J. McCombs, H. Al-Deek, A. Sandt, G. Gamaleldin, A. El-Urfali

Pages: 3-18

Abstract
The Highway Safety Manual (HSM) provides guidance for agencies on developing safety performance functions (SPFs) to predict traffic crashes. However, some states develop their own SPF methodologies due to data differences and a need for additional roadway categories. The Florida Department of Transportation has developed a context classification system that groups intersections into eight categories, allowing for the development of regional context-specific SPFs. To best utilize this system, safety engineers from departments of transportation (DOTs) across the United States were surveyed about their state’s current SPF development practices and context classification. Many states (64% of the 42 respondents) use HSM SPFs or SPFs calibrated to their jurisdiction. Although 62% of states had not heard of context classification, 67% of states expressed interest in it. One reason some states were not interested in using context classification was insufficient evidence of its benefits compared to the current HSM methodology. To showcase the increased accuracy of SPFs developed using this system, HSM SPFs [baseline, baseline with crash modification factors (CMFs), and calibrated with CMFs] for rural two-lane, two-way roads were compared with a context-specific SPF for C2T-Rural Town signalized four-leg intersections. These comparisons found that the HSM SPF with CMFs overpredicted crashes for Florida intersections (calibration factor of 0.87). The context-specific SPF better predicted total crashes, contained additional variables (including a regional variable), and performed statistically better than all three HSM SPFs. By implementing a similar context classification system, agencies could develop more accurate SPFs and identify regional differences, improving safety throughout their jurisdictions.
Keywords: safety performance functions; context classification; intersections; Highway Safety Manual; agency survey