The problem with a mirror is that it could cause you to react and crash yourself during close calls. Anyone who rides on the road has countless experiences where some idiot in a cage refuses to give you a few feet when he/she passes. On numerous occasions I have been passed so closely that I have thought, "I would not even have to extend my left arm and I could have touched that car." It sometimes scares the crap out of me even now. But, I don't ride with a mirror so I never know when a car is going to buzz me and can't overreact. I have never been hit by a car but I know a couple times I may have crashed myself through evasive action if I had known how close a car would be to me. Plus, I tend to agree with other posters that you probably don't have time to safely avoid being rearended by a car...and there are plenty of dangers to avoid ahead of you without worrying what's behind you.

As for people who ride on public roads and think they don't need a helmet because they ride slow, are you kidding me? If that is a serious opinion, maybe those people are right and should save their money on the cost of a helmet - it doesn't sound like they have much brain to protect anyway.