The hard shoulder is to be used as a running lane in peak periods on several more motorways, the Government will announce today.

Ministers are opting for a quick and cheap alternative to road widening because the Highways Agency’s road building programme has run more than £3 billion over budget.

Ruth Kelly, the Transport Secretary, will say that a trial on an 11-mile stretch of the M42 in Warwickshire has shown that opening up the hard shoulder to traffic helps to reduce congestion without compromising safety.

But campaigners believe that the Government has failed to allow enough time for any problems to emerge. The report covers only the first six months of the trial.

Cathy Keeler, head of campaigns at Brake, the road safety charity, said: “We are still concerned about access for emergency vehicles in the event of a crash. In a traffic jam paramedics can currently use the hard shoulder to jump queues in order to reach crash sites in the shortest possible time. In the event of all lanes, including the hard shoulder, being blocked, how will they get through?”

Paul Watters, head of roads policy at the AA, said: “In any major road improvement scheme the authorities would normally wait for three years of data before deciding whether it is a success or not. The most robust method of adding capacity to a motorway is to build another lane while retaining the hard shoulder. Improved traffic flow may encourage development that quickly undermines the benefit of extra room on the road.”

Mr Watters voiced concern that the safety measures used in the M42 trial would not all be used on other motorways. “Projects like this cannot be done on the cheap and there is a risk that, in rolling it out, there could be skimping, which would be counter-productive and even dangerous.”

On the M42, emergency refuges for broken-down vehicles have been built at 500-metre intervals. The limit for all lanes is reduced to a maximum of 50mph (80km/h) when the hard shoulder is in use and enforced by cameras.

Access to the hard shoulder is controlled using a series of gantries across the motorway. Sensors under the road surface detect when congestion is building up and send a message to the Highway Agency’s control centre. Screens on the gantries inform drivers that they can use the hard shoulder. The gantries display a red cross over the hard shoulder when it is closed and a speed limit sign when it is open.

The control centre uses a network of CCTV cameras to spot when a vehicle breaks down and cannot reach a refuge. Signs on the gantries are changed in seconds, telling drivers to leave the hard shoulder and alerting them to a hazard ahead.

The M42 scheme cost £100 million, compared with the £500 million that it would have cost to widen that section of motorway. Hard shoulders can be converted for use by traffic in only two years, compared with up to ten years to plan and construct additional lanes.

The scheme is expected to be extended to some of the busiest sections of the network, including parts of the M1, M5 and M6.