Incorporating Safety into Rural Highway Design

The objectives of this research were to explore ways to assess the safety performance of (predominantly two-lane) rural highways in New Zealand (NZ) and in particular identify driver/road/environmental factors affecting crashes on rural curves. Following a wide-ranging literature review, the Interactive Highway Safety Design Model (IHSDM) was identified as worthy of further investigation for adaptation to use in NZ. To help with this investigation, a comprehensive database was developed of road, traffic, crash and environmental data for all NZ State Highways, divided into variable-length road elements. A number of tasks were identified and undertaken to adapt IHSDM for general use here, including calibrating the Crash Prediction Module (CPM), developing a Design Policy file based on local agency standards, and developing an importing routine for NZ highway geometry and crash data. To assess the effectiveness of IHSDM for predicting the relative safety of rural road alignments, a series of tests were undertaken to confirm its appropriateness for use in NZ. These included “before and after” design consistency checks of a bridge replacement, a “before and after” crash comparison of a major highway realignment, and checks of actual versus predicted crash numbers along longer lengths of highway in varying terrain. These initial investigations have shown 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 accuracy in crash numbers, and appears to provide a better level of “local calibration” than by using sub-national (e.g. regional or terrain-specific) calibration parameters. Reported fatal/injury crash data generally provide more robust and precise measures than non-injury crashes. Correct specification of the extreme attributes of sub-standard elements (e.g. minimum radius, maximum roadside hazard) appears to be crucial to getting suitably accurate crash estimates on existing alignments. 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