Road geometry and drivers' speed choice

This paper discusses a recent research project that investigated the relationship between observed travel speed, road geometry and crashes using data collected on six nominally 20 km sections of New Zealand State Highway. The speed data collected was matched onto the highway geometry profile (radius and crossfall) in order to investigate the relationship between road geometry and drivers speed choices at the 488 curves covered by the study, and the impact this relationship has on crash risk. The research confirmed that drivers' speed choices are more strongly related to curve radius than curve design speed, and that the approach speed environment also has a significant impact on speed choice. While these findings support the approach to designing horizontal curves on rural roads, relationships derived from this research predict higher negotiation speeds that those currently used in Transit New Zealand's State Highway Geometric Design Manual. While the relationship between speed choice and crashes was less than conclusive, due to the relatively small number of crashes, indications are that the crash rate on a particular curve is a function of the difference between the negotiation speed and the design speed.