Unfortunately, I'll have to file this along with "my helmet saved my life".

I like using a mirror. It makes me feel safer, more aware. But the reality is that most of the cars that come close to running me over from behind don't do so, mirror or not. (Well, so far, 100% of them have failed to do so, and I hope that continues.) I could hit the ditch every time I think it might happen, but then I'd be hurting myself in the ditch quite often to avoid collisions that weren't going to happen.

Ultimately, the only reliable way I can think of to make sure that the car was going to hit you is letting it hit you. Anything else is just a guess.