I use a variety of mirrors on my different bikes, Mirrycles, Blackburns, and a 2" helmet mount. I'm hanged if the mirrors are any use in alerting me to stuff gaining on me unless I am looking in it. Mostly that is when I am checking to make sure its safe to pull out and pass someone in the bike lane or when I want to make a left turn etc. Sometimes I check just to check but LOTS of times stuff... big stuff slides right past me and I didn't even see or hear them coming up. Mirrors are great but they don't automatically put eyes in the back of your head. I've seen way too many posts in the last month that give that impression. Mirrors make it easier to check behind you once you decide that you need to do so. As a constant "monitor" of things taking place behind you your ears are WAY more useful.

H

When I see any situation ahead of me that will force my moving out in the lane, the first thing I do is check the mirror to see what's behind me and determine my action based on that. When I see a truck approaching in the opposite lane, I check to make sure another one behind me won't be passing it at the same time I am. I was once touring with a girlfriend, who was riding ahead of me, and I thought two trucks were going to pass her at the same time on a narrow road and started screaming her name, but she was fine.

I also find the mirror helpful for dealing with yahoos in redneck country, that being the sort of jerks who like to take a slap at you as they drive by. When I spot a suspect car approaching, I ride a bit further out than I need to, then use the mirror to move in just before they reach me, precluding any reaction on their part.

One way to try to get additional room is to hang some sort of bright/day-glo fabric on the left rear of your bike, which I realize is more mechanically difficult (and perhaps unwanted, based on your self-concept) on a roadbike. When touring once I found one of the orange flags that road workers use, and fitted it across the top of my rack so the flag portion hung behind my left pannier, sort of stating that was how much room I wanted. I noticed that cars definitely gave me more space after that.

I can't remember the last time I had someone do that. And, I live in an area where there are more cows than people. (Hey, maybe that's why.) But, there are a lot of idiots and even dangerous people out there. When I ride alone I usually carry.