Dr Richard Lambourn considers the scenario in which a person is stuck under a car: would the driver know?
Happily this is a rare event, but when it does occur, either accidentally or as a result of a deliberate assault, it is appropriate to ask, what awareness would a driver have that there was someone stuck under their car? One might imagine that it would be very obvious, and indeed in a recent case the press reported someone giving expert evidence as saying that it would be like driving with the handbrake on. I know nothing of the particular details of that case – in what fashion the victim was caught under the vehicle, or whether objective tests were carried out to assess the effect – but in general, the reality is that in cases where a person has simply become wedged underneath a car, the effect is small and may be barely noticeable to the driver.
This statement may be counterintuitive, and an unwillingness to accept or appreciate it might lead to a driver being held more culpable for an event than they actually were. I have had occasion to investigate several such incidents with practical tests, and these demonstrated how small the effect is. But before describing these, some simple physics may be helpful.
Most important is the (rather obvious) difference between the weight of a person and of a motor vehicle. A heavier person could have a mass of 90 kg; a small car could be 900 kg, which is to say, it would be 10 times as heavy. The drag on the car due to the presence of the person will come from the friction developed by their body rubbing against the road surface, and this will depend on their clothing and the state of the road, in particular whether it is wet or dry. The exact friction properties of the road (called the coefficient of friction) will depend on the construction of the road itself, but with ordinary clothing a typical wet coefficient would be in the region of 0.40 to 0.55, while a dry one might be around 0.70. With a car ten times the weight of the person, these figures mean that the retarding effect of the body would, on a wet road, be equivalent to slowing at only 0.040 to 0.055g, and on a dry road, at only 0.070g. With a lighter person and/or a heavier car, the effect would be even less.
For comparison, a car which is coasting will slow at a rate of around 0.03g, while very gentle braking would be about 0.10g. Therefore, the effect of a trapped body would be comparable to coasting and less than gentle braking – and not like driving with an applied handbrake!
Three investigations where I carried out actual tests have been described in a published paper[i]. It happens that all three associated incidents occurred on wet roads, where of course the effects would likely have been less than on a dry surface. In each set of tests an appropriate dummy was put either under or against the front of the car, and the vehicle driven to find the effect: in the test for the third incident the car was also instrumented to measure it objectively. The subjective experience in all three was that there was hardly any noticeable difference between driving with and without the dummy, the retardation, such as it was, being easily overcome by the engine. This subjective finding was confirmed by the measurements taken in the third set of tests.
The lessons to be learnt from these investigations, and also from two others that I have carried out, is, first, that one must not jump to the naïve conclusion that a body under a car would have a substantial and obvious effect on the driving, and second, that in any one incident actual tests would be of the greatest importance in establishing for the Court what the driver’s experience would probably have been: something which TRL would be able to arrange.
[i] Lambourn, R. and Manning, J., “Driving a Car with a Body Wedged Underneath,”
SAE Technical Paper 2017-01-1410, 2017, doi:10.4271/2017-01-1410.