The BMJ has an article on the effects of Canadian bike helmet legislation. Between 1994-2003, Canadian provinces with mandatory bike helmet laws saw a 54% reduction in bike-related head injuries as compared to only 33.1% in provinces without helmet laws.
So helmet laws work, right? Well, not exactly…
As the article notes, injury rates were already on a downward trend. And there are confounding factors (such as new bike facilities, fewer cyclists, enforcement, etc) that may also explain the reduction.
After taking baseline trends into consideration, however, we were unable to detect an independent effect of legislation on the rate of hospital admissions for cycling related head injuries.
Conclusions: Reductions in the rates of admissions to hospital for cycling related head injuries were greater in provinces with helmet legislation, but injury rates were already decreasing before the implementation of legislation and the rate of decline was not appreciably altered on introduction of legislation. While helmets reduce the risk of head injuries and we encourage their use, in the Canadian context of existing safety campaigns, improvements to the cycling infrastructure, and the passive uptake of helmets, the incremental contribution of provincial helmet legislation to reduce hospital admissions for head injuries seems to have been minimal.
Here are the time-series graphs. The vertical dotted line shows when the helmet law went into effect. As you can see, injury rates were declining anyway, with the law having no effect.