If a car is travelling at high speed and all the occupants are not belted up, the impact of a crash will cause all of them to be thrown from their seats.

It is likely that the driver will hit the steering wheel and suffer serious head injuries.

The front-seat passenger is also likely to suffer serious head injuries from smashing into the dashboard.

A child, who would be the lightest person in the car, will be sent flying through the windscreen from the impact, said Mr John Frederik Gronvall, manager of Volvo's Traffic Accident Research team in Sweden.

At speeds exceeding 40kmh, children who are not strapped in properly will most likely be flung out of the vehicle, he said.

"Children have small body mass. It's easy for them to be flung out the window, windscreen or an open door," he told The Sunday Times in a phone interview.

Volvo Cars runs a Traffic Accident Research Centre in Sweden which has studied more than 36,000 car accidents since 1970.

The main "killers", however, would be the back-seat adult passengers. They will be thrown forward, in the process collapsing the front seats and crushing those at the front, said Mr Gronvall.

"The dynamic force of the backseat occupant can be about three tonnes," he said. Three tonnes is roughly the weight of a female adult elephant.

There are usually three types of collision in an accident, said motoring journalist Leow Ju-Len.

First, the collision between the vehicle and an external object - say, a tree.

Second, the collision that happens inside the vehicle, between the passengers and the various parts of the car.

Third, the collision that happens within the bodies of the passengers - that is, between their internal organs.

"At 80kmh, the people in the car are carrying a lot of kinetic energy. So in a crash, their organs may collide against the other body parts," he said.

In 1997, for example, Britain's Princess Diana died from a tear in her pulmonary vein - a large blood vessel which carries oxygenated blood from the lungs to the heart, to be pumped to the rest of the body - after the car she was in crashed in a Paris tunnel. The tear was possibly caused by a bone fragment or a fractured rib.

Whether people in a car will die depends on what they hit upon impact, said Mr Gronvall. "But I can safely say the risk of fatality...is very high."

This article was first published in The Sunday Times on May 4, 2008.