Whenever a speed limit change is proposed there is a good deal of public debate. The British government’s recent call to allow drivers to do 80 miles per hour (129 kilometres per hour) on some of the country’s fastest roads, instead of the current 70mph (113kph), is no different.

Should we raise the speed limit to take advantage of the greater capabilities of modern vehicles? Would this increase casualties? How should limits be enforced? These are just a few of the questions that provoke endless debate. Speed is not the only factor in crashes – no one would argue otherwise. However, its importance for public health is that it is easily experimented on. Contrast that with driving while tired, which is less easy to measure and change.

There is no doubt that over the past 30 years vehicles have evolved to go faster with consummate ease. Before taking full advantage of this we might consider whether we, the drivers, have evolved much over the same period. Unfortunately our reaction times are not any faster, nor are our bodies any better at withstanding the forces involved in a crash.

A human who tries really hard can sprint at about 30kph. To reach even that speed, we have to put lots of energy into the system, our heart is pumping, we have the wind in our face and massive experience of movement – in other words, we have overwhelming biological feedback to tell us just how fast we are going.


Velocity blindness

When we drive a car, however, the energy input is the small movement of a large toe. The output, in contrast, is that we can easily travel at more than four times the maximum speed for which we have been designed – and with almost no experience of movement. The feedback to the brain from the legs, heart and lungs when we are driving is effectively that of no movement. Add to that the fact that prolonged exposure to speed reduces perceived velocity and that speed cues such as engine noise are systematically eliminated in modern vehicles, and it is no wonder there are some challenges in obeying limits.

We have the speedometer, of course, but this hardly provides visceral feedback – and, bizarrely, about half of the dial is devoted to illegal speeds. It is rare for any other product to broadcast its illegal capabilities like this.

For years, the general message from governments has been that, for safety reasons, a reduction in speed is good because it reduces casualties. But this has been difficult to get across. Messages such as “at 35mph you are twice as likely to kill someone as at 30mph” may be hard to appreciate if you assume that energy increases linearly with speed – in fact, it rises with the square of the velocity.

The transfer of that energy to the human body is the problem. The evidence on the relationship between speed and casualties is unambiguous whichever way it is examined. For example, raising the 55mph (89kph) speed limit to 65mph (105kph) in the US was estimated to have increased fatalities by 15 per cent (American Journal of Public Health, vol 79, p 1392). So what criteria should we use to define a limit? Two present themselves: functionality and survivability.

Survival speeds

Different types of road have different functions: access roads, which border residential and shopping areas; distribution roads, which need more entry and exit points; and through roads such as freeways and motorways which are for uninterrupted movement, with limited entry and exit.

Survivability refers to the body’s capacity to tolerate the energy transfer in accidents. Evidence shows that on access roads, where crashes involving pedestrians are likely, a 20mph (30kph) limit is appropriate. On distribution roads, where side impacts are likely – when a car might ram into the side of another that is pulling out of a side road, for instance – the limit should be 30mph (50kph). In situations without pedestrians and where side impacts and head-on collisions are improbable – motorways and freeways – the limit should be 60 to 70mph (100 to 110kph).

Getting drivers to stick to limits, be they new or old ones, is another thing. Deterrence is an obvious route. Deterrence theory, derived from the work of the 18th-century judicial theorist Cesare Beccaria and the 19th-century philosopher and social reformer Jeremy Bentham, emphasises the certainty, severity and imminence of punishment. The certainty of punishment has the clearest deterrent effect, which is problematic for speed enforcement because it relies on an uncertain police presence.

This can be solved by speed cameras, which have themselves stimulated a good deal of media debate. Controversy has focused on whether their goal is safety or revenue generation. Policy-makers can tackle this by emphasising casualty reduction: for instance, they can place cameras at accident locations, allocate fines to road safety, advertise the accident location by highly visible cameras and prior warning signs, and offer education for first-time offenders.

It may be important for politicians to distinguish between media debate and public concern on this issue. For example, Damian Poulter – a colleague at the University of Reading, UK – and I examined the UK government’s British Crime Survey to determine what people are concerned about in their local communities. In comparison with a range of antisocial behaviours such as race attack, drugs, intimidation and noisy neighbours, speeding was the top concern (Accident Analysis and Prevention, DOI: 10.1016/j.aap.2006.08.015).

So the challenge for governments who wish to change limits is complex. This is particularly so for those wishing to raise them, which is a less familiar path. What is clear, given the historical evidence and our biology, is that if they choose to permit faster driving, they must accept their part in the increased casualties that will follow.