D Walford writes:

With a visible number cyclists could be identified when they break the
road rules, couldn't have cyclists obey the road now could we.

My old Kronan 3vxl herrcykel came with a rear license plate.
It was a gimmick, of course.
https://en.wikipedia.org/wiki/Kronan_(bicycle)
Probably only feasible with a rear fender or rack.