|Road crashes at rural curves result in a high number of deaths and serious injuries. Speed has been identified as a significant contributor to such crashes, but some researchers have suggested that an understanding of other elements is also required. This study was designed to broaden the understanding of the mechanisms that lead to elevated crash risk at curves. A better understanding of these issues might provide insight into methods for improving safety at curves. This study included an assessment of the elements of curve design that are linked to side force and crash risk, including speed, curve radius and driver lane position. These issues were assessed for high risk and low risk curves. Differences between inexperienced and experienced drivers were also compared.
40 male drivers (20 inexperienced and 20 experienced) drove a set rural route in a test vehicle which was instrumented to collect data. Measures of speed, side force and lane position were taken on a continuous basis for each curve on the route. Information regarding the design elements for each curve was also collected, including curve radius, curve direction (left or right) and curve risk (calculated based on the difference between curve approach speed and minimum curve speed). Based on the calculation of curve risk 20 high risk curves, and 20 low risk curves were identified and data for each included in the study.
Differences in speed, side force and lane position through high and low risk curves were assessed, as were the differences between inexperienced and experienced groups of drivers on these same measures. This information was collected at key points on approach and through curves.
Results indicated that when comparing high risk curves with low risk curves speeds were lower for high risk curves; acceleration and deceleration levels were greater; side force was greater; and variance between drivers was greater. Deceleration continued through and beyond the curve mid-point for high risk curves. Given this is a high risk point within a curve (i.e. where the side force is greatest, and the chance to lose control highest) it is highly desirable that drivers will have already fully decelerated by this point.
There were substantial differences in lane position on approach and through high risk compared to low risk curves. Lane position was also statistically and substantively different for inexperienced compared with experienced drivers. For high risk curves, experienced drivers ‘cut the corner’, reducing side force to a greater degree than inexperienced drivers.
These results relating to speed (particularly the need to reduce speed at or before the curve mid-point) and lane position provide useful information on possible sources of risk to drivers through curves. The findings support the hypothesis that in order to understand risk at curves, factors such as speed and lane position should be considered in combination. The results presented provide additional opportunities to help improve safety. Earlier deceleration coupled with more appropriate lane position would act to reduce side force through curves, and therefore lead to improved safety outcomes.