This new type of speed camera simply uses ANPR cameras.
It will read your number plate whenever you drive past one.
Then, when you go past ANOTHER one anywhere else in the country, it will check to see where else you have been.
And it will see how long it took you to travel between the two cameras, and can work out if you were speeding anywhere along the journey.
I am not sure how this works, since as far as I am aware, they need to know EXACTLY what speed, and WHERE you exceeded the speed limit to be able to prosecute you...
Basically... average speed cameras, but nationwide :S
This system is called speed spike, and is currently being tested in two locations.
The first in
Southwark in London and another on the A374 between Antony and Torpoint in Cornwall.