I must live by a bunch of dumb-dumbs then since I almost never see adults wearing helmets. That is why I posted the original question. If i do see someone wearing a helmet they are cruising on a road bike. The fastest I have been on my MTB is 22 mph...no real hills to take advantage of by me, and I have meaty knobby tires. I usually cruise around 15 mph. I probably could have answered my own question without even asking. A head hit at 15 mph could certainly be fatal.

Most of the people in my area don't even wear helmets on motorcycles. Helmets on bikers are even rarer.

Thanks for all the input.