BMJ  2006;332:852 (8 April), doi:10.1136/bmj.332.7545.852

Letter

Do enforced bicycle helmet laws improve public health?

Determining true effectiveness of safety measures

EDITOR—Hagel et al have not grasped the main point of Robinson's analysis about mandatory bicycle helmet laws.^{1 2} Regardless of how effective a particular safety measure might be in theory, or even empirically from case-control studies, failure to demonstrate any real benefits in whole populations over time must necessarily call into question its true effectiveness—particularly when compulsion is involved.

Helmet proponents are often quick to claim any reduction in cycling fatalities as being primarily due to increased helmet use. However, they have been unable to demonstrate that these reductions are not better explained by reduced cycling, due at least partly to the imposition of helmet laws or other more general traffic safety measures, or both—for example, enforcement of lower speed limits, drink driving crackdowns, red light cameras.

Although there is no particular reason to suppose adult pedestrian and bicycling fatalities should necessarily track each other over time, considerable evidence shows that such fatalities have been strongly correlated in juveniles going back to the 1970s, at least in the absence of enforced helmet laws.

During 1979-2004 the correlation between cycling and pedestrian fatalities for children under 16 is an impressive 0.96 in Britain, and an even more impressive 0.99 in the US—strongly suggesting these trends have moved in virtual lockstep over the past 25 or more years.

Despite increasing helmet usage by juvenile cyclists in both countries over the past two decades, from virtually nil to 15-20% in Britain and at least 30-40% in the US, juvenile cycling fatalities declined by a slightly smaller proportion than juvenile pedestrian fatalities—77.3% v 80.5% since 1979 in Britain³ and 73.3% v 75.8% in the US.⁴

Even if we were to ignore the data from Australia and elsewhere and agree that bike helmets might still be substantially effective in preventing potentially fatal head injuries, the failure to observe any such beneficial effect in either short or long term time series data must imply that helmeted cyclists are still managing to get themselves killed at roughly similar rates to their formerly unhelmeted counterparts—presumably through some process similar to risk compensation.⁵

Credit: ED PRITCHARD/STONE/GETTY IMAGES

 

Institute for Traffic Safety Analysis, Arlington, VA 22204, USA rileyrgeary{at}comcast.net

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Competing interests: None declared.

References

1. Hagel B, Macpherson A, Rivara FP, Pless B. Arguments against helmet legislation are flawed. BMJ 2006;332: 725-6. (25 March.)[Free Full Text]
2. Robinson DL. No clear evidence from countries that have enforced the wearing of helmets. BMJ 2006;332: 722-5. (25 March.)[Free Full Text]
3. Department for Transport. Transport accidents and fatalities. STATS19. www.dft.gov.uk/stellent/groups/dft_transstats/documents/divisionhomepage/037791.hcsp (accessed 31 Mar 2006).
4. National Center for Statistics and Analysis. Fatality Analysis Reporting System (FARS) web-based encyclopedia. www-fars.nhtsa.dot.gov/ (accessed 31 Mar 2006).
5. Geary R. Electronic response to: Arguments against helmet legislation are flawed. bmj.com 2006. http://bmj.bmjjournals.com/cgi/eletters/332/7543/725#130841.

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