The risk compensation theory and bicycle helmets

The Cochrane review by Rivara and the Thompsons found evidence that if you bang your head the consequences will be less severe if you are wearing a protective helmet. Based on this review they recommend that cyclists should be “encouraged” to wear helmets. The form of encouragement that they favour is compulsion. We accept the principal finding of their review—that protective helmets protect in the event of an accident—but not the policy conclusions that they derive from it. The issue that divides us is risk compensation—does the behaviour of cyclists change as a consequence of wearing a helmet in ways that oVset the protective benefit of helmets in accidents? After briefly referring to selected references from the safety literature on cycling, motorcycling, and driving, Rivara and the Thompsons assert that “the empirical evidence to support the risk compensation theory is limited if not absent”. Certainly such evidence is limited or absent from the sources they choose to cite—with a notable exception which we discuss below. We find abundant evidence for risk compensation. It is important to distinguish between evidence for risk compensation in general— which is overwhelming, and evidence relating to cycle helmets—which is limited. Let us consider the general evidence first. Rivara and the Thompsons recommend readers to consult James Hedlund’s article in Injury Prevention entitled “Risky business: safety regulations, risk compensation, and individual behaviour”. We strongly support their recommendation. They quote James Hedlund: “I believe the evidence is overwhelming that every (our italics) safety law or regulation is not counterbalanced by compensating behaviour”. But Hedlund also makes clear that the evidence is overwhelming that some laws and regulations, as well as safety measures voluntarily adopted, are counterbalanced by compensating behaviour. He states: “We all change our behaviour in response to changes in our environment. Safety measures change our environment, so we may change our behaviour in response to them. . . . Never assume that behaviour will not change”. Hedlund helpfully sets out four rules for judging the circumstances in which behaviour might or might not change: (1) If I don’t know it’s there I won’t compensate for a safety measure. Bicycle helmets manifestly fail this test. (2) If it doesn’t aVect me, I won’t compensate for a safety measure. He poses the question “Do I feel safer wearing a bicycle helmet?” and suggests that if the answer is yes compensation is likely to occur. (3) If I have no reason to change my behaviour, I won’t compensate for a safety measure. Only if the behaviour of cyclists is completely unmotivated by concern for safety are they unlikely to compensate for a safety measure such as a helmet. (4) If my behaviour is tightly controlled I won’t compensate for a safety measure. He singles out driving as an activity that oVers very considerable freedom to compensate. Cycling oVers at least as much. Hedlund advises “to reduce or eliminate risk compensation, use measures rating low on at least one factor”. Cycling scores high on all four. Of all the cases Hedlund considers perhaps sports oVer the closest comparators. He observes: “Sports provide interesting examples of the interplay between injury prevention, compensation, and control. In many sports, such as ice hockey and American football, players are required to wear protective equipment. Some players have compensated by acting more violently within the confines of the rules. In some instances this has led to rules changes to control player actions more tightly”. Cyclists, like hockey and football players, are acutely sensitive to the likelihood that a miscalculation can result in serious injury, and govern their behaviour accordingly. We find it highly probable, in the absence of any change in propensity to take risks, that cyclists will respond like hockey and football players to measures that reduce the severity of the consequences of miscalculation. Hedlund oVers two further bits of useful advice: (1) Consider system eVects. Cycle helmet laws have led to a decrease in cycling; after it became compulsory to wear helmets in Australia, the level of cycling fell by about twice as much as did the number of cyclists admitted to hospital for the treatment of head injury. Other studies have found that the health benefits of cycling, measured in years of life gained, far outweigh the injury risks measured in years of life lost—by about 20 to 1. By concentrating attention on the need to protect cyclists from head injuries Rivara and the Thompsons encourage the view of cycling as an inherently dangerous activity. Other countries, most notably Denmark and the Netherlands, demonstrate that, by making proper Injury Prevention 2001;7:89–91 89