Understanding Safety Performance Functions (SPFs)
SPFs are statistical models used to predict the expected number of crashes on roadway segments based on traffic volume (AADT) and other characteristics. They are fundamental tools in the Highway Safety Manual (HSM) for proactive road safety management.
Key Components Explained:
- Annual Average Daily Traffic (AADT): The average number of vehicles passing a point each day. Higher AADT generally correlates with more crashes.
- Calibration Factor (a): Adjusts the model for local conditions (e.g., 0.015 for rural two-lane roads, 0.03 for urban arterials).
- Exponent (b): Reflects how crash risk scales with traffic volume (typically 0.5–1.2). Values >1 indicate disproportionate risk growth with traffic.
How to Use This Calculator:
- Enter the AADT (e.g., 10,000 vehicles/day).
- Input calibration factor (a) from local data or HSM defaults.
- Provide the exponent (b) (use 0.9 if unknown).
- Click calculate to get the predicted annual crashes.
Example Reference Values (HSM):
| Road Type |
Calibration (a) |
Exponent (b) |
| Rural 2-lane |
0.01–0.02 |
0.8–0.9 |
| Urban arterial |
0.03–0.05 |
1.0–1.1 |