Nathaniel Porter opined the following...
How about an effective means of delivering courtesy messages to other road
users? (i.e. I'm am ceading my priority, thank you and so on.)

Mostly needed on motor vehicles (other users can use hand signals to much
the same effect), but could be developed for use on other vehicles if demand
is there.

Would help make using the road easier and more pleasant.

On some occasions I have felt the need for a large sign across the back
window which could display messages to other drivers, such as "Sorry,
being a tit there!".

It could also be used to suggest to some drivers that they might wish to
use the vast and frequently empty tract of tarmac known as the inside
lane!

Jon