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Speed: even tiny changes save lives

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As debate continues to rage about lower speed limits in South Australia, researcher Craig Kloeden explains local and global research on the safety impact of reduced speed.

Lowering vehicle speeds will reduce the number of people injured and killed on the road and it only takes a small change in speed to have a big effect. Evidence for this is overwhelming.

Evidence

The Centre for Automotive Safety Research at the University of Adelaide conducted the first study in the world that quantified the risks of speeding in an urban area. It was found that travelling at 65 km/h compared to 60 km/h in a 60 km/h speed limit zone doubled the risk of involvement in a injury crash and that risk doubled again with each increase of 5 km/h in travelling speed.

Although this finding was literally incredible to many people it is not really surprising. The risk of being involved in a injury crash is about one in a lifetime, therefore, the risk on a given journey is very small. Doubling that risk will not be obvious to drivers but if all drivers double their risk the road toll would also double.

Comparing this result from the speed study to the risk associated with drink driving seems to make it more meaningful to many drivers: driving at 65 km/h in a 60 zone increases the risk of involvement in an injury crash to roughly the same extent as driving at the speed limit with a .05 blood alcohol level and travelling at 70 km/h is equivalent to a 0.1 blood alcohol level.

A follow up study by the Centre for Automotive Safety Research on high speed rural roads found that each 10 km/h increase in speed doubled the risk of an injury crash.

Actual reductions in speed limits have also been shown to reduce injuries and deaths on the roads. The National Maximum Speed Limit of 55 mph (89 km/h) introduced into the United States in 1974 in response to an oil embargo has been called the most effective traffic safety measure ever introduced with fatality rates falling by 34%.

For a more local example, when the default 50 km/h urban speed limit was introduced in South Australia in 2003, the average speed of vehicles fell by only 3.8 km/h, but the number of people injured on the affected roads fell by 26% and the number of fatalities fell by 37%. There are more than 50 people alive today who would be dead if the limit had not been lowered and many more have avoided serious injury.

When 1,100 km of South Australian rural roads had the speed limit lowered from 110 km/h to 100 km/h in 2003, the average speed of vehicles fell by only 2 km/h, but the number of people killed and injured on those roads fell by more than 20%. Since that time about 14 lives have been saved and hundreds of injuries prevented.

Enforcement

Speed limit enforcement can also be used to lower vehicle speeds. The highly respected Cochrane Collaboration reviewed studies on speed cameras from around the world and found cameras to be effective both in lowering vehicle speeds and reducing crashes and injuries. Where detection rates are high, drivers will modify their behaviour and slow down.

It is also worth noting that most speeding drivers exceed the speed limit by a small amount. While their individual risk of crashing is low, slowing them all down to the speed limit can reduce the road toll by as much as eliminating all the very high speed drivers.

Appropriate speed limits

The biology of the human body and the ability of current motor vehicles to protect occupants mean that the risk of serious injury or death increases rapidly for impacts speeds above 30 km/h for pedestrians, 50 km/h for side impacts and 70 km/h for head on and roadside object impacts. Impact speeds more than 20 km/h above these levels are almost certain to cause serious injury or death.

This is why many countries have or are moving to a 50 km/h general urban speed limit with a 30 km/h limit where there are numerous pedestrians and bicycles. Undivided two lane roads in the USA (similar to most Australian rural roads) typically have a speed limit between 70 and 90 km/h. Speed limits above 100 km/h are only considered appropriate where head on or roadside object crashes cannot occur such as on divided freeways.

The Bottom Line

While Australia is moving in the right direction with speed limit setting and speed enforcement, there are still major road safety gains that can be made.

If all drivers are willing to increase their travel time by a small amount by travelling at speeds that their bodies and vehicles can sustain then a large number of the road deaths and debilitating injuries in Australia can be prevented.

Craig Kloeden is a research fellow at the Centre for Automotive Safety Research at the University of Adelaide.

For more information on CASR’s speed and road safety research, go here.

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