I'm a bit surprised that no one mentioned rear view mirrors. I have them on both of my MBs and use them constantly. It's a lot safer to simply glance down at the mirror than to twist my head around to look behind me. I tried one of those eyeglass, clamp on thingies but for me it was a distraction and seemed to form a blindspot albeit a small one, there was an area of my peripheral vision that was blocked. I like the convex style of spot mirror, about 3 or 4 inches in diameter mounted on the left bar and with a short stem that keeps vibration from distorting the view. Most cars are quiet today and can sneak up on you before you know it. Having the ability to see what's approaching from behind is a big plus in the safety area. IMHO.
Tom
Tom