Blackburn helmet mirror. The mount to helmet isn't really secure enough, so I add my own glue. I bring a spare on tour, just in case. I much prefer helmet or glasses mirrors so that one can scan the area behind.
Racers don't use mirrors because the deal is that safety is the responsibility of the rider following. Thus those recreational riders who fancy themselves racer boys or girls also seldom use mirrors. The issue is that with large metal objects instead of bicycles behind one, it doesn't matter whose responsibility it is, you're just as dead or injured either way.
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Results matter