Gatso cameras are used on traffic lights, to catch people who go through red lights, and also on main roads, at accident black spots to catch people speeding.
When they were first introduced in the Sheffield area, early 1990's there were just six traffic signal cameras and no speeding cameras. Now there are about 15-20. The cameras are designed such that the box and flash equipment are installed permanently at the side of the road, and the internal gubbins is swapped around different locations in the area. The boxes can be purchased independently and it is much cheaper to have lots of extra boxes, which act as a deterrent anyway.
     The local police have to administer the taped evidence from inside the box, which is on a video cassette. In some parts of the country the video cassettes can be filled up in a matter of hours because the speeding laws are frequently ignored.
     The evidence collected on the tape is basically two pictures of the back of the offending vehicle, both with a time and date stamp, and the location. These show the offence clearly being committed. As the first shows the lights at red with the car crossing the line, and the second the car well over the line with the lights still red and the information of the speed of the vehicle and the number plate clearly lit up with the flash gun.

     The Gatso Cameras are generally installed on A roads in towns and cities, especially at accident black spots. Until recently there were hardly any on motorways because their safety record is generally good, compared with the commuting accidents you get on a high street.
     However in the London area, where traffic congestion leads to daily gridlock, the Gatso is king. The M25 is littered with all sorts of licence endorsing gadgets. In one stretch there is a variable speed limit which goes down, as the weight of traffic gets higher, and there are Gatsos on the gantries above each lane of the carriageway. Every driver behaves!
     And there is a live video speed detecting system also in use which can be monitored in real time via a remote viewing installation, and these are endorsing driving licences as you read this.
 

There are many more Gatso installations around that I haven't noticed recently, because I haven't been around those areas for a couple of years. I'll get them checked out soon.
 

The speeding boxes in some parts of the country are positioned in some downright scandalous places, hidden behind large road signs etc.  This is definitely not cricket! But often the result of a very sour faced civil servent who probably doesn't drive, and is also responsible for a number of idiotic, illogical one way systems.

But they are not likely to be working unless they are backed up with the road graduation markings in white paint. And the position of the large camera boxes, the camera warning signs and the white paint should be caution enough.

     On the A1 in the last stretch between St Neots and London, there is a large swinging boggle eyed monstrosity which can do either carriageway, depending upon the time of day.
     In North East Derbyshire they really went overboard with the boxes to such an extent that the roads between Staveley, Eckington and Bolsover have Gatso boxes every half a mile. But none of them appear to be wired up, and some of them still don't even have the white calibration marks on the road.