NZ Transport Agency (NZTA) enlisted Abley’s help to produce priority lists of high-risk local rural roads and high-risk urban arterials. The iterative approach was required because of the broad range of road types that exist in rural speed environments (from very low volume remote rural roads through to arterials on the fringe of urban areas carrying significant volumes of traffic) and the best means for measuring risk varies across these categories.
A composite Collective Risk, Personal Risk and Infrastructure Risk Rating (IRR) metric was explored initially. This mix of reactive and proactive metrics was intended to reduce bias away from roads at either extreme of the spectrum. Work from other projects being progressed for the NZ Transport Agency (nationwide speed management) and ACC (identification of out-of-context curves) was also evaluated for use in the project to inform the identification and prioritisation of high-risk rural roads and high-risk urban arterials.
A key target of the approach was to ensure that the length of network targeted as being high-risk accounts for a disproportionately high percentage of estimated Death and Serious injury (DSi) casualty equivalents. The process worked towards achieving the iRAP target of 10% of the network length accounting for 50% of the risk.
The nature of the project was such that the analysis needed to be run over a region or regions at each iteration to ensure the outputs were sensible/intuitive. Once the approach was delivering sensible/intuitive results the methodology was applied to the national network and assessed. Further refinement occurred following nationwide assessment to address anomalies, outliers and outcomes that were unexpected, as well as revisiting the appropriateness of thresholds developed at a regional level.
The methodology produced results that aligned with the results of the prioritisation process and other prioritisation schemes such as the Speed Management Framework and State Highway Gap identification. This gives confidence that the prioritisation process is robust and can be used to inform work schemes for low-cost safety interventions. The results will help the Transport Agency to effectively target road safety spending to areas with the greatest potential safety savings.