The purposes of this paper are:

  • to show the association between speed and crash outcomes;
  • to describe the current speed limits in Western Australia; and
  • to outline the safety benefits if future speed limits were more consistent with the underlying principles of Western Australia’s   Towards Zero road safety strategy.

Travel speed, speed limits and safety

It has been consistently found that the higher a vehicle’s travel speed (even when driving at or under the legal limit), the greater the likelihood of crashing and the more serious the crash outcomes1. Even relatively small changes in vehicle speed can result in substantial crash reductions. As an example: a study in Adelaide found that one-third of pedestrian fatalities would probably have survived if vehicles had been travelling only 5 km/h slower – and one in ten pedestrians would not have been hit at all2.

This association can be easily explained, as higher vehicle speeds: 

  • allow less time to recognise hazards; 
  • increase the distance the vehicle travels while reacting to hazards; 
  • increase the vehicle’s stopping distance after braking in response to the hazard; 
  • reduce the opportunity for other road users to avoid a collision; 
  • make it more likely that a driver will lose control of the vehicle; and 
  • increase the impact forces in the event of crash, making severe injuries more likely.

Much of the association between travel speed and crashes is explained by stopping distance. Stopping distance is the distance travelled by the vehicle during the time it takes the driver to react to a hazard plus the distance travelled once the brakes are applied. The importance of travel speed and stopping distance is illustrated by the following example2:

  • a car travelling at 50 km/h is being overtaken by a second car travelling at 60 km/h; 
  • at the moment of overtaking, a child runs onto the road about 40 metres in front of both cars; 
  • the car travelling at 50 km/h will be able to come to a total standstill within the 40 metres and the child will escape injury;
  • the car overtaking at 60 km/h will hit the child while travelling at 44 km/h and there will be a more than 50 percent probability that the child will be killed or severely injured.

Because vehicle travel speed is heavily influenced by the speed limits set by transport authorities, speed limits also have direct safety consequences. To be more specific, increased speed limits are associated with increased frequency of crashes of greater severity, whereas reduced limits are associated with fewer crashes and of less severity – with the changes usually being greatest for crashes involving fatalities3.

The recent reduction in the urban speed limits in Australian jurisdictions illustrates this association. Until 2001 the speed limit for local, neighbourhood streets in all Australian jurisdictions was generally 60 km/h. Jurisdictions then moved to a default 50 km/h limit, with the move having been closely evaluated for safety and other outcomes. The different evaluations consistently show that the lowered speed limit is associated with reduced crash numbers and reduced crash severity. In Western Australia for example4, reduced crash frequencies in metropolitan streets were found for: crashes involving pedestrians (51 percent reduction), crashes involving young drivers (19 percent) and crashes involving older drivers (18 percent) – with an overall reduction of 21 percent for all casualty crashes. It was also found that community support for the reduced limit increased once it was implemented, consistent with support for reduced excessive speeding in 50 km/h zones in both metropolitan Perth and regional Western Australia.

Many other studies can be cited to support the safety benefits of reduced speeds – and conversely, the increases in crashes that accompany increases in speed limits. Despite these findings, Australian and Western Australian speed limits, especially in urban areas, are amongst the highest in the world5.