I am super super strict about bike helmets. I am about the only mom on my block who is. Friends have said "oh, they just don't listen. " (Same thing about seat belts. I always comment if I see someone I know with children not strapped in properly. )
DS (4) just learned how to ride a two wheeler. I realized that I had been lax about teaching him to always wear a helmet, so he was wearing one when he felt like it. He is a good rule follower when there are consequences lol. So I told him about my rule, gave him one reminder, and the next time he rode his bike without a helmet I took his bike away for the day. I went a little drastic on him because I wanted to drill it in asap, all because I was negligent in drilling it in from day one. But no matter, it worked. He will not get on his bike without his helmet. And that little show was a good reminder for my older kids, who I never even remind because they are thoroughly habituated. I've heard too many stories of helmets saving lives- even when the riders weren't riding in traffic. I just heard a bizzare story where a car jumped a curb and ran over a kid riding a bike on the sidewalk. He was badly hurt but was wearing a helmet.
I just don't understand the parents who claim their kids don't listen. If you don't teach and enforce rules then don't expect them to listen! I am "that mom" who will roll down my window and ask kids where their helmets are. I see so many teens riding in traffic without helmets. It really upsets me.