Under most conditions, I can usually hear well in advance when a vehicle is approaching me from the rear. But when I'm pedaling into a strong headwind, all bets are off. That's when gravel trucks catch me completely by surprise.
I bought a helmet mirror to remedy this situation. When I attached it to my helmet, I realized immediately that I was in trouble. My vision is skewed so that one eye is near-sighted and the other far-sighted. Working together they compensate amazingly well -- at 44 I still don't yet need glasses. Unfortunately, it's my left eyeball that's far-sighted, and the mirror is designed to prop four inches away from it.
I took it for a tour, figuring maybe this challenge would force my deficient lens to exercise some flexibility. But no, that was not going to happen. Instead, the mirror became a serious distraction and I finally ripped it off.
Since the design deficiency is my own, I'll not bother with a brand name. I'm told there are cyclists who swear by their helmet mirrors.
Filzer dZ4L Cycling Computer:
I gambled $16 on this baby, and I'm glad I did. It isn't a necessity for cyclists, unless they take their driving directions from Google Maps. But for those of us who wonder how fast we're going, and how far, this little gadget pays off in spades. It's a snap to mount (took me 45 minutes of consulting the instructions and attaching the gizmo to the appropriate spot on the bike). Negatives: it's impossible to read in the light of high noon; it, uh, can puncture the ego, somewhat (trips I thought were the equivalent of a spin to the Big Apple turned out to be only 16 kms).