Roadway Departure Safety: A Manual for Local Rural Road Owners

Appendix C: Crash Rate Calculations

The crash rate for roadway departure crashes on a roadway is calculated as:

The variables in this equation are:

• R = Roadway Departure crash rate for the road segment expressed as crashes per 100 million vehicle-miles of travel,
• C = Total number of roadway departure crashes in the study period
• V = Traffic volumes using Average Annual Daily Traffic (AADT) volumes
• N = Number of years of data
• L = Length of the roadway segment in miles

This equation relies on having traffic volume information To determine how to obtain actual and estimated traffic volumes for a particular roadway, a local agency can contact its State highway agency, LTAP representative, or other state agencies.

Example 1. Crash Rate by Vehicle Miles Traveled

In this example, two roadways have the same number of crashes but different traffic volumes. By factoring in exposure, the calculation indicates that Route B may be more susceptible to future crashes. However, before any decision is made, other factors such as roadway geometrics, cross section, and other potential differentiating factors should be considered. There could be other issues not related to traffic volume that affect crash rates.

Route A has experienced 17.1 crashes per 100 million vehicle-miles traveled on that roadway. Route B has experienced 27.4 crashes per 100 million vehicle-miles traveled. This data can be used to compare the two roadways. In this case, even though both routes had the same number of crashes, Route B is more susceptible to crashes based on the level of exposure. The practitioner could consider Route B a more promising candidate for a safety treatment than Route A due to its higher crash rate.

Example 2. Crash Rate by Route Length

In this example, two roadways have the same number of crashes but different roadway lengths. Traffic volume data is not available.

A "crashes per mile" rate for road segments is calculated as:

Where:

• R = Crashes per mile for the road segment expressed as crashes per each 1 mile of roadway per year.
• C = Total number of crashes in the study period.
• N = Number of years of data.
• L = Length of the roadway segment in miles.

In this example, Route A has experienced 0.14 crashes per roadway mile. Route B has experienced 0.09 crashes per mile of roadway. In this case, even though both routes have the same number of crashes, Route A may be more susceptible to future crashes. Therefore Route A may be a more promising candidate for safety treatments.