Appendix E: Crash Rate and HIN HIN developed in this study was based on the Sliding Windows Analysis which used the crash frequency to identify locations with high concentrations of crashes. Crash frequency or number of crashes was often used for network screening in safety studies as it can help identify locations that need the most improvement to reduce the number of crashes and eventually reach the goal of Vision Zero. Crash rate, which is the number of crashes normalized by traffic volume, is another measure that can be used to identify locations that may merit safety improvement. However, crash rate tends to be impacted by the change in traffic volume, and having a relatively low crash rate doesn't mean those locations are safe. For example, higher volume roadways with large numbers of crashes may have relatively low crash rates but they are exactly where safety improvements are particularly needed as many people depend on those roadways for their daily travel. The traditional way of calculating crash rate also often doesn’t consider crash severity which is not consistent with the Safe System approach's principle of focusing on preventing fatal and serious injury crashes. To take into consideration of crash rate and use it as a measure for project location prioritization, we followed the typical way of calculating crash rate per the following formula:
total number of crashes x 100,000,000 AADT x 365 x 5 years x road way segment length
𝐶𝑟𝑎𝑠ℎ 𝑅𝑎𝑡𝑒 =
We then overlaid the roadway segments with medium to high crash rates based on Natural Breaks with the overall HIN map. The results are shown in Figure 43 . The overall HIN overlaps with many roadway segments with higher crash rates. There are segments with medium or above crash rates that are not part of HIN as well as HIN segments with relatively low crash rates. Both the HIN results and the crash rate results, in combination with several other factors such as social vulnerability considerations, public input, overlap with existing projects, etc. will be used as metrics for project location prioritization. This can help make sure locations need the most safety improvement either due to a high number of crashes or a high crash rate will be accounted for.
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