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Highway Safety Behavioral Strategies for Rural Areas (2023)

Chapter: Appendix B: Crash Incidence Explanatory Model (Task 2)

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Suggested Citation:"Appendix B: Crash Incidence Explanatory Model (Task 2)." National Academies of Sciences, Engineering, and Medicine. 2023. Highway Safety Behavioral Strategies for Rural Areas. Washington, DC: The National Academies Press. doi: 10.17226/27196.
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Page 139

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139 Appendix B: Crash Incidence Explanatory Model (Task 2) The research team used a negative binomial regression model to identify patterns in safety outcomes in different types of rural counties and along different rural road types. Negative binomial regression is appropriate for analyzing crash data due to its ability to work with over dispersed count data (i.e., data with high variability). We specified our regression model using crash data from two states: Minnesota and Washington. As explained earlier, these states offered the most complete data and included necessary county and road type identifiers. In each dataset, we included a filter for metropolitan and rural counties to identify differences between the two. We used HSIS data for this analysis for the reasons described earlier. Our primary dependent variable was crash incidence, which is a continuous variable. The following shows the fitted regression equation: 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶ℎ𝑒𝑒𝐶𝐶 = exp (𝐵𝐵0 + 𝐵𝐵1(𝑆𝑆𝑒𝑒𝑆𝑆𝑆𝑆𝑒𝑒𝑆𝑆𝑆𝑆𝑆𝑆𝑒𝑒𝑆𝑆𝑆𝑆𝑆𝑆ℎ × 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 × 𝑆𝑆𝑆𝑆𝑒𝑒𝑒𝑒𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆) + 𝐵𝐵2(𝑁𝑁𝑁𝑁𝑆𝑆𝑁𝑁𝑒𝑒𝐶𝐶𝑆𝑆𝐶𝐶𝑆𝑆𝑒𝑒𝐶𝐶 × 𝑆𝑆𝐶𝐶𝑆𝑆𝑒𝑒𝐿𝐿𝑆𝑆𝑆𝑆𝑆𝑆ℎ) + 𝐵𝐵3𝑅𝑅𝑆𝑆𝑆𝑆ℎ𝑆𝑆𝑆𝑆ℎ𝑜𝑜𝑁𝑁𝑜𝑜𝑆𝑆𝑒𝑒𝐶𝐶𝐿𝐿𝑆𝑆𝑆𝑆𝑆𝑆ℎ + 𝐵𝐵4𝐴𝐴𝑒𝑒𝐶𝐶𝐶𝐶𝐶𝐶𝑆𝑆𝑆𝑆 + 𝐵𝐵5𝑅𝑅𝑁𝑁𝐶𝐶𝐶𝐶𝑜𝑜/𝑈𝑈𝐶𝐶𝑁𝑁𝐶𝐶𝑆𝑆) Where Crashes = Annual crashes (averaged over a 3-year period) on a given road segment SegmentLength = Road segment length in miles AADT = Annual average daily traffic SpeedLimit = Posted speed limit (for Washington only) NumberLanes = Number of lanes LaneWidth = Lane width in feet RightShoulderWidth = Right shoulder width in feet Terrain = Three-level variable relating to the frequency and steepness of hills; values are coded to be either Level, Rolling, or Mountainous (for Washington only) Rural/Urban = Binary dummy variable denoting whether the road segment is located in a census defined urbanized area or cluster which meets certain population density requirements (U.S. Census Bureau, 2010).

Next: Appendix C: Counties with Significant Crash Reductions (Task 2) »
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Rural roads have a higher risk of fatality or serious injury than urban roads due to factors such as varying terrain, wildlife, and long distances between services.

BTSCRP Web-Only Document 4: Highway Safety Behavioral Strategies for Rural Areas, from TRB's Behavioral Transportation Safety Cooperative Research Program, documents the overall research effort that produced BTSCRP Research Report 8: Highway Safety Behavioral Strategies for Rural and Tribal Areas: A Guide. Supplemental to the document is a PowerPoint presentation that outlines the project.

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