No clear evidence from countries that have enforced the wearing of helmets

Objective observation of helmet use is essential

14 May 2006

Robinson finds no measurable useful effect of cycle helmet laws, contradicting several case-control studies1. These have consistently found that cyclists without head injuries are much more likely to report helmet use than cyclists with head injuries2. From this, a strong protective effect of helmets has been inferred.

Polystyrene caps may or may not have some useful protective effect, but many sources have tried to impress upon cyclists their duty to wear one3. Helmets are inconvenient, costly, and uncomfortable. In some areas, an injured cyclist who admits to riding without a helmet may face legal penalties for doing so. These factors may lead to self-reported rates of helmet wearing much higher than observed rates. For example, in one case- control study based in Seattle, 7% of the cyclists with head injuries reported wearing helmets, as compared with 24% of the emergency room controls and 23% of community cyclists who had had an accident4. However, less than 6% of cyclists on the Seattle streets at the time were actually observed to wear helmets5. Either helmets are a serious cause of accidents, or self-reports of helmet use are not valid.

This points to the fatal flaw in published case-control studies; cyclists without head injuries will report rates of helmet wearing much higher than their true rate. Cyclists with recent head injuries, as in the case-control studies, will not be able to deceive themselves or their interviewers, and will report much lower, truer rates of helmet wearing. This bias is enough to account for all the positive findings in case- control studies; excess accidents caused by helmets remain an additional possibility. Studies based on self-reporting of helmet use are not valid and cannot sustain valid conclusions. Any future studies of the effect of cycle helmets must include objective observation of helmet wearing if they are to be of any scientific value.

Robinson presents the best available evidence derived from objective assessment of helmet wearing. Her demonstration that cycle helmet laws do not work is likely to remain the definitive answer.

1. Robinson DL. No clear evidence from countries that have enforced the wearing of helmets. BMJ 2006;332:722-725

2. Thompson DC, Rivara FP, Thompson R. Helmets for preventing head and facial injuries in bicyclists. The Cochrane Database of Systematic Reviews 1999, Issue 4. Art. No.: CD001855. DOI: 10.1002/14651858.CD001855.

3. http://www.cyclesense.net

4. Thompson, Rivara & Thompson. A case-control study of the effectiveness of bicycle safety helmets. NEJM 1989;320:1361-7.

5. DiGuiseppi CG, Rivara P, Koepsell D, Polissar L. Bicycle helmet use by children. Evaluation of a community-wide helmet campaign JAMA 1989;262:2256-2261.

Competing interests: Cyclist

Competing interests: None declared

Richard M Keatinge, General practitioner

Gwalchmai Surgery LL65 4RS

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