Assessing International Transferability of the Highway Safety Manual Crash Prediction Algorithm and its Components

The Highway Safety Manual (HSM) provides an algorithm, and associated knowledge, for predicting crashes for different facility types. This algorithm requires calibration to current local conditions to enhance transferability, using a procedure that is prescribed in the HSM. However, there is no procedure for assessing transferability. To fill this void, this paper is focused on the methodology for assessing the transferability of the key HSM algorithm components, the baseline Safety Performance Function (SPF) and the Crash Modification Factors (CMFs), using the Italian road network for an illustrative case study. The calibration of the HSM crash prediction model is investigated with a dataset for two-lane two-way rural highways, to demonstrate tools that could be used by jurisdictions around the world for assessing the validity and compatibility of the CMFs and base models, as well as the performance of the complete algorithm. A comparison with the results from a similar study carried out in Canada is provided in supplementing the conclusions on the transferability of the HSM algorithm outside the United States.