The Safety Risks of Out-of-Context Curves: Three Decades of Rural Curve Research in NZ

The rural road network of New Zealand contains many horizontal curves that are inconsistent with their surrounding environment. These “out-of-context” curves – where the safe negotiation speed is significantly lower than the prevailing approach speed – are associated with higher crash risk than in-context curves of otherwise similar geometry. Over the past three decades, New Zealand researchers and transport agencies have developed a robust body of work to understand and address this issue, although it remains under-explored internationally. 

This paper reviews the evolution of rural curve safety research in New Zealand, including the development of high-resolution road geometry datasets, operating speed models, and crash prediction models. It also highlights how these insights have informed national project evaluation guidance and safety prioritisation frameworks, and some recent adaptation of this research to international contexts, including the United States. In particular, it was found that traditional crash prediction models such as those in IHSDM and the Highway Safety Manual can underestimate the observed numbers of crashes around out-of-context curves by at least 30%, and potentially up to 60%. 

The recent application of New Zealand curve context modelling to US rural roads through the SafeCurves software tool addresses this limitation. This review aims to demonstrate the value of incorporating curve context into safety analysis and prioritisation, and encourage broader application of these methods to reduce crash risk on rural roads worldwide.

With industry colleagues Joe Corbett-Davies / Shane Turner (Abley) and Fergus Tate (WSP), Glen co-authored this paper that was presented as a poster at TRB by Steve Abley.