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...

But importantly by looking backwards you make the other drivers aware that you know they are there.

As such the mirror does not aid in communication in the same way. It does not enable eye contact so easily.