Intersection Crash Rate Calculator
Calculate Crash Rate per Million Entering Vehicles (MEV)
Understanding Intersection Crash Rates
In traffic engineering and road safety analysis, calculating the crash rate at a specific intersection is critical for identifying high-risk locations. Unlike raw crash counts, the crash rate accounts for exposure by relating the frequency of crashes to the volume of traffic entering the intersection.
The MEV Calculation Formula
The standard unit of measurement for intersection safety is Crashes per Million Entering Vehicles (MEV). The mathematical formula used by transportation departments (DOTs) is:
- R = Crash rate per million entering vehicles.
- A = Total number of crashes during the study period.
- V = Average Annual Daily Traffic (AADT) entering the intersection from all directions.
- T = Number of years in the study period.
Why Use Crash Rates Instead of Raw Numbers?
Raw crash data can be misleading. For example, an intersection with 10 crashes per year might seem dangerous, but if it handles 50,000 vehicles daily, its rate might be lower (and therefore statistically safer) than a rural intersection with 5 crashes per year but only 2,000 daily vehicles. Rates allow engineers to prioritize funding for safety improvements where they will have the greatest impact relative to traffic exposure.
Example Calculation
If an intersection had 12 crashes over a 3-year period and the total entering traffic (AADT) is 10,000 vehicles per day:
- Total exposure = 10,000 vehicles/day × 365 days × 3 years = 10,950,000 vehicles.
- Crash Rate = (12 × 1,000,000) / 10,950,000.
- Result = 1.096 MEV.
Interpreting Your Results
While "acceptable" rates vary by jurisdiction and intersection type (signalized vs. unsignalized), many agencies consider rates above 1.0 MEV as an indicator that the location requires further safety investigation or potential countermeasures like improved lighting, signal timing adjustments, or geometric changes.