Effective use of clear zones and barriers in a Safe System's context

This paper describes an exploratory study that re-examined the use of clear zones as the preferred rural roadside treatment to address fixed object crashes, as compared to barrier treatments, within the context of a Safe System’s approach to road safety. Traditionally, a clear zone of up to nine metres is recommended on a straight section of road. A sample of crashes from the Centre for Automotive Safety Research (CASR) in-depth crash investigations in South Australia was analysed to determine the typical dynamics of vehicles in single vehicle run of road crashes, with a particular focus on the lateral departure distance and the departure angle of the vehicle. A number of these crashes were simulated using advanced computer techniques to determine the relative merits of clear zones and barrier protection. The relationship between the speed of the vehicle and its lateral distance from the edge of the road throughout the departure event was investigated. The merit of barrier protection was assessed by determining the barrier normal velocity at differing barrier protection offsets. Initial analyses showed that in crashes where no fixed object was struck, all vehicles travelled well beyond the nine metre clear zone, with several travelling over 20 metres laterally. Simulations suggest that adequate clear zones to ensure noninjurious impact speeds can not be provided in most situations. Roadside barrier protection in combination with narrower clear zones may provide the most cost effective way to treat rural roadsides to achieve a Safe System.