Cycle Training

A long time ago, in a galaxy far far away there was thing mounted to every bike sold. It was called a rear view mirror.
There were also factory fitted lights and reflective pedals, but hipsters decided, that it's a square "dad's" look and thou shall to strip the bike from all distracting unstylish props...

Mirrors? Bollocks.

When were mirrors a common fitment on bikes?
They are shit and give you a limited rear view.
Headchecks ftw