you could just paint the serial number up the front fork or along the back so if it’s size you’re after. easily identifiable and with no cost to the cyclist.
i don’t see how a registration system for bikes is a bad thing. as i said before, if your bicycle was stolen you’d reference the serial number to the guards. if someone is cycling the wrong way up the south mall then the guards should also be notified of that.
Front fork? What if one has front panners?
Also, do we paint a reg on a car?
I love these stupid ideas coming out of the wood work. Very Trumpian and simpleton world view some people have.