Large four-wheel-drive vehicles are twice as likely as other cars to roll over in crashes because of their higher centre of gravity, a study has found.

Their occupants are also more likely to be killed or injured if they hit a roadside safety barrier.

The findings, from a study for the Highways Agency by the Transport Research Laboratory, challenge the perception that the “Chelsea tractors” are always safer to drive.

The researchers studied thousands of crashes in the past decade and found that 4x4s, also known as sports utility vehicles, offered more protection in collisions. Their greater mass and higher riding position mean smaller vehicles tend to come off much worse, especially when a 4x4 overrides their side-impact protection systems. In cars involved in crashes in 2001, at least one occupant was injured in 61 per cent of non-4x4s compared with at least one in 48 per cent of 4x4s.

But the study also found that 4x4s were more vulnerable to turning over after hitting objects such as kerbs or when swerving violently. Injuries tend to be much more severe when vehicles overturn.

Only 2.9 per cent of other cars involved in a crash in 2001 overturned, compared with 6.2 per cent of 4x4s. The gap widens when a crash involves a collision with safety barriers, with 41 per cent of 4x4s overturning compared with 11 per cent of other cars.

The study noted the doubling in annual sales of 4x4s in the past decade and said that they tended “to be bought for image value and for a perceived sense of safety on the road”. It said many modern 4x4s had “questionable off-road abilities” and added: “The closest many of them get to an off-road environment is the super-market car park.” The authors concluded that many people bought them to transport children to school because they believed they were more likely to survive unscathed in collisions.

But the authors said that perception may change as owners became aware that the growing number of larger vehicles on the road meant they were more likely to have a collision with an equally heavy and tall car. The total forces in such a collision would be greater.

Andrew Howard, the head of road safety at the AA, said 4x4 drivers tended to be lulled into a false sense of security about safety. He added: “But they are not always better off, especially if they hit an object which causes them to overturn.”

Mr Howard said that 4x4 owners were also more likely to think they could drive as fast in wet or frosty conditions. “The sense of protection which the vehicle gives them may encourage recklessness, even in situations where they are in greater danger because of the slippery road conditions.”

The study found that safety barriers, designed to stop lighter cars, did prevent 4x4s and people carriers from crossing over into oncoming traffic.

The problem of 4x4s rolling over has been well documented in the US, where models tend to be larger. Jeffrey Runge, the head of the National Highway Traffic Safety Administration in the US, said that he would not drive some “if they were the last vehicles on Earth”.

In hot seats

Sales of 4x4s fell from their peak last year after a decade of uninterrupted growth, but more than 176,000 were sold against 78,000 in 1996

Glass’s Guide to secondhand car prices said 4x4s were depreciating much faster last year. A one-year-old BMW X5, purchase price £63,397, is worth less than £38,800

The Mayor of London wants to increase the congestion charge for large 4x4s to £25 from 2010

Imperial College, London, found 4x4 drivers were four times more likely to use mobile phones than other drivers. A third more shunned seat belts

Source: Times database