Cameras that detect average speed will be deployed on hundreds of miles of motorway under a Government plan to force all drivers to comply with variable limits.

Drivers will no longer be able to exceed the limit knowing that police rarely prosecute anyone driving at less than 85mph on motorways. They will also no longer be able to slow down when they spot a camera.

Unlike Gatso or Truvelo cameras, which typically cover less than 100 yards, average-speed cameras work in pairs to cover several miles. They use numberplate recognition to record the time it takes a driver to travel between any two points and work out the average speed.

The Department for Transport (DfT) is developing plans for the rapid installation of average-speed cameras, starting with the 500 miles of motorway where it is planning to turn the hard shoulder into a running lane. The limit will be reduced to 50mph or 60mph, with gantries over the motorway carrying signs and cameras.

At present the Highways Agency uses average-speed cameras only on motorways where there are contraflows during roadworks.

A DfT report recommends “moving away from ‘spot’ speed measurement and control, to ‘average’ speed measurement and control”.

The DfT is considering “how quickly could we migrate from spot to average-speed enforcement and how extensively could average-speed enforcement be applied”.

The report says that average-speed cameras are needed to ensure a high degree of compliance as the Highways Agency introduces the concept of “managed motorways”.

This involves extracting the maximum capacity from the motorways by reducing the limit at busy times, as already happens on the southwestern section of the M25.

It also entails converting the hard shoulder into a running lane and, from 2010, turning one lane into a toll lane with a higher speed limit. The DfT says that compliance with the limit is essential when the hard shoulder is not available for breakdowns.

The DfT is also considering making the outside lane of the busiest motorways bi-directional to increase the number of lanes in the direction in which traffic is heaviest. A machine would move the central barrier across the lane each time the direction of travel was reversed.

The report claims that there is “broad support” among motoring organisations and road safety groups for average-speed cameras on motorways.

But the AA said that the measure could undermine safety because drivers would become frustrated by vehicles adhering strictly to the limit and would drive too close to them.