The risk compensation theory and bicycle helmets EDITOR,-It has come to our attention that a number of readers have been mystified by our contribution to the debate about bicycle helmets published in the June issue (2001;7:86-91). In particular, those familiar with our previous writings on the subject were puzzled by the claim of the Thompsons and Rivara, in what appeared to be the conclusion, that we agreed with them that "bicycle helmets are eVective in decreasing head injuries to cyclists". The confusion was caused by the fact that the responses were published in the wrong order. For those wishing to clear up the mystery, we recommend returning to the published debate and reordering the contributions as follows: 1. Risk compensation theory should be subject to systematic reviews of the scientific evidence (Thompson, Thompson, and Rivara 1 2 By reducing cycling and, hence, diluting the eVect of "safety in numbers", compulsory helmet laws could have the perverse eVect of increasing serious injury rates among those who continue to cycle.Nearly all fatal cycling crashes involve motorists. But there is evidence that the rate of bicycle-motor vehicle crashes declines as the amount of cycling on a road or in a region increases. This "safety in numbers" eVect is thought to occur because as cyclists grow more numerous and come to be an expected part of the road environment, motorists become more mindful of their presence and more respectful of their rights.The implication is that adding cyclists to the road makes it less likely that a motorist will strike an individual cyclist and cause serious injury; and, conversely, removing cyclists from the traYc stream raises the risk to those who continue to cycle. One empirical estimation of this eVect, preliminary and sitespecific, pointed intriguingly toward a cyclist safety-volume "power law" of approximately 0.6. 3 According to this relationship, the probability that a motorist will strike an individual cyclist on a particular road declines with the 0.6 power of the number of cyclists on that road. Say the number of cyclists doubles. Then, since two raised to the 0.6 power is 1.52, each cyclist would be able to ride an additional 50% without increasing her probability of being struck. (The same phenomenon can be expressed as a 34% reduction in per cyclist crash risk per doubling in cycling volume, since the reciprocal of 1.52 is 0.66.)A confident estimate of the precise value of this safety-volume relationship will require further study, but two other studies report similar relationships, one for cyclists 4 and the other for pedestrians.
5This suggests an important thought experiment regarding compulsory helmet legislation:Suppose that (i) cyclists currently are split between helmet wearers (one third) and bareheaded cyclists (two thirds); (ii) there is no self selection or other confounding diVerence between bareheaded and helmeted cyclists as regards their risk of injury-causing accident; (iii) a helmet law provokes one third of the bareheaded cyclists to quit cycli...
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