About twenty years ago, New Zealand passed legislation mandating (§11.8) that cyclists wear a helmet while cycling. The premise behind this law is pretty self-evident: protecting one's cranium in the event of a cycling accident. What I would like to do is a) figure out whether mandatory bicycle helmets have major impacts on the rates of cycling injuries, and b) whether the government mandate is necessary.
Looking at the studies on the efficacy of bicycle helmet mandates, the effects are not clear. The British Medical Journal (BMJ) published a study on the effects of cycling-related head injuries in six Canadian provinces (2013) and found that the "overall rates of head injuries were not appreciably altered by helmet legislation." When looking at Australian road deaths, which is relevant because Australia was the first country with a mandatory bicycle helmet law, there would have been a greater decrease relative to other road deaths, but there was no such decrease. According to a BMJ cost-benefit analysis (2002), there were some cost-savings for those under 19 years old, but considerable costs for adults in New Zealand. Even in the United States, cycling fatalities and injuries have decreased without mandatory helmet laws (Department of Transportation).
The reason why the mandatory bicycle helmet laws do not produce the "desirable results" is because other more primary factors can be attributed to the decrease in injuries, including speed limit laws, improved traffic laws, better lighting of streets, increased education, and decreased bicycle ridership as a result of the new law (see study here; also see here and here).The thing with helmets is that they do not prevent collisions, but provide a last line of defense in the event of a crash.
Cycling is a relatively safe activity. The benefits of cycling outweigh any risks or costs that might take place (see meta-study by Teschke et al, 2012; de Jong, 2010), and to enact a policy such as mandatory helmet laws that decreases cycling rates because of a relatively small probability of collision is senseless. Furthermore, motorists and pedestrians are more likely to get into an accident, so why don't we mandate that everyone wear helmets whenever traveling from Point A to Point B?
I'm not here to say that bicycle helmets do not provide some sort of protection, because they do (NIH, 2001). If an individual infringes upon another individual, then yes, a case can be made for government intervention. I have a problem when the government gets into its paternalistic mode by thinking it's justified in protecting individuals from themselves. John Stuart Mill created the "Harm Principle," which states that one should not interfere with competent adults who take risks with their own health. Whether it's smoking cigarettes, donating one's organs, eating trans fats, health care choices, and that includes wearing a bicycle helmet, the choice should be up to the individual, not the Nanny State.