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Development of Crash Modification Factors for Uncontrolled Pedestrian Crossing Treatments (2017)

Chapter: Appendix E - Safety Performance Functions for the Before-After Evaluation

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Suggested Citation:"Appendix E - Safety Performance Functions for the Before-After Evaluation." National Academies of Sciences, Engineering, and Medicine. 2017. Development of Crash Modification Factors for Uncontrolled Pedestrian Crossing Treatments. Washington, DC: The National Academies Press. doi: 10.17226/24627.
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Suggested Citation:"Appendix E - Safety Performance Functions for the Before-After Evaluation." National Academies of Sciences, Engineering, and Medicine. 2017. Development of Crash Modification Factors for Uncontrolled Pedestrian Crossing Treatments. Washington, DC: The National Academies Press. doi: 10.17226/24627.
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Page 83
Suggested Citation:"Appendix E - Safety Performance Functions for the Before-After Evaluation." National Academies of Sciences, Engineering, and Medicine. 2017. Development of Crash Modification Factors for Uncontrolled Pedestrian Crossing Treatments. Washington, DC: The National Academies Press. doi: 10.17226/24627.
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Page 84
Suggested Citation:"Appendix E - Safety Performance Functions for the Before-After Evaluation." National Academies of Sciences, Engineering, and Medicine. 2017. Development of Crash Modification Factors for Uncontrolled Pedestrian Crossing Treatments. Washington, DC: The National Academies Press. doi: 10.17226/24627.
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Page 84

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81 A P P E N D I X E This appendix documents the safety performance functions (SPFs) that were estimated as part of the Empirical Bayes (EB) before-after evaluation using the data from a reference group of 474 sites that included information for 3,842 site-years. The SPFs were estimated using negative binomial regression. The functional form of the SPFs was the following: exp 0 1 Y Xi i i n ∑= β + β  = where Y = the number of crashes per year, b0 = the intercept, Xi = the independent variables, bi = the coefficients for these independent variables, and n = the number of independent variables. The estimation was done using PROC GENMOD in SAS. Tables E-1 through E-4 provide the SPFs that were estimated for total, rear-end, sideswipe, and pedestrian crashes. The tables show the coefficients, standard errors of the coefficients, the over- dispersion parameter (k), and the scaled deviance divided by the degrees of freedom. As is the norm, for the categorical variables, one of the levels or categories was included as a reference level. It is important to note that unlike the regression models that are documented in the main body of this report and used for estimating the CMFs, these SPFs are meant for prediction and not for making inferences regarding the safety effects of certain roadway features. Safety Performance Functions for the Before-After Evaluation

82 Development of Crash Modification Factors for Uncontrolled Pedestrian Crossing Treatments Variable Level or Category Coefficient S.E. Intercept -8.5602 0.4051 ln(major AADT) 0.8243 0.039 ln(pedestrian volume) 0.1098 0.013 City Alexandria -0.4509 0.3144 Arlington -0.7981 0.1656 Cambridge 0.0887 0.1035 Charlotte 1.2773 0.0595 Chicago 0.4627 0.0976 Eugene -0.6029 0.1448 Miami 0.3052 0.0888 Milwaukee 0.1719 0.1157 New York -0.2646 0.1656 Phoenix 0.5687 0.0933 Portland 0.2588 0.097 Scottsdale 0.6593 0.1304 St. Petersburg 0.2119 0.0938 Tucson ---- ---- Year 2004 ---- ---- 2005 -0.0449 0.0876 2006 -0.0861 0.0838 2007 -0.0738 0.0815 2008 -0.1504 0.0805 2009 -0.3778 0.082 2010 -0.4724 0.0829 2011 -0.4784 0.0831 2012 -0.444 0.0832 2013 -0.4887 0.0919 Intersection or Midblock Intersection 0.5626 0.0508 Midblock ---- ---- Crosswalk Type No crosswalk ---- ---- Other 0.2975 0.064 PLC 0.229 0.0859 Area Type Urban 0.21 0.0807 Suburban ---- ---- One-way or Two-way One-way 0.464 0.1138 Two-way ---- ---- School Crosswalk Yes -0.3267 0.0731 No ---- ---- k 0.5843 0.0281 Scaled Deviance/df 1.0775 Note: SPF was estimated based on information from 9,352 total crashes. Table E-1. SPF for total crashes.

Safety Performance Functions for the Before-After Evaluation 83 Variable Level or Category Coefficient S.E. Intercept -14.3684 0.6908 ln(major AADT) 1.3678 0.0708 ln(pedestrian volume) 0.0816 0.0192 Crosswalk length -0.012 0.0024 City Alexandria -0.7634 0.4725 Arlington -0.9307 0.2662 Cambridge -0.3598 0.166 Charlotte 1.1519 0.0895 Chicago 0.1151 0.1475 Eugene -0.6767 0.2467 Miami -0.1524 0.149 Milwaukee -0.1074 0.1807 New York -0.3886 0.3018 Phoenix 0.508 0.1298 Portland 0.3115 0.1443 Scottsdale 1.0372 0.2135 St. Petersburg 0.0264 0.1544 Tucson ---- ---- Year 2004 ---- ---- 2005 -0.0646 0.1219 2006 -0.1001 0.1179 2007 -0.1885 0.1156 2008 -0.1801 0.1138 2009 -0.3712 0.1166 2010 -0.5288 0.1194 2011 -0.461 0.1185 2012 -0.5994 0.1218 2013 -0.5577 0.1305 Intersection or Midblock Intersection 0.5718 0.0765 Midblock ---- ---- Lighting Yes 0.2261 0.0707 No ---- ---- Crosswalk Type No crosswalk ---- ---- Other 0.5612 0.0946 PLC 0.5152 0.1268 k 0.7882 0.0588 Scaled Deviance/df 0.8196 Note: SPF was estimated based on information from 2,989 rear-end crashes. Table E-2. SPF for rear-end crashes.

84 Development of Crash Modification Factors for Uncontrolled Pedestrian Crossing Treatments Variable Level or Category Coefficient S.E. Intercept -13.2532 0.9558 ln(major AADT) 1.0215 0.0909 ln(pedestrian volume) 0.1094 0.0263 City Alexandria -0.7589 1.0282 Arlington -0.3505 0.4407 Cambridge 1.3105 0.2043 Charlotte 1.9124 0.1483 Chicago 0.8885 0.2201 Eugene -0.4492 0.4449 Miami 0.1643 0.2518 Milwaukee 1.347 0.2322 New York -21.4792 22962.31 Phoenix 1.5857 0.1822 Portland 0.3786 0.2448 Scottsdale 2.0354 0.2612 St. Petersburg -0.1471 0.3281 Tucson ---- ---- Year 2004 ---- ---- 2005 0.0796 0.1632 2006 -0.0946 0.1652 2007 0.3011 0.1522 2008 0.0205 0.156 2009 -0.1311 0.1607 2010 -0.2377 0.1635 2011 -0.3732 0.1677 2012 -0.3416 0.1664 2013 -0.2531 0.1758 Intersection or Midblock Intersection 0.2571 0.1002 Midblock ---- ---- One-way or Two-way One-way 1.0262 0.1987 Two-way ---- ---- Crosswalk Type No crosswalk ---- ---- Other 0.5137 0.1451 PLC 0.4959 0.1738 School Crossing Yes -0.7122 0.1732 No ---- ---- k 0.5721 0.1015 Scaled Deviance/df 0.5757 Note: SPF was estimated based on information from 1,015 sideswipe crashes. Table E-3. SPF for sideswipe crashes. Variable Level or Category Coefficient S.E. Intercept -12.4454 1.5093 ln(major AADT) 0.8448 0.1432 ln(pedestrian volume) 0.3158 0.0513 City Alexandria 0.3544 1.0511 Arlington -1.2284 0.7367 Cambridge 0.1865 0.2864 Charlotte 0.1855 0.1943 Chicago -0.1748 0.326 Eugene -2.124 1.0173 Miami 0.3153 0.2571 Milwaukee -2.4065 0.7494 New York 0.1166 0.3808 Phoenix -0.3435 0.3473 Portland -0.4141 0.3079 Scottsdale -0.8352 0.7541 St. Petersburg -0.3729 0.4011 Tucson ---- ---- Area Type Urban 0.8911 0.2393 Suburban ---- ---- k 1.2039 0.4162 Scaled Deviance/df 0.2858 Note: SPF was estimated based on information from 272 pedestrian crashes. Table E-4. SPF for pedestrian crashes.

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TRB's National Cooperative Research Program (NCHRP) Report 841: Development of Crash Modification Factors for Uncontrolled Pedestrian Crossing Treatments quantifies the safety benefits of four types of pedestrian crossing treatments—rectangular rapid flashing beacons, pedestrian hybrid beacons, pedestrian refuge islands, and advanced YIELD or STOP markings and signs—and presents a crash modification factor (CMF) for each treatment type. This information, which is suitable for inclusion in the American Association of State Highway and Transportation Officials (AASHTO) Highway Safety Manual, the U.S. Federal Highway Administration's (FHWA's) CMF Clearinghouse, and other guidance, will be valuable to transportation agencies in choosing the appropriate crossing treatment for uncontrolled pedestrian crossings.

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