Speed cameras are to be installed on large sections of the motorway network under a government plan to reduce congestion and vehicle emissions by cutting and strictly enforcing the speed limit at peak times.

The plan was announced as the Department for Transport published forecasts that congestion will rise by at least 28 per cent by 2025. It believes that “active traffic management” — in which the speed limit varies according to the conditions — will be a cheaper way of accommodating rising traffic levels than widening motorways.

Proposals to widen the M6 between Birmingham and Manchester and sections of the M1 may now be dropped. The department will instead build gantries at intervals across congested sections of the motorway network. The gantries will carry cameras and digital signs displaying the limit, which will be reduced to between 40mph and 60mph depending on the volume of traffic.

A trial of the variable-speed-limit system on the M42, southeast of Birmingham, has shown that more than 95 per cent of drivers comply. The limit on the 11-mile stretch of motorway is enforced by eight cameras rotated among 96 locations.

Police have tended to carry out very little speed-limit enforcement on motorways and a government survey found that more than half of cars on the motorway network exceeded the 70mph limit last year.

The M42 trial, and a similar scheme on the southwest section of the M25, found that reducing the limit resulted in faster average journeys because vehicles were less likely to brake sharply, which causes tailbacks. Overall fuel consumption fell by 4 per cent and vehicle emissions by 10 per cent.

The trial included use of the hard shoulder as a running lane in peak times and found that this reduced average journey times on the northbound carriageway by more than a quarter.

Ruth Kelly, the Transport Secretary, confirmed yesterday that the hard shoulder would be used as an extra lane on two short sections of the M6. She also said that variable speed limits would be introduced on other parts of the motorway around Birmingham and that a feasibility study would take place into introducing them across much of the motorway network.

In an interview with The Times, she said: “The trial shows there is a real culture change. People are focusing on the way they are driving and there is almost complete compliance with the limit. It’s about trying to encourage a regular flow of traffic rather than stop-start conditions. It reduces emissions and many people would prefer it to taking land to widen a road.”

She said that the study would consider introducing different speed limits for different lanes. On a four-lane motorway, the limit on the two outside lanes could be 60mph while on the two inside lanes, where traffic joins and exits, the limit could be reduced to 40mph or 50mph.

The Transport Department forecasts show that, even assuming improvements to traffic flow on motorways, journeys on roads in England will take 4 per cent, or three seconds, longer per kilometre by 2025.

Theresa Villiers, the Shadow Transport Secretary, said that Mrs Kelly was seeking to distract attention from cuts in the roadbuilding programme. “Conservatives welcome more hard-shoulder driving and we have repeatedly called for more active traffic management and efficient use of existing road space,” Ms Villiers said. “However, such schemes should not be used as an excuse to put the brakes on the much-needed improvement to the worst road bottlenecks.”

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