What is the best practice to ensure the accuracy of output from a Safety Performance Function (SPF)?

• A. Calibrate with local information on crashes, traffic volumes and geometric characteristics of the roadway
• B. Use national averages only
• C. Rely on local data without calibration
• D. Use data from another city

Answer: (A)

Calibrating an SPF with local crash history, traffic volumes, and roadway geometry is essential because the relationships between exposure, road design, and crash risk vary by location. An SPF links how much traffic you have and how the road is built to the number of crashes you can expect, but local conditions—driver behavior, enforcement patterns, weather, and specific design features—shape those relationships. By adjusting the model to reflect local crash patterns, accurate traffic volumes, and the true geometric characteristics of the roads, the SPF produces predictions that match reality more closely. This makes outputs more reliable for identifying high-risk sites and evaluating safety treatments.

Using national averages, data from another city, or data without calibration can lead to predictions that don’t fit local conditions, resulting in biased or inaccurate estimates.