Between 1994 and 2008, Canada had 66,716 hospital admissions for cycling accidents. 30% of those were head injuries. Cyclists are vulnerable road users and head injuries account for 75% of cycling-related deaths. It's a dangerous way to travel and so the debate has long been whether or not helmet legislation makes any difference in injuries.
During that time, there was a substantial and consistent fall in the rate of hospital admissions for cycling related head injuries - and reductions were greatest in provinces with helmet legislation - but that trend had been happening before the law was enacted.
Taking baseline trends into account, helmet legislation didn't save people. If safety is paramount, just ban cycling, but pulling people over for not having a helmet isn't saving lives, it's wasting public safety resources. Most people used helmets before they were mandatory and still would. No one denies helmets reduce injuries and should be encouraged but the impact of legislation seems to be minimal and therefore not worth the cost. Basically, not all safe conduct can be made mandatory and the cost of controlling all behavior may not be worthwhile.
"When baseline trends in cycling related injury rates were considered, the overall rates of head injuries were not appreciably altered by helmet legislation," the authors write. "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."
Citation: Jessica Dennis, Tim Ramsay, Alexis F Turgeon, Ryan Zarychanski, 'Helmet legislation and admissions to hospital for cycling related head injuries in Canadian provinces and territories: interrupted time series analysis', BMJ 2013; 346 doi: http://dx.doi.org/10.1136/bmj.f2674 (open access)