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

• A. Use only national averages for crashes and volumes.
• B. Calibrate the SPF with local information on crashes, traffic volumes and geometric characteristics of the roadway.
• C. Apply the SPF across all roadway types.
• D. Use a single year's data regardless of variability.

Answer: (B)

Calibrating a Safety Performance Function with local crash history, local traffic volumes, and the roadway’s geometric characteristics ensures the model reflects how this specific network behaves. This local tuning adjusts the SPF so its predicted crashes align with what’s actually happening in the area, improving accuracy and reliability for planning and safety decisions. Using national averages can misrepresent local risk because it doesn’t capture the particular mix of road types, driver behavior, or exposure here. Applying the same SPF to all roadway types ignores important differences in how arterials, local streets, and other facilities contribute to crashes. Relying on a single year of data ignores natural year-to-year variability and trends that affect crash counts. For these reasons, calibrating with local information provides the most accurate and actionable SPF outputs.