I don't understand what you gain with an awareness of approaching cars. I assume there are cars behind me constantly, so there's no need to adjust anything. Unless you use the mirror to actively avoid the car coming up behind you, which I'll ask again, at what point do you take action when you are watching that car in your mirror approach? 20 feet? 10? 2? When do you make the call that they aren't going around you, but going into you?

Take Steve Barbour, Columbus, OH super-cyclist, randonneur, ride leader, UBER-Fred, yellow vest, reflective crap everywhere, mirror, "mr. safety first", the works... Mowed down from behind by a drunk driver the other month, dead.

http://www.bikeforums.net/showthread.php?t=564176

What did the mirror do for him?

Unless, of course, you head for the ditch the moment a car is coming at you at around, oh say, 20 / 30 feet or so, all you are "aware of" is your future mowing down.