Readers of a recent article in the BMJ  may have puzzled over the failure of evidence to confirm the intuitive theory that bicycle helmet legislation saves lives. How to explain such a finding when, given a standardised blow to the head, helmets reduce damage in the cranium? One possibility is risk compensation whereby cyclists armed with a helmet are overbold. Another is that drivers are more careful when overtaking a bare-headed cyclist. Support for the latter hypothesis is produced by Walker’s study of 2007, based on telemetric measurements of vehicle distance from bicycles whose riders were, or were not wearing helmets. Beware of theories that are too tidy – subsequent re-analysis of the same data shows that the effect size is tiny after controlling for other variables, and disappears if driver distances are dichotomised into ‘safe’ and ‘unsafe’ at one metre. It is doubtful if the small differences in Walker’s paper could account for the lack of effectiveness observed for bicycle helmets. Should we start again and carry out further empirical work of cyclist-related variables that may affect driver behaviour? Occasionally I see a parent riding with a child to school and I like to think that drivers take extra care in such a situation.
— Richard Lilford, CLAHRC WM Director
- Dennis J, Ramsay T, Turgeon AF, Zarychanski R. Helmet legislation and admissions to hospital for cycling related head injuries in Canadian provinces and territories: interrupted time series analysis. BMJ. 2013; 346: f2674.
- McIntosh AS, Lai A, Schilter E. Bicycle Helmets: Head Impact Dynamics in Helmeted and Unhelmeted Oblique Impact Tests. Traffic Inj Prev. 2013; 14(5): 501-8.
- Walker I. Drivers overtaking bicyclists: objective data on the effects of riding position, helmet use, vehicle type and apparent gender. Accid Anal Prev. 2007; 39(2): 417-25.
- Olivier J, Walter SR. Bicycle helmet wearing is not associated with close motor vehicle passing: a re-analysis of Walker, 2007. PLoS One. 2013; 8(9): e75424.