I respectfully disagree, I think helmet laws are not a good idea at all. They discourage people from cycling by signalizing that cycling is a dangerous activity, which it isn’t. (For a sense of perspective, walking the stairs is much more dangerous than cycling.)
The helmet law in (some parts of) Australia apparently has slightly reduced the number of cycling-related head injuries, but at the cost of drastically reducing the number of people who cycle at all, such that the reduction in number of injuries may just come from much less distance cycled.
And fewer people cycling (and driving instead) is bad not only for the climate, but also from a health point of view: every-day cycling as a moderate activity is healthy. So, on a society level, any lives saved by mandatory helmet-wearing may very well be countered by the lives lost or shortened due to an inactive lifestyle of the population.
If you do not want people to cycle, impose a helmet law. If you want people to cycle, build proper cycling infrastructure. (To see what proper cycling infrastructure is, see http://www.aviefromthecyclepath.com.) Nobody wears a helmet in the Netherlands, and it has the lowest number of cycle injuries per distance cycled in the world (apart from those countries where this rate is zero undefined, because nobody cycles).
