I just sold a road bike. I would wear a helmet when I rode it. Because, it was fast and the tires are quite thin. Accidents can more easily happen. When I rode my mountain bike with slick tires around, I never felt the desire or need to wear a helmet. It was such a sturdy safe bike. I now ride a hybrid, and I am willing to ditch my helmet permanently. I still have it in case I want it for side of the highway long rides. But, it is such a safe bike, I don't feel the need.
I mean just think about horses. Man has been riding the horse for thousands of years. And, he never wore a helmet. And, a horse is much more dangerous that a bicycle in my opinion.
If a bicyclist obeys the traffic laws, cars are not a problem. Cars and drivers treat me so well. My path is like open sesame.