Safety Performance Functions (SPFs)
In the predictive method, the appropriate safety performance functions (SPFs) are used to predict average crash frequency for the selected year for specific base conditions. SPFs are regression models for estimating the predicted average crash frequency of individual roadway segments or intersections. Each SPF in the predictive method was developed with observed crash data for a set of similar sites. The SPFs, like all regression models, estimate the value of a dependent variable as a function of a set of independent variables. In the SPFs developed for the HSM, the dependent variable estimated is the predicted average crash frequency for a roadway segment or intersection under base conditions and the independent variables are the AADTs of the roadway segment or intersection legs (and, for roadway segments, the length of the roadway segment). Below table shows two sample default SPFs from HSM:
Facility | SPF Equations | k |
Rural, 2-lane segments | Nspf rs= AADT*L*365*10^-6*e^(-0.312) | 0.236/L |
3-leg Stop controlled intersections | Nspf 3ST =exp(-9.8+0.79*ln(AADTmaj)+0.49*ln(AADTmin)) | 0.54 |
Nspf rs = predicted total crash frequency for roadway segment base conditions;
Nspf 3ST = estimate of intersection-related predicted average crash frequency for base conditions for three-leg stop-controlled intersections;
AADTmaj = AADT (vehicles per day) on the major road;
AADTmin = AADT (vehicles per day) on the minor road;
AADT = average annual daily traffic volume (vehicles per day);
L = length of roadway segment (miles);
k = Overdispersion parameter
Although, k, overdispersion parameter, is not used directly in predicting number of crashes, it is used in calculation of various performance measures in the screening.
