"Handheld guns - Used by an officer standing at the side of the road. They point it at the numberplate, pull the trigger and get a readout of your speed. They then wave you in as you pass, or stop you with another vehicle further down the road. These are not easy peices of kit to use, so they won't always get a read. They can not do a postal prosecution for speeding based on these. If you're speeding, they will pull you over.
Traffic cars (parked) - you may see a traffic car with a camera (normally a black round one stuck to the rear roof) watching traffic, often on motorways and dual carriageways. This is ANPR (automatic numberplate recognition). It is for checking your number plate against the Police National Computer (PNC). This reports if your car is taxed, tested, insured, registerred to someone with a license, not generally got anything dodgy about it, eg been seen near a crime scene. This is not a speed measurement device. To measure your speed they need to use a handheld gun or VASCAR.
Traffic cars (in motion) - Traffic cars are equipped with calibrated speedometers and odometers. Traffic car follows you a set distance behind. Sets a stopwatch and trip counter. Follows you for a set distance. Average Speed = distance divided by time, which is calculated by VASCAR. Blue lights come on, you get pulled over and given a ticket.
Note - not all traffic cars are marked.
Mobile speed cameras ("Talivans"). A van (normally a van) with a speed camera in the back, often pointing out of a hatch in the back door down the road, or out of the side door if the vehicle is parked, for example, on a motorway bridge. You go past too fast and a ticket automatically comes in the post."