In the pantheon of Britain's great engineering feats, it remains relatively modest; the widening of the M6 motorway along a 51-mile stretch between Birmingham and Manchester. But in terms of sheer cost, the UK may never have seen its like before.

Every inch of the proposed new road is estimated to cost £897. And when construction inflation has been built in - currently 9% a year - the likely figure will top £1,000. Either will make it the most expensive piece of tarmac ever laid, with the entire project, according to the Highways Agency's own figures, expected to cost £2.9bn and take three years.

Yesterday road builders tried to explain how widening a road by just one lane could cost twice as much money as Britain gives to Africa in a year.

"It is a very difficult way to build a road," said Roger Bailey, of engineering consultancy Faber Maunsell. "In a greenfield site you are in control of your construction planning. But on a live road you have to work round more traffic." The M6 widening, he said, will involve fitting the work around traffic, night shifts and widening dozens of bridges and culverts.

The Royal Institution of Chartered Surveyors said major project construction costs are now being driven up even further by the Olympics and other major schemes in Europe. "There's a lot of road building going on. The price of construction is going up because there is a lot of work around. Road building is an international market. In the last 10 years costs have gone up 7-9% a year," said a spokesman.

But transport and environmental groups said they were "staggered" at the cost, which has quadrupled from a relatively modest £670m in 2002.

Rebecca Lush, a campaigner with Transport 2000, said: "This must be the most expensive roadworks in history. Britain is spending £13bn on new roads and next to nothing on reducing road traffic or railways. This is a complete waste of resources which will only increase the numbers of cars on the road and make climate change worse. £1,000 an inch is a scandal. The money should be put towards rail schemes or projects which would reduce climate change emissions rather than increase them."

Motorway construction has certainly come a long way since the transport minister Ernest Marples opened the first stretch of the M1 in 1959. Back then, 5,000 workers built the first 80 miles of British motorway at a rate of nearly one mile every eight days for under £50m. Today that would not be enough money to widen one mile of the M6.

The M1 itself is also under renewed scrutiny from environmental groups.

Figures revealed in a report ordered by the Department of Transport show that the cost of widening the M1 south of Luton has now hit £5.1bn.

The eye-watering sums for motorway widening have forced the government to consider other options, such as converting hard shoulders into improvised driving lanes. A trial project on an 11-mile stretch of the M42 near Birmingham is proving a success and the M1 is one of the favourites for any extension of the scheme.

But underlining all this is the fact that in the UK, cars are still king, despite efforts to wean people off them.

Britain is now one of the most car dependent countries in the world. Between 2005 and 2006, says the Highways Agency, cars in Britain travelled 506bn km - up 7bn from the year before - despite government intentions to reduce traffic. According to the agency, car traffic has increased nearly 12% since 1996 and 851% since 1955.

Bicycle traffic was estimated to be 4.6bn km in 2006, an increase of 5% from 2005, but a fraction of Germany's 30bn km.

Last week the government confirmed a shift in policy by stating that passengers will foot 75% of the cost of operating the rail network, an increase from £5bn a year now to £9bn by 2014. The Transport Salaried Staffs' Association said it would "price people off rail and on to the roads" at a time when discontent over fares has reached record levels, with four out of 10 rail users saying their ticket does not represent value for money.

A spokesman for the Highways Agency said the M6 business proposal was still being prepared and would be considered by ministers "in due course".

· This article was amended on Wednesday August 1 2007. The M1 is being widened south of Luton, rather than south of London. This has been corrected.