Britain could get its second toll motorway, along a 51-mile route between Birmingham and Manchester, under plans announced by the government yesterday despite fresh evidence that the only existing toll road has become a racetrack for drivers.

Alistair Darling, the transport secretary, launched a consultation exercise yesterday on a toll motorway to run alongside the existing M6 north of Britain's second city, relieving severe congestion along the route, which carries 140,000 vehicles a day.

Private firms are likely to be asked to tender for a contract to build and operate the road, which could cost between £500m and £1bn.

The initiative follows the opening, in December, of the M6 Toll around the east of Birmingham, which was intended to provide a fast-track route for north-south traffic. Mr Darling said he considered the M6 Toll to be a "success".

But its operator, Midland Expressway, has angered hauliers by charging high prices for lorries. The road is notorious in the Midlands for its scant enforcement of speed limits.

Figures from the Highways Agency yesterday revealed that the motorway is so clear of traffic that speeding has become epidemic, with vehicles travelling at an average of 75.8mph on weekdays.

Just 42,000 motorists use the M6 Toll daily, a fifth of the traffic on the parallel free motorway. A three-month breakdown of figures by the agency revealed that drivers were completing a 43-mile route through the Midlands in just 34 minutes.

The Highways Agency said it was "aware of the problem" of chronic speeding. "We are working with Midland Expressway and the police to address the problem," the agency said.

Mr Darling's proposal for a second toll motorway was a fresh step by the government towards a nationwide strategy of charging drivers for using roads.

The Department for Transport is due to publish a feasibility study this month on using satellite technology to charge motorists for every journey they take.

A new motorway is considered preferable to widening the M6 because it would be cheaper and less disruptive to traffic. By getting a private contractor to foot the bill, the Treasury could avoid meeting the bulk of the cost. In addition to the new motorway, Mr Darling set out plans for a trial of special lanes reserved for vehicles carrying two or more people on either the M62, M3, M1 or M61.

"Decades of under-investment and a growing economy have put our roads under severe pressure," he told the Commons. "Our approach is to tackle congestion through a range of measures, both in road and rail."

Motoring organisations gave the proposals a cautious welcome.

Edmund King, director of the RAC Foundation, said: "It's a realisation by ministers that they have got to do something about our road network.

"For the first two or three years of this government, integrated transport did not include the roads. There was this utopian view that if you improved public transport, you need not do anything for motorists."

But John Dawson, director of the AA Motoring Trust, said tolls should be coupled with a review of fuel tax levied on drivers. He said this would be "socially progressive" because fuel taxes were a particularly heavy burden on low-income rural motorists who depended on their cars.

Environmentalists said the plan for parallel motorways north of Birmingham was equivalent to a "10-lane super-highway" through Staffordshire and Cheshire.

Stephen Joseph, director of Transport 2000, said the proposal was a betrayal of Labour's 1997 pledge that roads would only be built as a "last resort". Friends of the Earth questioned the wisdom of encouraging more people to use cars when a £10bn upgrade of the west coast mainline was nearing completion, which would provide a twice-hourly service between Birmingham and Manchester.

But Professor David Begg, chairman of the Commission for Integrated Transport, said the plan was a shift in policy towards rationing road space.

"If this is the start in a change in the way we pay for road use, it is a good initiative," he said. "We've got an opportunity for a charge which will control the demand for road space."