Cycle helmets and the lawBMJ 1994; 308 doi: https://doi.org/10.1136/bmj.308.6943.1521 (Published 11 June 1994) Cite this as: BMJ 1994;308:1521
- L Evans
Any discussions of the law and the use of bicycle helmets will be helped by focusing on three questions. Firstly, how much do cycle helmets affect the risk of injury in a crash? Secondly, how much do laws requiring cyclists to wear helmets affect casualties? And, thirdly, should wearing of bicycle helmets be required by law?
Answers to the first two questions have objective answers that can be sought from specific empirical studies and what is already known about traffic safety.1 Three unrelated sources of evidence consistently show that cycle helmets reduce risk substantially in a crash. The science of biomechanics shows that a helmet reduces the peak acceleration forces that are associated with injury. Many published epidemiological studies, including that by Maimaris and colleagues in this issue of the BMJ and the references it cites,2 find that helmets reduce injury and the risk of death. Such studies fall short of the methodological ideal of comparing harm in matched treatment and control populations - so less direct methods must necessarily be used.
For example, examining the ratio of head injuries to nonhead injuries in cyclists wearing and not wearing helmets indicates that helmets reduce the risk of head injury by more than half. One important way in which these studies fall short of the ideal is due to selective recruitment,1,3 the tendency of people who wear protection devices to differ in many ways from those who do not. Estimates of effectiveness will be biased if they ignore the reasonable expectation that crashes will be more severe for cyclists who do not wear helmets than for those who do.1,3 The third source of evidence is the analogy with motorcycle helmets. The effectiveness of a motorcycle helmet has been determined by comparing outcomes for a driver and a passenger riding the same motorcycle, and thereby having the same crash, but one wearing and the other not wearing a helmet.1,4 The large sample sizes available in data from the United States gave a precise estimate of the effectiveness in reducing death of 28% (SE 8%).
Similar effectiveness was found for drivers and passengers and for men and women. Independent examination of the ratio of head to non-head injuries provides results comparable to those in pedal cyclists - suggesting similar effectiveness for bicycle and motorcycle helmets. A universal property of all protection devices is that effectiveness declines as severity increases,1,3 so bicycle helmets are less effective for the higher severities that result from crashes with other vehicles.
Even if cycle helmets protect in a crash casualties need not necessarily decline if more cyclists use them. There is abundant evidence that human behaviour can reduce, negate, or even invert the expected outcomes of changes in traffic systems.1 The mass of evidence, mainly from laws governing the use of safety belts in cars, shows that any change in risk taking by drivers is small and there is more likely to be a reduction in risk if wearing seat belts is mandatory.1 For laws on cycle helmets the evidence is difficult to interpret because of small sample sizes and the absence of the sudden increases in rate of use generated by some belt wearing laws. One good evaluation found reductions in casualties associated with the law in Victoria, Australia.5 Although an ideal evaluation has not been possible for laws on cycle helmets, an event in the United States provided a unique opportunity to evaluate a law on motorcycle helmets. Under pressure from the federal government nearly all 50 states passed laws making motorcycle helmets mandatory in the mid-1960s. Congress removed that pressure in 1976, and soon after about half of the states repealed their laws. This made possible a near optimum natural experiment, the results of which showed that repeal led to a 25% (4%) increase in deaths of motorcyclists.1
This effect is, if anything, larger than expected, thus ruling out the possibility that wearing helmets leads to any large increase in risk taking. Laws on wearing motorcycle helmets had the expected effect, and it seems hard to imagine plausible reasons why the case for cycle helmets would be all that different. The evidence is fairly compelling that passing a law making the wearing of cycle helmets mandatory will result in appreciable reductions in casualties.
Accepting that a law would reduce casualties does not inexorably require that such laws ought to be passed. Many unappealing laws could reduce traffic casualties. Successfully prohibiting passengers from travelling in front seats of cars while any rear seat remained unoccupied would prevent many casualties because of the substantially lower risk in rear seats.1 The advantages of social interaction and a better view make it unlikely that any such law will find support. Being compelled to wear a bicycle helmet does not incur such large negatives, but it does require a diminution in freedom. No scientific investigation can ever lead to the conclusion that a law ought to be passed. Such questions should properly be decided through the political process, the appropriate forum for taking into account disparate interests, values, and alternative approaches (p 1534).6 It would be presumptuous for me to express a preference for whether Britain should or should not pass a law making the wearing of cycle helmets mandatory - my experience goes back more than 30 years to commuting by bicycle without wearing a helmet, as was then the universal custom, to universities in Belfast and Oxford.
My plea is that the discussion of whether or not to pass a law should take full account of the scientific information that research has uncovered. Robert Oppenheimer and Edward Teller disagreed passionately over whether to develop thermonuclear weapons, but they had no disagreement over the physics on which they were based. Discussions on whether to require cyclists to wear helmets would become more productive if everyone would accept that it is well established that helmets substantially reduce risk in a crash, and that passing laws making wearing them mandatory would substantially reduce casualties.