If a driver was faced with a car of similar speed (100km/hr) coming straight at the driver on the wrong side of the road, and the drivers only options were to hit the car or a solid concrete bridge abutment, which should the driver choose?

John Raine, a mechanical engineer at Massey University, responded.

This would be a case of being caught between a rock and a hard place. It is an almost impossible situation in that any decision would have to be made in a second or two, which gives extremely little time to look for other options and, if there were none, to assess the variables, e.g. the speed, mass and age of the oncoming car, in order to make an informed decision.

The key is to look at the momentum change on impact for the vehicle in which our unfortunate driver is travelling. The change in speed on impact and the associated deceleration is what dictates the injury trauma to the vehicle occupants. Hitting the bridge abutment from 100 km/hr and decelerating the driver to rest over a distance of, say 1.5 metres as the front of the vehicle crushes (likely to be fatal because of penetration into the occupant space) would involve an average vehicle deceleration of approximately 26g, with a higher peak deceleration, and the occupant would likely experience more severe decelerations through hitting the interior of the vehicle.

For two identical vehicles hitting head-on, the result should be similar to the above. However if our driver is in a large heavy car weighing 2000kg all-up and the car hit head-on is a small car of only 1000kg all-up, then after impact both vehicles would travel in the original direction of the heavier car at approximately 33 km/hr. The average deceleration of the vehicles would depend on the stiffness of the two vehicles but for the larger car would for this impact be of the order of 12.5g (reasonably survivable), and for the driver of the smaller car around 25g. Peak decelerations would be perhaps twice these levels and the collision would very likely be fatal for the driver of the smaller car.

For this very simplistic example, if you were in the larger car you would be better to go for the head-on collision, but in the smaller car the trauma would be similar to that from hitting the bridge abutment. A problem with hitting the bridge abutment is that if it is narrow it will severely penetrate the vehicle and is likely to cause huge trauma to occupants. If the driver was alone then impacting the bridge abutment or the oncoming car on the passenger half of the front of the car would appear to be the best odds the driver could have.

The age of the cars would also be involved. Modern cars are designed to crumple progressively at the front while providing a protective shell around the occupants. Front crumple zones can reduce the impact deceleration.