Would you be ok with a mandatory helmet law for car drivers and passengers? The risk of a head injury that a helmet would prevent is much higher in a car, and carrying a helmet around in your car is more convenient than on your bike (e.g. it's much less likely to get stolen). Nevertheless, we don't have mandatory helmet laws for people in cars; why is that?
Sure but you have to normalize it somehow. Americans drive way more than they bike. I imagine if you calculated this per-time or per-distance, driving would still come out much safer for the brain than cycling without a helmet.
I don't remember the source, take that as you will. Cars go faster which means a higher risk of collisions (less time to react etc.), and also a higher risk of head injury per collision.
