Transferability of the Highway Safety Manual Freeway Model to the Italian Motorway Network

The currently available release of the AASHTO Highway Safety Manual (HSM) does not include a model for freeways and interchanges. NCHRP Project 17–45 has recently developed specific crash prediction models for this type of infrastructure. Key to making the HSM a standard for road owners and managers worldwide is the transferability of prediction models to different networks. An extensive study conducted on the primary Italian motorway network evaluated the potential issues in application of this methodology to a network characterized by environmental conditions, road characteristics, driver attitudes, and crash reporting systems different from those for which the HSM models were developed. The freeway network considered in this study, approximately 6,700 km long, was represented by 56 freeway sections characterized by an average length of about 12.5 km, covering 700 km of freeway distributed throughout Italy. Four calibration factors were obtained for freeway segments, according to crash severity and type, and two calibration factors were obtained for speed-change lanes, according to crash severity. The results show a good transferability of the analyzed models to the Italian network, especially the freeway models for fatal and injury crashes, although a tendency to underestimate locations of high crash frequency was observed. Some improvements could be made concerning variable calibration factors within the data sets or local calibrations for crash modification factors. Improved localization of the crash data on the Italian road network needs to be highlighted, mainly for speed-change lanes.