bringing up statistics and chances of this sort of thing happening, when I see how many people tour without a mirror, it makes me think that the chance of having an incident of being passed super close by a car, truck or RV and not being aware of this incoming danger is in the thousands and thousands more times likely to be a real threat, with potential every time you ride.
True indeed, I love my Safe Zone helmet mirror. 5.7 cm diameter gives nice clear view. Looks kinda nerdy but so what. A couple of people have asked about it since
AFAIK most bike shops don't carry it.