Four-wheel drives might be safer for owners, but not for others on the road

The rise in numbers of suburban four-wheel-drive utility vehicles has fuelled a fiery debate over road safety. Supporters say that one of the biggest attractions of these cars is their safety. Opponents say they make the roads more dangerous for other drivers. Who is right?

The answer, it seems, is both. According to a new report from the Monash Accident Research Centre, which analysed data from over a million Australian crashes, four-wheel-drives both reduce injury to their owners, and raise the risk to everyone else.

The researchers considered a range of factors, including the speed of the crash, when and where it occurred, and the age and sex of the drivers. The study did not take into account the risk to pedestrians.

From the perspective of their occupants, our-wheel-drives helped save lives. For every 1000 crashes they were involved in, 32 drivers of four-wheel drives were killed or hospitalised. For every 1000 crashes drivers of medium-sized cars were involved in, 36 suffered the same fate.

So by buying a four-wheel-drive instead of a medium-sized car, your risk of death or serious injury in a crash falls by four in 1000.

Next, the researchers looked at "aggressivity" - the damage different types of vehicles inflicted on other road-users.

For every 1000 crashes involving four-wheel-drives, 32 drivers of other vehicles were killed or put in hospital. By contrast, only 21 other drivers were killed in or hospitalised after an accident with a medium-sized car.

So if you crash after trading in your modest medium-sized car for a chunky four-wheel-drive, the chance that you will kill or hospitalise the other driver increases by 11 in 1000.

The net result? Four-wheel-drive buyers are making themselves safer, but the cost is being borne by other road-users. For every serious injury or death that is saved by buying a four-wheel-drive vehicle, nearly three more result.

This simple statistic explains much of the rise in sales of four-wheel-drives. In 1980, one in 50 new cars sold was a four-wheel-drive utility vehicle. Today the figure is one in seven, and most new sales are in urban areas. The more four-wheel-drives there are on the road, the more other drivers begin to wonder whether they should buy one too.

In the United States, this has led to the proliferation of larger and larger vehicles, in what University of California, San Diego, researcher Michelle White has dubbed "the arms race on American roads".

At the core of the problem is that four-wheel-drive owners do not internalise the full costs that their vehicles impose on society. Our publicly funded health-care system spreads the costs of hospital care across taxpayers, while in Victoria, Tasmania and the Northern Territory, no-fault or part-no-fault insurance spreads the cost of accidents across all drivers.