> I'm willing to bet that a lot of the same people who think that not wearing a bike helmet (and especially allowing a child not to wear one) should be a criminal offense would think the idea of having to wear a helmet driving is just silly.

Although I don't think biking without a helmet should be a criminal offense for adults, I think the comparison with cars isn't quite right. We do require people to use safety equipment like seatbelts and airbags when riding in a car. A helmet probably won't do much for you in a car crash but can if you wreck your bicycle just like a seatbelt would be silly on a bike.