Further investigations into the effective use of clear zones and barriers in a Safe System’s context on rural roads

This paper describes further research that re-examines 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. A previous study investigated the implications of providing clear zones for vehicles that departed straight sections of road. The current study extends this work to consider vehicles that depart roads on bends. Current guidelines allow for a clear zone of up to 10.5 metres on straight sections of road. Although there is provision to increase this distance on bends, in practice this is rarely applied. A sample of crashes from the Centre for Automotive Safety Research (CASR) in-depth crash investigations in South Australia were analysed to determine the typical dynamics of vehicles in single vehicle run of road crashes on bends, 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. Differences between departures on straights and bends were noted and a discussion of clear zone requirements to prevent injury during a road departure is made in the context of the Safe Systems approach.