Further investigation into the effective use of clear zones and barriers in a safe system's context on rural roads

Abstract

This paper extends an exploratory study that assessed the traditional implementation of clear zones and compared their effectiveness at reducing injury severity with that provided by protective barriers. The results presented in this paper support the findings of the original study in challenging the conventional approach of pursuing wide clear zones. A sample of 132 crashes from the Centre for Automotive Safety Research’s in-depth crash investigations were analysed to determine the typical dynamics of vehicles in single vehicle run off road crashes. The mean departure angle of vehicles that drifted off the road without losing control was only 7.3 degrees while for vehicles that were out of control it was 17.6 degrees. Only three of the 18 vehicles in the cases where no fixed object was struck did not travel beyond a traditional nine metre clear zone. Simulations were performed for 15 of the cases. The cases chosen for simulation represented typical vehicle departure dynamics. Each case was simulated with the driver attempting to recover by steering input and then again with the driver employing emergency braking half a second after departing the road. These simulations revealed that it would rarely be feasible to provide a clear zone wide enough to accommodate a vehicle that has left the road out of control. Clear zones can accommodate vehicles that simply drift off the road without losing control. The simulations were also used to assess the appropriateness of barrier protection. Barrier protection has the potential to meet the requirements of a safe system. Ideally the barrier would be placed as close to the edge of road as practical to reduce the angle at which it may be struck.