft (12 m). Illinois 0.5 99.5 Table 3-3 shows the proportion of the total length of two- lane PRPM treatment sites where PRPMs were implemented New Jersey 1 unknown unknown at a spacing of either 40 ft (12 m) or 80 ft (24 m). On freeways New York 11.1 88.9 and expressways, the general practice is to implement one- Pennsylvania 5.3 94.7 way, white PRPMs on the lane line only at a spacing of 80 ft. 1 The spacing of PRPMs at treatment sites in New Jersey Table 3-4 provides a summary of the all treatment site data could not be confirmed from the videolog recordings received from New Jersey DOT. used in the analysis. 3.2.2 Reference and Comparison Group Sites calibrated for each year of the analysis period. Table 3-5 sum- marizes the reference group data used for this study. For states in the study group that have selective implemen- In states where PRPMs were installed nonselectively (e.g., tation policies (i.e., Pennsylvania and New York), samples of for four-lane freeways in Wisconsin, Missouri, and Penn- untreated roadways of the relevant roadway types were iden- sylvania and for two-lane roadways in Illinois and New Jer- tified to compose a reference group from which safety per- sey), the reference group information used for calibrating formance functions (SPFs) or crash prediction models were SPFs made up the before-period data collected for sites with TABLE 3-4 Summary of treatment site data used in the analysis State/ Miles Before Period After Period Road Type (Sites) Mile- Average Crash Count Mile- Average Crash Count Years AADT Years AADT Total Fatal and Total Fatal and (veh/ Injury (veh/ Injury day) day) All two-lane roadways 983 5153 NA 8970 3011 2615 NA 6006 2166 Illinois two-lane roadways 460 2755 2850 2783 706 1139 2650 1133 292 New Jersey two-lane roadways 174 348 10944 1522 656 696 10951 2508 1219 New York two-lane roadways 82 409 9140 1431 1000 164 9650 1121 424 Pennsylvania two- lane roadways 267 1641 5486 3234 649 616 5887 1244 231 All four-lane freeways 2713 17201 NA 42472 11906 6330 NA 16058 4074 Missouri four-lane freeways 1441 10929 14007 25565 8271 3488 16844 9195 2720 New York four-lane freeways 37 185 15390 326 180 74 16370 335 91 Pennsylvania four- lane freeways 779 3807 24995 5750 741 2312 29920 3640 501 Wisconsin four-lane freeways 456 2280 20900 10831 2714 456 22970 2888 762 All four-lane expressways 251 1228 NA 2899 487 471 NA 1122 210 Pennsylvania four- lane expressways 106 503 13810 725 126 326 16200 531 86 Wisconsin four-lane expressways 145 725 11770 2174 361 145 12590 591 124