Rationalisation of speed limits within the Safe System approach

The goal of the Safe System approach to eliminate death and serious injury puts a focus on the speeds (and masses) of vehicles involved in crashes. The principal role of vehicle speed is the build-up of kinetic energy which needs to be dissipated in each crash, no matter how caused, often resulting in injury to the humans involved. However the dissipation of kinetic energy only becomes relevant if a crash occurs. There are high quality road environments that minimise vehicle interactions by grade-separation, median barriers and lane discipline, and prevent fixed-object collisions by very adequate roadside clearance. Such freeway-type road environments already exist and high speed limits are allowed on them because crash risks are minimal. A Safe System should aim for as much of its road system as possible to be at this high quality standard, particularly in rural areas. An optimum speed limit could be set that provides maximum benefit from reduced travel times and minimises the costs of road trauma, air pollution emissions and vehicle operating costs. This paper summarises the calculation of the optimum speed limits for the range of Australian rural road types: rural freeways, mult-lane divided roads, and single-lane undivided roads, with and without shoulder-sealing. The system-wide impacts if cars and trucks were to travel at their optimum speeds, as a basis for setting speed limits in each road environment, are then calculated. Depending on the method used to value serious road trauma, it was found that crash costs across the full rural road system would decrease by at least 10% (though increase on rural freeways) and total travel time would increase by 1%. It is suggested that such a rationalisation of speed limits to reflect the innate expectations of drivers, but also based on rational analysis of all the costs and benefits, may result in greater compliance with speed limits and reduce the need for intensive speed enforcement.