Safety Performance Functions for Florida's Freeways

Safety performance functions (SPFs) are a required input to the newly released SafetyAnalyst software tool. Using statistical goodness-of-fit measures and visual plots, this paper compares the performance of locally calibrated SPFs using Florida data with the default SPFs from SafetyAnalyst for both freeway interchange influence areas and basic freeway segments. SafetyAnalyst defines an interchange influence area as one that extends 0.3 miles upstream and 0.3 miles downstream of the respective gore point. Using four years of local crash data (2007-2010) from Florida, SPFs for both types of segments for both urban and rural areas were developed using the negative binomial (NB) regression model. The results showed that Florida-specific SPFs generally produced better-fitted models than the calibrated SafetyAnalyst-default SPFs. This was clear in that the majority of Florida-specific models had higher Freeman-Tukey R-square (R²_FT), as well as lower mean absolute deviance (MAD) and mean square prediction error (MSPE) estimates.