I assume you mean wearing a helmet while being in a car to reduce the risk of car related injuries and deaths. I actually looked into this and from what I remember, helmets do more harm than good. They have the benefit of protecting you from hitting your head against something but the issue with accidents comes much moreso from the whiplash, and by adding more weight to (the top of) your head, helmets have the cost of making whiplash worse, and this cost outweighs the benefits by a fair amount.
I assume you mean wearing a helmet while being in a car to reduce the risk of car related injuries and deaths. I actually looked into this and from what I remember, helmets do more harm than good. They have the benefit of protecting you from hitting your head against something but the issue with accidents comes much moreso from the whiplash, and by adding more weight to (the top of) your head, helmets have the cost of making whiplash worse, and this cost outweighs the benefits by a fair amount.