Britain needs a new network of motorways and A roads to cope with 12 million extra cars and a 43 per cent increase in traffic over the next 30 years, according to a new study.

It adds that, without a significant investment in new routes, most of the motorway network will degenerate to conditions experienced on the western section of the M25, which is congested throughout the day.

The study, by Imperial College, London, commissioned by the RAC Foundation, recommends that 373 miles (600km) of new lanes be added to the strategic road network every year. This is the equivalent of 100km of motorway with three lanes in each direction. The Government has approved an average of just over 100km of new lanes a year until 2015.

The study, entitled Roads and Reality, also recommends the introduction of road pricing on motorways and A roads, an idea that the Government has been reluctant to pursue since 1.8 million people signed a petition against it this year.

Stephen Glaister, professor of transport at Imperial and the lead author of the study, said that drivers would be more willing to accept road pricing if some of the £80 billion projected annual revenue were used to build more roads on congested corridors.

The study includes a map showing roughly where the new roads should be built. It includes three routes from London to the North and West and a giant new South East motorway ring well beyond the M25.

It says that some of the new capacity could come in the form of extra lanes on existing routes. But it favours the construction of new roads because they are cheaper, cause less disruption than widening and can be built to higher standards. It suggests mitigating some of the impact of the new roads by building them in tunnels through sensitive areas. It also recommends making some car-only.

The study acknowledges the depth of public feeling against new roads but points out that the strategic network, which carries a third of all traffic, occupies only 0.16 per cent of the country.

Professor Glaister said: “The Government cannot use the possible future introduction of road pricing as a reason to ignore the need to improve the strategic road network.”

David Holmes, chairman of the RAC Foundation, said it was a myth that new roads simply filled up with traffic. He admitted that they did encourage some extra journeys but said that they also reduced congestion and removed traffic from less suitable, less safe roads. He said: “Our children and grandchildren will not accept poorer standards of service from transport than we have today.”

Using DfT forecasts and data, the study predicts that the number of cars will rise from 26 million at present to 38 million by 2041. It proposes a road-pricing system of varying charges depending on the level of demand. It says that the average charges would be 9p per km in northern and western regions and 11p per km in the Midlands and South East. In cities, the average charges would be much higher.

The Campaign for Better Transport, which is partly funded by bus and rail companies, said that the proposals would result in a large increase in climate-change emissions. It published the result of a YouGov poll that it commissioned in which 2,000 people were asked if they would prefer the Government to invest in public transport or road building. Only 30 per cent voted for road building while 62 per cent chose public transport.

Peter Hendy, commissioner of Transport for London and chairman of the Government’s Commission for Integrated Transport, said that all new roads should have tolls.