Slightly tangential, but there are a couple of local villages near where I live where local residents operate calibrated speed guns on roads where there have been accidents caused by speed in the past.
They do not try to pretend that they are police (their average age is 60+ which rather gives that away anyway!) and so a number of drivers ignore them, considering that the inevitable arm-waving and hard-staring is the only consequence of their speeding past them. However, if they are 'caught' twice within 12 months, the owner of the car receives a very strongly-worded letter from the local constabulary enclosing the offending pictures and confirming that their car reg. has been added to a list of cars for 'heightened-monitoring' and circulated to all local police officers.
Whether there is such a list is a matter of conjecture, but when these letters started to be received it certainty caused a stir locally and I understand that the frequency of those speeding has considerably reduced.
Given the funding pressures on plod I think that this initiative by locals has real worth.