Crash Severity Prediction Functions on Italian Rural Roads

The Highway Safety Manual (HSM) published by the American Association of State Highway and Transportation Officials (AASHTO) provides predictive models for rural two- lane highways giving estimates for total crashes and it presents predictive methods for urban or suburban arterial facilities to estimate the expected average crash frequency, crash severity and collision types. According to HSM methodological issues, the objective of the research is to calibrate and validate safety performance functions (SPFs) that predict the number of injuries and deaths per year (frequency of injuries and deaths) on homogeneous road segments of two-lane rural roads by investigating local conditions. More than 1,400 mi of road network were analyzed. Crash data over an 8-year period (2003- 2010) were used (1,597 injury crashes with 2,592 injuries and 67 deaths) of which 5 years (2003 – 2007) of crash information were used to calibrate SPFs while the remaining 3 years were used to check the reliability of SPFs. Reflecting HSM procedure, a base regression model was performed to predict the frequency of injuries and deaths and then multiplicative coefficients were introduced to account the differences between the first base conditions and the proper road geometric features for transferability to different networks. Three SPFs were performed as follows: a) one equation to predict the frequency of only injuries per year; b) one equation to predict the number of only deaths per year; c) one equation to predict the total injuries+deaths number per year.