daihard, it also has some information about bicycle-car crashes. Interesting to me, about half of the serious injuries in bike-car collisions involved a head/brain injury. About half did not.

Six Jours might be pleased to note that the "risk factor" in car-bike collisions, when multiple injuries are considered, is only 0.18 higher when not wearing a helmet. It's only 157 of those injuries per billion kilometers however, so it's easy to understand why the Netherlands riders aren't that gung-ho on wearing helmets.