Calibration of Highway Crash Prediction Models for other Countries - a Case Study with IHSDM

The use of suitable crash prediction models (CPMs) is an increasing feature of rural highway design practice around the world. Given the significant undertaking required to develop these, there is a certain appeal in investigating how countries can jointly develop crash prediction models and calibrate them for each jurisdiction. Research recently explored ways to assess the safety performance of (predominantly two-lane) rural highways in New Zealand (NZ). The Interactive Highway Safety Design Model (IHSDM) from the US was identified as worthy of further investigation, and a number of tasks were undertaken to adapt IHSDM for use in NZ. Importing routines were developed to export NZ geometry and crash data into formats suitable for IHSDM. IHSDM's CPM was calibrated to match NZ crash patterns, both nationally and for a series of subsets defined by traffic volume, number of lanes, region and terrain. A series of validation tests were then undertaken, using actual sections of NZ highway including a "before and after" realignment case study. These investigations showed that IHSDM is a promising tool for safety and operational assessment of highway alignments (both existing and proposed) in NZ. Incorporating crash history data generally improves IHSDM’s crash estimates, and appears to provide a better level of “local calibration” than by using sub-national calibration parameters. However, IHSDM’s current lack of consideration for bridges and inconsistent adjacent elements are notable omissions that limit the ability of the CPM to assess sub-standard existing routes with as much accuracy as well-designed newer alignments