Mum, in reply to your unasked question, "they don't". I live on the busiest road in Clacton on Sea, and I have been working on Essex CC for the last 2 years to try and slow down the traffic on it, the police did a survey in response to my pleas and came up with 2000 VEHICLES WERE EXCEEDING 38 MPH PER DAY on a 30 mph road, and Essex CC said, and I quote, "we don't go on police surveys, we do our own when it's necessary, and here it isn't."
ps at the same time 85000 vehicles passed the "little black box" in 6 days.