I think the polystyrene and the actual ‘something on the head’ works for a small range of potential injuries; that 12 to 15 mph tumble type of thing.
I think the problems begin when people start to view them as the bicycle equivalent of seatbelts, and assume accordingly that not having a helmet equals certain death.
I don’t think that a cycle helmet can be the difference between walking away or ending up severely brain damaged.
If it were the case, we would see clear evidence that head injuries in cyclists have declined, and that evidence simply doesn’t exist.
It’s a very subtle, very complex problem, which is unfortunately presented as a black or white, helmet or death issue.
Making helmets compulsory will cause a drop in the number of cyclists, but the only thing that seems to improve cycling safety is having more bums on bikes….