• bicycle helmet
• risk compensation theory

Editor,—The recent exchange about risk compensation and bicycle helmets overlooked an important dimension of the issue.^1,2 By reducing cycling and, hence, diluting the effect of “safety in numbers”, compulsory helmet laws could have the perverse effect 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” effect 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 …