I often travel on the M6, at least twice a month. The M6 constantly has roadworks in which they impose a speed limit of 40 mph, and is monitored by average speed checks.
I’m always amazed when I travel through these sections of roadworks by the number of people who do not understand the word “Average”. Loads of drivers acknowledge the speed cameras and slow down for just the section in which the camera is, however the average speed checks do exactly what it says on the tin!

How it works is, CCTV cameras monitoring the traffic recognise your numberplate as your enter the speed check zone and record it along with the time. Further along the route other CCTV cameras repeat this operation producing a list of times for each vehicle at each camera. The computer controlling the CCTV cameras knows the distance between each camera, and therefore can easily calculate your average speed.
So, if you enter the zone at 12.00 and leave the zone at 12.01 and the distance between each point is 1 mile, you have an average speed of 60mph.

A common misconception is that each camera takes your speed and then an average of the two is taken. This is wrong, it doesn’t matter how fast you pass the cameras. For example, you could be going at 60mph as you enter the zone and continue at this speed for 30 seconds, then drive at 20mph for 30 seconds and your average speed would still only be 40mph.

There really is no way of cheating these cameras!