More accidents happen on "local" roads (30mph or 40mph) than on motorways.
So how do you control cars going above 30mph in a 30mph limit road?
Do you perhaps fit a device to EVERY car that controls its speed limit.
And do you then fit devices in EVERY street that stops cars going above the speed limit for that street.
I am sure if Mr Cameron announced this and said it was going to cost 10 billion pounds or whatever he would be very popular.
Tinkerbell, suggest you THINK about things before making flippant suggestions.