I only ever see casual riders not wearing helmets. I figure my helmet makes my head incredibly hot, but the sweat all in my hair is way less gross than if it were blood after my head hits the pavement. I can't imagine riding my bike faster than 6 mph without a helmet.

I'm constantly telling my students that they should wear a helmet because first of all, it's the law. Second, looking like a dork for a bit while on a bike/skateboard/scooter is way better than spending the rest of your life with a severe head trauma and not being able to feed and/or bathe yourself.