Click for next page ( 28


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 27
166 APPENDIX G GLM Analysis Results for Effect of Shoulder Rumble Strip Offset and Recovery Area on Safety This appendix presents the companion tables to the four where a (i.e., intercept), b, and c are the coefficients whose cross-sectional generalized linear model (GLM) analyses inves- estimates are shown in Table G-1. The companion coefficients tigating the effect of shoulder rumble strip offset. for rumble strip placement, d, at two levels (edgeline vs. non- edgeline) as compared to no RS, are shown in Table 42. Table G-1 presents the GLM results to investigate the effect Table G-2: The statistics shown for all sites and states of shoulder rumble strip placement (edgeline vs. non-edge- combined include the estimate for: line) on SVROR FI crashes based on all site types; it is the Intercept companion table to Table 42. ADT (on natural log scale) Table G-2 presents the GLM results to investigate the overall Outside RHR effect of shoulder rumble strip placement on SVROR FI Overdispersion parameter crashes across all sites in all states; it is the companion table to Table 43. The single regression model is represented by the following Table G-3 presents the GLM results to investigate the effect equation: of shoulder rumble strip offset (at three levels) on SVROR Expected total crashes mi yr = FI crashes based on all site types; it is the companion table to Table 44. exp {( a + b lnADT + c RHR Out + d RS Placement ) Table G-4 presents the GLM results to investigate the com- IRoadway typeState } bined effect of shoulder rumble strip offset and recovery area on SVROR FI crashes based on all site types; it is the where a (i.e., intercept), b, and c are the coefficients whose companion table to Table 45. estimates are shown in Table G-2. The variable IRoadway typeState is an indicator variable with value 1 for a particular roadway Number of sites, number of site-years, offset, and offset type state combination in the table, and zero otherwise. The recovery area statistics for each model are provided in the companion coefficients for rumble strip placement, d, at two corresponding Tables 42 through 45. levels (edgeline vs. non-edgeline) as compared to no RS, are Table G-1: The statistics shown for each roadway type and shown in Table 43. state (combined or single) include: Table G-3: The statistics shown for each roadway type and state (combined or single) include: Intercept: estimate and standard error Intercept: estimate and standard error ADT (on natural log scale): estimate, standard error, and ADT (on natural log scale): estimate, standard error, and p-value (i.e., significance level) Outside RHR: estimate, standard error, and p-value p-value (i.e., significance level) Outside RHR: estimate, standard error, and p-value Overdispersion parameter: estimate and standard error Overdispersion parameter: estimate and standard error Each regression model is represented by the following Each regression model is represented by the following equation: equation: Expected total crashes mi yr = Expected total crashes mi yr = exp ( a + b lnADT + c RHR Out + d RS Placement ) exp ( a + b lnADT + c RHR Out + d Offset )

OCR for page 27
Table G-1. GLM estimates for SVROR FI crashes based on all sites--rumble strip placement analysis. Intercept lnADT Outside RHR Overdispersion Roadway type State Estimate SE Estimate SE pvalue Estimate SE pvalue Estimate SE Urban freeways PA 6.92 1.15 0.67 0.11 <.0001 0.20 0.07 Combined 8.76 1.46 0.79 0.15 <.0001 0.21 0.11 0.0622 0.19 0.06 Rural freeways MO 7.02 1.79 0.67 0.18 0.0002 0.20 0.07 a PA Combined 7.25 1.47 0.59 0.16 0.0003 0.30 0.10 0.0024 0.44 0.09 Rural multilane MN 9.73 1.47 0.90 0.15 <.0001 0.20 0.07 divided highways (nonfreeways) MO 15.06 3.93 1.55 0.42 0.0002 0.58 0.20 PA 0.05 0.28 0.57 0.32 Combined 5.46 0.77 0.31 0.10 0.0014 0.41 0.06 <.0001 0.84 0.13 MN 4.49 1.20 0.25 0.15 0.09 1.15 0.33 Rural twolane roads a MO a PA b Combined 5.38 0.79 0.31 0.10 0.003 0.41 0.06 <.0001 0.86 0.14 Rural two-lane roads MN 3.83 1.20 0.17 0.15 0.28 1.21 0.46 a LM algorithm did not converge. b Excludes 53 Minnesota nontreatment cross-sectional sites.

OCR for page 27
168 Table G-2. GLM estimates for SVROR FI crashes based on all sites-- overall rumble strip placement analysis. Intercept Roadway type State (or state effect) lnADT Outside RHR Overdispersion Urban freeways PA 3.70 0.62 0.07 MO 4.29 0.66 0.12 Rural freeways PA 6.82 0.87 0.27 Rural multilane MN 6.00 0.82 0.07 divided highways MO 11.11 1.42 0.13 0.29 (nonfreeways) PA 2.14 0.38 0.39 MN 0.93 0.06 0.43 Rural twolane roads MO 6.01 0.45 0.98 PA 3.01 0.25 0.0008 where a (i.e., intercept), b, and c are the coefficients whose where a (i.e., intercept), b, and c are the coefficients whose estimates are shown in Table G-3. The companion coefficients estimates are shown in Table G-4. The companion coeffi- of offset distance, d, at three levels as compared to no RS, are cients, d, for the combination offset recovery area, at five shown in Table 44. levels as compared to no RS with narrow shoulders, are Table G-4: The statistics shown for each roadway type and shown in Table 45. state (combined or single) include: Tables G-1 through G-4: For states that treat both sides of a divided highway as separate sites (i.e., Missouri and Intercept: estimate and standard error Pennsylvania), the RHR variables in the models represent the ADT (on natural log scale): estimate, standard error, and values for a single side of the divided highway. When both sides p-value (i.e., significance level) of a divided highway are treated as a single site (i.e., Minnesota Outside RHR: estimate, standard error, and p-value sites), the RHR variables in the model represent average values Overdispersion parameter: estimate and standard error for both directions of travel. Similarly, the RHR variable in the models for rural two-lane roads represents the average RHR for both sides of the roadway. Each regression model is represented by the following No GLM results are shown in those cases where the algo- equation: rithm did not converge. Empty cells in those cases where the GLM algorithm did converge indicate that the corresponding Expected total crashes mi yr = coefficient estimate is not statistically significant at the 0.15 exp ( a + b lnADT + c RHR Out + d Offset RA ) level or that the coefficient's sign is not in the expected direction.

OCR for page 27
Table G-3. GLM estimates for SVROR FI crashes based on all sites--offset analysis. Intercept lnADT Outside RHR Overdispersion Roadway type State Estimate SE Estimate SE p-value Estimate SE p-value Estimate SE Urban freeways PA 6.92 1.15 0.67 0.11 < .0001 0.20 0.07 Combined 8.73 1.51 0.78 0.16 < .0001 0.21 0.12 0.068 0.19 0.06 Rural freeways MO 6.74 1.77 0.65 0.18 0.0004 0.20 0.07 a PA Combined 7.36 1.56 0.60 0.17 0.001 0.29 0.10 0.003 0.43 0.09 Rural multilane MN 8.90 1.47 0.82 0.15 < .0001 0.19 0.07 divided highways (nonfreeways) MO 15.06 3.93 1.55 0.42 0.0002 0.58 0.20 a PA Combined 5.48 0.77 0.31 0.10 0.001 0.42 0.06 <.0001 0.84 0.13 MN 4.32 1.20 0.23 0.15 0.123 1.13 0.33 Rural two-lane roads a MO a PA a GLM algorithm did not converge. Table G-4. GLM estimates for SVROR FI crashes based on all sites--combined rumble strip offset and recovery area. Intercept lnADT Outside RHR Overdispersion Roadway type State Estimate SE Estimate SE pvalue Estimate SE pvalue Estimate SE Urban freeways PA 5.85 1.21 0.60 0.11 <.0001 0.18 0.07 Combined 0.54 0.26 0.07 Rural freeways MO 7.02 1.79 0.67 0.18 0.0002 0.20 0.07 PA 0.54 0.24 0.11 Combined 6.59 1.49 0.60 0.16 0.0003 0.29 0.10 0.004 0.43 0.09 Rural multilane MN 9.73 1.47 0.90 0.15 <.0001 0.20 0.07 divided highways (nonfreeways) MO 15.11 3.91 1.55 0.42 0.0002 0.58 0.20 PA 0.81 0.00 0.48 0.30 Combined 5.45 0.78 0.34 0.11 0.0021 0.39 0.07 <.0001 0.81 0.12 MN 4.50 1.33 0.25 0.19 0.1774 1.14 0.33 Rural twolane roads MO 0.65 0.75 0.23 PA 0.89 0.59 0.15