Originally Posted by
Slaninar
Because:
1) you're usually going slow enough to have the time to turn around and look.
2) Because you're in the open space on a bicycle, no roof, back seets etc. blocking your view.
3) Because when I ride 30 km/h and a vehicle approaches me with 80 km/h, I have no chance to avoid them if they decide not to swerve to pass me - too high a speed difference.
I used to have a mirror. It was good when riding with a rain hood on - less complete turn arounds before left turns/lane changes. Apart from that, mostly useless. For me.
With a properly adjusted mirror one has a constant view to the rear in their peripheral vision which improves ones situational awareness. A mirror doesn't replace a "head check", but the constant situational awareness a mirror provides can be invaluable when there isn't time for one.
My mirror is helmet mounted, and I sometimes ride the local MUT without wearing it, so yes I understand one can still maintain an acceptable level of safety without one, but it comes at the cost of being less focused on whats ahead, and expending more effort to maintain the same level of awareness.