3Rd Eye Pro Mirror Adhesive (Black) (Fits Most Helmets)

I put one of these on every helmet I own. I like it much more than the handle bar mounted mirrors since your field of vision is bigger and you can quickly scan everything behind you with a quick twist of the head. I commuted 25 miles to work two or three times a week through Silicon Valley traffic and I can't say enough good things about this mirror in heavy traffic.

I prefer this to turning your head since you aren't taking your eyes off the road in front of you for multiple seconds while you turn your body, shoulders, and head to take a look behind you. Granted, turning your head does indicate to the car behind you that you are interested in moving left but frankly, so does a left turn hand signal, and if there is a car within your danger zone, you shouldn't be moving left anyway.

I have had some issues with the adhesive not sticking to some helmets after some months of use, but double sided tape easily and permanently fixes that. This is clearly a perfect solution to a nagging problem.