A safety performance function (SPF) is an equation used to predict the average number of crashes per year at a location as a function of exposure and, in some cases, roadway or intersection characteristics.

For intersections, exposure is represented by traffic volumes on the major and minor intersecting roads.

Generally, SPFs more realistically demonstrate the relationship between crashes and traffic volume. SPFs account for the regression to the mean by using an Empirical Bayes statistical method. The advantages of using this method are more accurately calculating the potential for safety improvement and acknowledging the complex, non-linear relationship between crash frequency and volume.

SPFs are regression equations that estimate crash frequency as a function of traffic volume.

The predicted number of crashes for a given traffic volume is found on the SPF curve (1). But for any specific intersection, the observed number of crashes (2) is likely to be above or below the predicted number calculated by the SPF. The observed crash count is corrected using the EB method resulting in the expected number of crashes (3) at that location. The difference between the expected number and the predicted number gives the potential for safety improvement (4) also known as potential crash reduction.

A high PCR indicates a poorly performing intersection.