Objective observation of helmet use is essential
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.
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.
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
Competing interests: No competing interests