But a helmet will help even more. It costs nothing, what is there to reject?
A competent car helmet is probably a motorcycle helmet, which is more like $100 to start, but it impacts hearing, reduces vision in all directions, is a significant weight, usually doesn't adjust much for sizing, takes longer to put on (especially if you wear eyeglasses).
A bike helmet in a car would likely be more trouble than anything, it would interfere with the headrests and probably increase neck injuries.