128.5 Crash Rate Calculation

A crash rate is calculated for each project and included in the conceptual study.

The formulas for the crash rates are:

Segment Crash Rate

$\text{Segment Crash Rate}=\frac{\text{no. of crashes x 100,000,000}}{\text{no. of yrs. x 365 x weighted ave. AADT x length in miles}}$

Range lengths of more than one mile should be used in the calculation, if practical. As seen in the rate calculation, an accident rate for a route less than one mile long will result in an artificially high crash rate.

The segment crash rate yields a result in crashes per hundred million vehicle miles traveled (HMVMT). The number of crashes is the total number of crashes in the study period. For conceptual reports a five-year study period is typically used, utilizing the last five full years of traffic crashes. The AADT and crash data are obtained from the TMS database maintained by Transportation Planning. The calculated crash rate is compared to the five-year average statewide rate for a similar class of highway as obtained from Transportation Planning or found in their Traffic Crash Statistics.

Intersection Crash Rate

$\text{Intersection Crash Rate}=\frac{\text{No. of crashes x 1,000,000}}{\text{No. of years x 365 x Entering AADT}}$

The intersection crash rate yields a result in crashes per million entering vehicles. Entering AADT includes all legs of the intersection. The number of crashes is the total number of crashes in the study period. For conceptual reports a five-year study period is typically used, utilizing the last five full years of traffic crashes. The AADT and crash data are obtained from the TMS database maintained by Transportation Planning. The calculated crash rate is compared to the five-year average statewide rate for a similar class of highway as obtained from Transportation Planning or found in their Traffic Crash Statistics.