Hi again Mary:
This is what used to happen: You would, quite rightly in my view, report such an incident, the Police would then trace the car owner through the registration number(as long as the motor was actually registered - sadly, not all are!), pay them a visit, find out who was driving at the time, and give them a good old fashioned "b*llocking" which at least made the little scr*tes aware that they'd been brought to Police attention.
There was no need to disclose the informant's details and the matter was usually resolved without the hassle and cost of a Court case. Simple, eh?
Ahhhhhhh, those were the days !