Funny - I thought this might come up again after Wiggins' comments. There is a very strong opinion piece in the Times by Christian Wolmar arguing against compulsion - making some of the same points as have been made in this thread - it is pay-walled so here is an excerpt:
First, it would reduce considerably the number of cyclists on the roads, as evidence from Western Australia, where helmets have been mandatory for 20 years, shows. Cycle use in Perth rose by 10 per cent a year between 1983 and 1989. But by 1994, two years after the law was introduced, the number of cyclists had fallen by 50 per cent.
Cost and practicality are deterrents, as is style, especially for the young. As Dr Mark Porter, chairman of the BMA Council, has said: “My children refuse to wear helmets. I would prefer them to ride without than not cycle at all as the benefits far outweigh any risks that would be mitigated by a helmet.”
And why not make motorists wear helmets? As 50 per cent of car occupants’ deaths are caused by head injuries, wearing a head band with a hard shell, such as the one designed at Adelaide University, would save many lives. But motorists balk at the idea. When I tweeted it yesterday, Edmund King, of the AA, said it was “a silly idea as drivers have airbags, crumble [sic] zones etc”. But they still die in droves.
Second, cycle helmets may increase accidents because of the well-documented phenomenon of risk compensation. Remarkably, in Western Australia, despite the fall in cycle use after the legislation, hospital admissions of cyclists remained the same. The reason may not be just that cyclists feel less vulnerable and take more risks, but because motorists treat cyclists wearing helmets differently.
In 2006 Dr Ian Walker, a traffic psychologist from the University of Bath, cycled in Salisbury and Bristol recording data from overtaking motorists. They came closer when he wore a helmet because drivers felt he was likely to be more experienced and gave him a wider berth when he wore a blonde wig, pretending to be a woman.
There is a wider point. Making helmets compulsory suggests that cycling is dangerous and needs special equipment and protection. Only when old ladies and pregnant women feel safe cycling in our cities, as they do in the Netherlands and Denmark, will the aim of protecting cyclists have been achieved.