My research on SPECs says it is used on motorways and dual carriage ways. It times drivers across two known points and so determines the speed - which is fine if the drivers are likely to be at a constant speed.

This is clearly not a good method for normal roads in london, journeys on which are continual stop start - not many drivers get around london near 30mph, but they would certainly have driven well over the limit to do so.

However, I am in favour of a system that is digital, automatically reads number plates, uses video evidence, and can record evidence against a number of cars simultaneously.
Maybe one camera could produce 5000 notices a week!

Plus it seems reasonable to check that cars are taxed at the same time - there would almost certainly be a strong link between speeding cars, untaxed/uninsured cars and other criminal behaviour.

Come-on cameron! Make it so.