The Conservatives were seeking today to turn transport thinking upside down with a claim that more road-building would ease congestion on Britain's roads.

Despite widely-researched evidence that new roads fill up as quickly as they are built, the Conservative leader, Iain Duncan Smith, today said that the government's multi-billion pound road-building scheme announced this month did not go far enough.

Reasserting the Tories' traditional position as the party of the motorist, Mr Duncan Smith pledged to increase spending on new motorways - although he ruled out road tolls on all but new roads.

Speaking ahead of a transport summit in London, the party's transport spokesman, Tim Collins, said the party had done research which "nails the assertion, often expressed, that building more roads simply creates more traffic".

"Over time we need to close the gap which has widened considerably between the amount of high quality roads, trunk roads and motorways, compared with the rest of Europe," Mr Collins said.

"We are the number two economy. We are the number two or three country in terms of population in Europe.

"But in terms of our motorway network we are about number 13." Mr Duncan Smith said the increase in cars used on the roads corresponded to an increase in the number of licences issued.

The increase in the number of licences over the last 10 to 15 years was down to women learning to drive and finding new freedom - they should not be "punished" for this, he said.

But that assumption does not take into account that more drivers increases out-of-town development, which in turn increases the number of journeys per car.

Speaking on GMTV, the Tory leader said the road building programme had "completely collapsed" over the last five years - and 2001 was the only year on record where "no tarmac was laid at all" by a government.

He said: "They always blame the motorists, they say that more roads means more cars. We do not believe that is true at all.

"We have been looking at this - in Europe it does not work like that, there are many more miles of roads per head of the population, and their traffic congestion is much lower than ours."

He added that commuting times had gone up and traffic congestion had got worse as a result of the "collapse" of road building.

He again promised that the Tories would be looking at speed limits - including a higher limit on the motorways and lower limits around schools and homes for the elderly.

"People want people to travel very slowly near schools and near old people's homes but they want to be able to move faster on motorways," he said.

Mr Duncan Smith's remarks were made as the Tories prepared to unveil a new policy document entitled Free To Travel at a transport summit in London.

Major new road-building projects recently announced by the government do not go far enough, Mr Collins told BBC Radio 4's Today programme.

"If you look actually at what the government has done they have simply restored a number of projects they axed in 1997 or 1998.

"It still remains one of the smallest road-building programmes in western Europe

"Even after it is completed, we still have many, many fewer miles of motorway than France, Germany or even smaller countries."

He continued: "We are not talking obviously of building all this in one year any more than the government are.

"But we are saying we do think it is vital that we recognise the government's principle asset is the £62bn road network in this country.

"It has been significantly under-invested over many years. I would not pretend this just started in 1997.

"We have a gap we have to close with the rest of Europe.

"If we don't, our business will face real competitive difficulties."

Road tolls would be considered but only to pay for new roads, he added.

Mr Collins said the Conservatives would come forward with specific proposals for new roads and improvements to existing motorways nearer the general election.

But he added: "We are committed to removing the M4 bus lane and returning it to normal traffic."

Tony Bosworth, transport campaigner for Friends of the Earth, said: "The Tories' analysis doesn't add up.

"And worse than their analysis is their proposed solution: build more roads and raise speed limits. Pandering to the motoring lobby might win some quick votes but it won't help solve transport problems."