The voice of him that crieth in the wilderness

Isaiah 40:3

But for me, it's bicycle mirrors.

Despite the hype, there's evidence both for and against bicycle helmet use. I can find no good research about the use of bicycle mirrors (riders either seem to have a mild prejudice against them, or are rabid, no-room-for-discussion supporters of mirrors for bikes).

I believe in bicycle mirrors. By the time your helmet comes into play, it's too late: something drastic has already happened. My contention is that bicycle helmets really protect everybody else: riding clubs, schools, municipalities; they either can say that the helmet saved you, or they can tut-tut if you get into an accident and you weren't be-helmeted. But the mirror, once you've gotten used to it, and you know its limitations (and it both requires getting used to, and has limitations), saves you from getting into the accident in the first place. If helmets protect everybody else, mirrors protect you. I can say with certainty that my bike mirror has saved my life at least twice.

I have a hanging-from-my-eyeglass-frame mirror that I use when I get all suited up for a ride. I've learned to use it, and I know its limitations (its not useful if someone is riding to my right rear, for example). I do enough riding of my road bike in my street clothes, thought, that I've also installed a handlebar-end mirror; this one is convex, so "objects in mirror are closer that they appear", and it's not as useful as the other - but it's way better than nothing.

If I had my way (and it's probably a good thing I don't), little kids would be required to wear bike helmets... but kids over 8 would be required also to have mirrors (they could be incorporated into the helmets themselves), to teach them how to use a bike mirror. Within a generation, I bet riders would just expect to use mirrors, like we do in cars - and I bet accidents would drop by great bunches, because I'll bet regular use of a mirror teaches riders to be aware of ALL their surroundings: ahead, behind, and on all sides, as well. That, at least, has been my experience.