Bike mirrors - should they be mandatory?

This question comes up from time to time: "Why bikes don't have wing mirrors fitted? Surely mirrors would warn of approaching cars?"

CAN's response
In New Zealand bicycles are required to have effective brakes, reflectors and at night, lights. Although mirrors can be useful in some situations, they are no substitute for looking over your shoulder and having a good look around. This also signals to other road users that you are planning to make a turn or change lanes. Mirrors can be useful, but the case for compulsion is weak.

This suggestion was also raised during the Cycle Safety Panel period, in 2014. There seems to be an assumption that having a mirror will enable riders to notice a vehicle bearing down on them from behind and be able to to dive out of the way (or some such evasive manoeuvre). This thinking overlooks two key issues:

1. Contrary to popular belief, most cycle crashes (esp. in urban areas) occur with vehicles in FRONT of you. I.e. it is vehicles crossing, pulling out, opening their doors, etc; not from overtaking you. So on balance it is better to spend most of your time keeping your gaze ahead of you.

2. The notion that riders should use a mirror to take responsibility for avoiding errant vehicles behind them is inconsistent with how other road users use mirrors. Mirrors are primarily there for checking before changing lanes and other lateral manoeuvres (and even then, you are expected to still physically turn your head to check before shifting over). When I'm driving I'm not constantly scanning my mirror to watch for wayward vehicles that *might* run into me (again, my biggest risk is vehicles in front of me, so I'd better pay most attention there). Remembering too that, unless I make a sudden manoeuvre, the legal onus on not hitting me rests with the vehicle BEHIND me.

Practically speaking too, how is a mirror really going to help me avoid a crash with a vehicle from behind me? If I'm riding on a road, I expect that 99.9999% of adjacent motor vehicles will pass me sufficiently well to avoid hitting me. How do I quickly ascertain from a mirror that the *one* exception to this is actually veering into my path, and not just whizzing beside me? Not easy to do, and (as mentioned above) nor should it be my responsibility to do anything about it if I am legally riding in a consistent manner.