Low curve radius

Average crash rates are higher on horizontal curves than on straight sections of rural two-lane highways. Radius or degree of curvature consistently tops the list of geometry variables that most significantly affect operating speeds and crash experience on horizontal curves. The crash rate increases with lower curve radii (tighter curves), with strong increase for radii smaller than 200 metres. In general, sharp curves in combination with long straight sections, sharp vertical sag or sharp crest curves, and a sequence of gentler curves are factors that increase risk in curves. For specific groups of drivers, such as motor cyclists and truck drivers, curves with low radii may be more risky than for other drivers and may require additional risk mitigating measures. The analysis of coded studies confirmed that curves with low radii have a higher crash risk. Moreover this analysis showed that crash modification functions for curve radius are very different for curve radii < 200 metres, with particular steep functions for Germany and USA. Based on USA rural highway studies, the analysis of coded studies found steeper crash modification factors for fatal/injury crashes than for Property Damage Only (PDO) crashes. It was also found that low curve radius is especially risky in interaction with vertical sag or crest curves, and that curve radius was the strongest predictor for motorcycle-to-barrier crashes. On the basis of these results, it is expected that the measure of increasing horizontal curve radius will have analogous positive effects on road safety.