> Passengers would in fact have a lower chance of injury in a car crash if they were wearing helmets.

I once thought so. Someone pointed out that helmets might make whiplash worse (because they extend the head backward, so will change the angle at which the head impacts the headrest) and that we're essentially using airbags for (a.o.) the same functionality (protection from direct strikes to the head). So, I'm ambivalent on the topic now.