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37 TABLE 4-2 Index of effectiveness model for two-lane roadways (nighttime crashes) Model Applicable Condition Estimate Standard p-value Parameters Error Constant AADT 5000 1.1573 0.0260 < 0.001 Degree of curvature 3.5 AADT 2 5000 < AADT 15000 -0.1700 0.0395 0.003 AADT 3 15000 < AADT 20000 -0.4004 0.0607 < 0.001 Degree of Degree of curvature > 3.5 0.2736 0.0824 0.011 curvature TABLE 4-3 AMFs (nighttime crashes) derived not be undertaken for two-lane facilities and for four-lane from Table 4-2 freeways and expressways because of the nonselective imple- AADT (veh/day) AMF when AMF when mentation policies. The nonselective policies resulted in too DOC 3.5 DOC > 3.5 small samples of potential spillover sites. 05000 1.16 1.43 Using data for Pennsylvania two-lane roadways, several 500115000 0.99 1.26 nonoverlapping locations within 2 miles of a given PRPM 1500120000 0.76 1.03 installation were identified. The results of the spillover analy- sis are shown in Table 4-4. DOC = Degree of curvature. According to the results of the statistical analysis for the sample of two-lane roadways in the state of Pennsylvania, analysis that, generally, PRPMs are more effective on higher- there were no significant spillover effects to adjacent roadways volume roadways (possibly a reflection of the higher design to those roadways where snowplowable PRPM were installed. standards of these highways) and on roadways with more gen- tle curvature. For example, at AADTs ranging between 15,000 4.4 RESULTS OF ANALYSIS and 20,000 on a roadway with a degree of curvature less than FOR FOUR-LANE FREEWAYS 3.5, a decrease in nighttime crashes of 24.3 percent follow- ing PRPM installation can be estimated from the model as 4.4.1 Composite Analysis noted above. At lower AADTs and sharper curvature, PRPMs can in fact be associated with an increase in crashes. For exam- Table 4-5 shows the results of the composite safety evalua- ple, for PRPMs installed on roadways with AADTs between tion of PRPMs on four-lane freeways. Statistically significant 5,000 and 15,000, an increase in nighttime crashes of 26 per- results (at 95-percent confidence level) are shown in bold. cent can be estimated from the model. That PRPMs are more As mentioned in Chapter 3, the widespread implementa- effective on roadways with more gentle curvature (i.e., where tion of PRPMs on four-lane freeways and expressways dur- the degree of curvature is less than 3.5) is contrary to a belief ing 1999 meant that Wisconsin DOT could only provide held by many. One possible explanation is that PRPMs may promote an increase in operating speeds and that the speed TABLE 4-4 Results of spillover increase is a greater safety concern on a sharper curve. analysis: two-lane roadways in Pennsylvania (total crashes) 4.3.4 Spillover Analysis Crash Pennsylvania Two-Lane Spillover Type Sites The same before-and-after evaluation methodology used # Sites1 = 5227 for PRPM locations was applied to a sample of road seg- # Miles = 306.55 ments found immediately surrounding the treated road seg- Obs2 % Ch4 ments to examine possible migration and spillover effects. Exp3 s.e. As discussed in Chapter 2, if a significant spillover effect Total 1447 1.048 were found, it would have been necessary to consider this 4.8 1381 0.030 effect in assessing the net effect of PRPM installations. 1 A site is a homogeneous segment of road Pennsylvania two-lane roadways were selected for the represented by a set of attributes (shoulder width, spillover study because their PRPMs were installed selec- type, lane width, AADT, terrain, guide rails, tively, and the state DOT had the required data to support a horizontal alignment, etc.). spillover analysis study. New York, despite its selective pol- 2 Obs = Observed crash frequency. icy for PRPM installation, did not have sufficient data for a 3 Exp = Expected crash frequency. 4 Ch = change. spillover analysis. In New Jersey, spillover analysis could
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38 TABLE 4-5 Results of safety evaluation of four-lane freeways (nonselective implementation) with snowplowable PRPMs Crash Type Missouri Freeway New York Freeway Pennsylvania Freeway # Sites1 = 1327 # Sites = 64 # Sites = 1629 # Miles = 1441.80 # Miles = 36.49 # Miles = 778.93 Obs 2 % Obs % Obs % 3 4 5 Exp s.e. Ch Exp s.e. Ch Exp s.e. Ch Total 9195 0.979 335 1.031 3640 0.943 -5.7 -2.1 3.1 9394 0.012 324 0.074 3860 0.019 Fatal and Injury 2720 0.946 91 1.179 501 1.000 0.0 -5.4 17.9 2876 0.021 77 0.141 501 0.047 Daytime 5955 0.979 177 1.046 2155 0.935 -6.5 -2.1 4.6 6080 0.015 169 0.100 2305 0.024 Daytime Fatal and Injury 1801 0.938 55 1.195 293 1.023 2.3 -6.2 19.5 1919 0.026 46 0.183 286 0.062 Nighttime 3240 0.991 158 0.900 1485 0.960 -4.0 -0.9 -10.0 3269 0.020 175 0.090 1547 0.028 Nighttime Fatal and Injury 919 0.975 36 0.951 208 0.988 -1.2 -2.5 -4.9 942 0.035 38 0.171 211 0.070 Dry 6343 1.046 167 0.997 2228 0.956 -4.4 4.6 -0.3 6066 0.016 167 0.100 2329 0.024 Wet 2852 0.872 -12.8 161 0.974 -2.6 1404 0.946 -5.4 3270 0.019 165 0.096 1484 0.027 Guidance-related 3870 0.897 Sample too small 834 0.986 -1.4 -10.3 4315 0.017 845 0.038 1A site is a homogeneous segment of road represented by a set of attributes (shoulder width, type, lane width, AADT, terrain, guide rails, horizontal alignment, etc.). 2 Obs = Observed crash frequency. 3 Exp = Expected crash frequency. 4 Statistically significant results (at 95% confidence level) are shown in bold. 5 Ch = change. comparison group data (i.e., roadways without PRPMs) for a 2000 (after period) crashes. This data preparation was total of 43 miles of four-lane freeway. Additional crash data undertaken for all crash types (see Appendix A, Table (total and fatal and injury) were collected for urban Interstate A-10). highways without PRPMs in Milwaukee County as an alter- · Comparison Group 2: Milwaukee County. The urban native comparison group. Both comparison groups were used Interstate highways without PRPMs in Milwaukee for composite analyses, and the results were compared. The County were all illuminated and had a lower posted speed methodology that was applied to estimate the annual factors limit (45 mph or 72 km/h) when compared with four- for the two comparison groups is as follows: lane freeways (55 mph or 86 km/h) on which PRPMs were installed. The crash data showed an increase in · Comparison Group 1: 43 miles of four-lane free- 2000 (after period) of 27 percent for total crashes and ways. The crash data for the 43 miles of freeway were 16 percent for fatal and injury crashes when compared used to derive a ratio between the crash counts for 2000 with the same crash types for 1994 to 1998 (before and the average annual crash counts for 1994 to 1998 period). Thus, for the composite analysis, these per- (before period). This ratio was used as a multiplier to centages were applied for crash types accordingly (see determine an annual calibration factor for the year Appendix A, Table A-10).