In my experience, mirrors are dangerous. If you're going to make a left turn, and you're concerned that there is someone behind you that's going to mow you down, pull off to the right, and walk your bike across. If you're concerned that someone is going to come up behind you and mow you down - you're screwed anyway. I've ridden tens of thousands of miles, and totally don't understand what mirrors do for people other than distract them and give them a false sense of security.