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22 environments where the road diets were implemented. The stop-controlled and yield-controlled; and four-leg signalized, sites in Iowa ranged in AADT from 3,718 to 13,908 and were stop-controlled and yield-controlled) and on five types of predominately on U.S. or state routes in small urban towns roadway segments (i.e., all segments combined, rural two-lane with an average population of 17,000. The sites in Washington segments, rural multilane segments; urban two-lane segments, and California ranged in AADT from 6,194 to 26,376 and were and urban multilane segments). Appendix D provides the predominately on corridors in suburban environments that details associated with this evaluation. The target crash types of surrounded larger cities, with an average population of interest in the intersection analyses included the following: 269,000. In addition, in Iowa there appeared to be a calming effect as evidenced in a study (44) of one site that revealed a 4 to Total, 5 mph reduction in 85th-percentile free flow speed and a Wet road, 30-percent reduction in percentage of vehicles traveling more Dry road, than 5 mph over the speed limit (i.e., vehicles traveling 35 mph Rear end, or higher). The researchers' speculation is that this calming Rear end wet, effect would be less likely in the larger cities in the HSIS study, Right angle, and where the approaching speed limits (and traffic speeds) might Right-angle wet-road. have been lower to start with. The "new" Iowa results also seem to be incompatible with The target crash types considered for segments included: those in the earlier Iowa analysis of the same treatment site data (44). However, the 25-percent reduction reported in that Total, study was based on average effects per mile derived by com- Wet road, paring average crashes per mile after treatment with expected Dry road, average crashes per mile without treatment. These results are Rear end, not comparable to the "new" results since sites of different Rear-end wet-road, lengths were weighted equally. (The "new" results are overall Rear-end dry-road, effects that provide more weight to sites of longer length.) In Single vehicle, and addition, the "new" results use a much larger comparison Single-vehicle wet-road. group than the previous study, which used an equal number of treatment and comparison sites. Data Used The data for this study were provided by the New York State Increasing Pavement Friction DOT and included crash, geometric, and AADT data for on Roadway Segments treated and untreated intersections and segments during the and at Intersection Approaches period of 1994 to 2003. Data were included for 256 treated in- tersections and 3,993 untreated reference intersections, as well Description of Treatment and Crash Types as for 36.3 miles (118 segments) of treated non-intersection of Interest locations and 1,242.4 miles (2,108 segments) of untreated This analysis examined the safety impacts of improving reference locations. pavement skid resistance using data from the state of New York. The New York State DOT has implemented a skid acci- Methodology dent reduction program (SKARP), which identifies sections of pavement with a high proportion of wet-road accidents, per- The general analysis methodology used was the EB before- forms friction tests on these locations, and treats those with after analysis, as previously described. SPFs and annual correc- both a high proportion of wet-road accidents and low friction tion factors were successfully developed for each of the site numbers (below the Programmatic Design Target Friction type/crash type combinations noted above. Number, FN40R, of 32). The treatment generally involves a 1.5-in. resurfacing or a 0.5-in. microsurfacing using non- Results carbonate aggregates. This treatment is applied principally on Intersection Treatments the major road approaches at intersections, but is often extended some distance away from the intersection as well. Estimates of the AMFs for the crash frequency analyses for The goal of this analysis was to develop separate AMFs for intersection skid-reduction treatments are given in Table 9. different crash types occurring at seven different intersection Results that are statistically significant at the 95-percent level types (i.e., all intersections combined; three-leg signalized, are in shown in boldface type.

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23 Table 9. Crash frequency AMFs (and standard error) by crash type for intersection skid-reduction treatments. Total Rear-end Right- Right- Intersection Wet-road Rear-end Dry crashes wet angle angle wet Type (s.e.) (s.e.) (s.e.) (s.e.) (s.e.) (s.e.) (s.e.) 0.799 0.426 0.582 1.149 0.322 1.045 0.799 All (0.028) (0.030) (0.034) (0.051) (0.041) (0.078) (0.123) Three-leg 0.667 0.372 0.554 0.959 0.261 0.787 0.470 signalized (0.050) (0.053) (0.065) (0.093) (0.066) (0.125) (0.161) Three-leg stop- 0.819 0.355 0.586 1.302 0.335 0.828 0.828 controlled (0.048) (0.046) (0.057) (0.095) (0.075) (0.218) (0.218) Three-leg yield- 0.590 0.217 0.304 1.392 0.221 n/a n/a controlled (0.114) (0.103) (0.086) (0.321) (0.161) Four-leg 0.797 0.546 0.585 0.992 0.361 0.898 1.105 signalized (0.052) (0.070) (0.068) (0.081) (0.084) (0.117) (0.294) Four-leg stop- 1.271 0.597 0.943 1.754 0.482 1.687 0.829 controlled (0.143) (0.137) (0.188) (0.242) (0.215) (0.323) (0.351) Four-leg yield- 0.589 0.361 0.504 0.651 n/a n/a n/a controlled (0.216) (0.371) (0.248) (0.273) The results show statistically significant reductions at Table 10. Results that are statistically significant at the almost all types of intersections in total crashes; wet-road; 95-percent level are in bold. rear-end; and rear-end, wet-road crashes. As expected, the In general, the results show statistically significant reduc- largest effects were on total wet-road crashes (i.e., 40-percent tions in total crashes and in wet-road; rear-end; rear-end, wet- to 78-percent reductions) and rear-end, wet-road crashes road; single-vehicle; and single-vehicle, wet-road crashes for (i.e., 52-percent to 78-percent reductions). There was very lit- most roadway categories. The only exception was for two-lane tle effect on wet-road, right-angle crashes. Overall, dry road rural roads, where no significant decreases or increases in crashes showed a statistically significant 14-percent increase. frequency were found. As expected, the largest statistically sig- However, this did not negate the effects on wet-road crashes, nificant effects were on total wet-road crashes (i.e., 46-percent as shown by the statistically significant 20-percent decrease in to 74-percent reductions), on wet-road, rear-end crashes total crashes when all intersection and crash types were com- (i.e., 36-percent to 66-percent reductions) and on wet-road, bined. To see if the principal benefits of improved skid resist- single-vehicle crashes (i.e., 38-percent to 71-percent reduc- ance on wet-road crashes declined over time, the effect on tions). The only statistically significant increase found was for wet-road accidents was analyzed by year after treatment. The dry-road crashes on urban multilane roads (i.e., a 13-percent analysis indicated no discernable decreasing trend over the increase). However, that increase did not negate the overall 6 years of after-treatment-period data. treatment effect in that there was a 14-percent reduction in total crashes (i.e., dry plus wet) on these roads. A final analysis examined changes in the overall propor- Segment Treatments tions of wet-road crashes before and after the treatments. It Estimates of the AMFs for the crash frequency analyses found a statistically significant reduction in the proportion of for segment-based skid-resistance treatments are given in wet-road accidents at intersection locations (i.e., 40 percent Table 10. Crash frequency AMFs (and standard errors) by crash type for segment skid-reduction treatments. Single- Rear- Rear- Total Single- vehicle Segment Wet-road Rear-end Dry end wet- end dry- Crashes vehicle wet- Type (s.e.) (s.e.) (s.e.) road road (s.e.) (s.e.) road (s.e.) (s.e.) (s.e.) 0.764 0.434 0.828 1.003 0.575 0.977 0.698 0.399 All (0.023) (0.024) (0.043) (0.043) (0.055) (0.068)) (0.040) (0.039) Rural 2 0.964 0.852 1.047 1.167 0.971 1.235 1.078 1.125 lanes (0.073) (0.126) (0.149) (0.114) (0.256) (0.219) (0.141) (0.287) Rural >2 0.684 0.346 0.776 0.875 0.474 0.838 0.588 0.292 lanes (0.032) (0.028) (0.068) (0.061) (0.079) (0.098) (0.046) (0.038) Urban 2 0.599 0.260 0.612 0.992 0.344 0.695 0.921 0.523 lanes (0.082) (0.066) (0.142) (0.195) (0.145) (0.216) (0.232) (0.247) Urban > 0.862 0.538 0.866 1.132 0.640 1.120 0.800 0.615 2 lanes (0.038) (0.045) (0.059) (0.065) (0.084) (0.099) (0.083) (0.115)