Safety Risks of Out-of-Context Curves: Three Decades of Rural Curve Research in New Zealand

Abstract

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 underexplored 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 prioritization frameworks, and a 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 the Interactive Highway Safety Design Model and the Highway Safety Manual can underestimate the observed number of crashes around out-of-context curves by at least 30%, and potentially up to 60%. The recent application of New Zealand curve context modeling to U.S. 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 prioritization, highlight New Zealand research in this area, and encourage broader application of these methods to reduce crash risk on rural roads worldwide.