"Personally, I find the 'danger' idea quite silly. The idea that people who have been cycling for any length of time suddenly think to themselves, "Oh my, we've got to wear a helmet? Streuth, this must be dangerous. Best stop." No.
Personally I'm not so sure. Suppose participation in school sports was accompanied by leafleting and poster etc. campaigns on the dangers of sports, photographs of mangled children and the admonishment that you MUST equip your child with armour if you want them to take part - or opt out. I think there would be a sizeable decline even though most parents are smart enough to understand that children do not explode when taken out of doors.
If you took someone from Mars into the streets and asked them if they thought cycling looked very dangerous, the scattering of riders in luminous bin-man suits with body armour on would paint a very different picture to, say, continental Europe where crowds of cyclists just look like pedestrians with wheels. Personally I think this (rather than a careful assessment of casualty rates) is what puts the boot in to participation.
At the end of the day, hardly anybody dies cycling (for instance, an average year in Scotland sees a total of zero night time cycling deaths, but people are particularly afraid of cycling after dark) so the whole thing is based on a shifting sand of perception in the first place.
"And to be honest, I think cost is not a significant enough factor. Even in the 80s helmets were not that expensive. They were horrible, cumbersome but even that I find quite unconvincing, except among the type of people who worry more about looking cool than about their safety. "
Again, is it not about marginal decisions? When petrol goes up by just 1p per litre, the amount of driving going on falls, although I cannot imagine any scenario where I would not travel because each mile cost me an extra 1/13th of a penny.
I can easily imagine doing less cycling in a helmeted world. The car keys are hanging at the door, but it's a nice day so you think about getting the bike out of the garage but you can't remember where you put your helmet when you last came in. For what proportion of people would the delay of finding it (or even if you know where it is, going to get it) be outweighed by just grabbing those dangling keys and walking out the door?
It's interesting that (apart from in the UK) the pace of change seems now to be set against helmet use. Laws have been repealed in Mexico and Israel to support urban cycle use while the Australians are coming around - increasing calls for repeals because their cycle hire schemes are such a dismal failure compared with... everywhere else in the world.
If helmet use didn't discourage cycling, and everybody is actually enjoying just the same enthusiastic cycling environment as before their unrelated but contemporaneous "blips" in participation - why are people going to the trouble of campaigning over it?