People have threatened to kill themselves because of it. Celebrities have been arrested for showing their anger over it. People have lived in trees to prevent it happening. But road-building is set to inflame passions like never before as the biggest shake-up in Britain's transport policy begins this week.

Against a background of impassioned debate, a power ful group of motoring and business groups has united to launch a nationwide advertising drive calling for nearly 1,000 miles of motorway to relieve chronic and growing congestion across the UK.

The £250,000 crusade is being mounted partly in response to mounting pressure from a rival coalition of 29 environmental and social justice groups which have deluged ministers with postcards lobbying for more sustainable transport.

They are pressing to stop road building, launch new congestion charges, and gain more priority for public transport and schemes to get people out of cars.

Another spur for the roads campaign is the mounting cost of railways, which threaten to use up any spare transport money in the next government spending round this summer.

It also comes at a critical time for future policy, with ministers planning this summer major statements on rail reform and road pricing, and a new 10-year transport strategy.

Inside government, politicians and officials are said to be divided over whether to step up road building to ease the gripping congestion problem on key arteries and around towns and cities.

Those in favour claim conventional economic analysis shows the benefits of building roads far outweigh all but a few public transport investments.

Others argue the roads will 'fill up' with new traffic within years, or even months, of opening.

At the same time there are critical decisions to be made about road charging: on any new motorway capacity, in urban areas, or as a national satellite-based system.

Towering over these debates is the growing political muscle of the motoring lobby - which claims to represent 26 million car owners - and the vociferous environmental movement, which is already angry about big airport expansion plans.

Tony Bosworth, transport campaigner for Friends of the Earth, said: 'July is going to be crunch time; it's going to be a big decision for government about the future of transport policy.'

The campaign next week is headed by the Freight Transport Association, supported by the AA, RAC, Confederation of British Industry, British Chambers of Commerce and the Road Haulage Association.

They are calling for 470 miles of the M1, M4, M6, M25 and M62 to be widened by adding an extra lane - 'just 12 foot wide' - in each direction.

The schemes - treble the government's current motorway programme - would cost about £12bn.

Richard Turner, the FTA's director general, said the driver behind the campaign is Britain's continuing congestion crisis - estimated to cost the economy £20bn a year.

This has inspired the campaign slogan: 'Jams Ahead, Economy Behind'.

It also persuaded the campaign backers to come together in an historic unison, which onlookers believe reflects a similarly significant shift in government where a return to road building is seen by some as economically and politically imperative.

'This is the first time in my memory a group of users of roads have got together in this way,' said Turner.

'We have almost got to out road building: people have been trying not to mention it because it's been a dirty word. It mustn't be - our economy does need roads.'

It is in this context that the campaign was designed to appeal to both sides of the debate in government: a big increase in building, but targeted at the most critical pinch-points on the nation's trade routes.

The coalition believes it has three trump cards: the plan, they say, is the quickest, most cost effective and most eco-friendly way of tackling the country's single greatest transport problem.

Firstly, the proposed schemes could be done by the end of 2010 and would solve 80 per cent of motorway congestion - a critical leap forward while the country waits for more long-term plans for national road pricing to tackle congestion.

Secondly, on conventional economic analysis, roads projects typically create 'benefits' of 2-4 times their cost - compared to only one railway scheme (the £10bn proposed London Crossrail line) which is known to have a positive cost/benefit ratio.

Thirdly, by widening existing trunk roads, the plan would reduce the extra fuel burn and local blight caused by congestion with the mini mum impact on the landscape.

'We're highlighting congestion, we're highlighting the problems caused by congestion, and we're highlighting a solution: relief from congestion is just 12 foot wide,' said Turner.

There appears to be less agreement among the partners on how the new roads will be paid for however. Some insist road users should not pay more than the current £38bn a year of fuel duty, tax discs and VAT. But John Dawson, director of the AA Motoring Trust, said ideas for US-style charges on the extra 'faster' lanes could be considered.

'Providing there's choice, and you introduce extra choice, you can do [such] things,' he said.

Last night a department for transport official dismissed the latest campaign as 'blatant lobbying in advance of the spending review'.

The official also played down the political appetite for road building on the scale proposed. However the door was left open to increase the rate of construction in the revised 10-year plan, which will run from 2005 - possibly with a more visionary strategy for 10 to 15 years set out.

'What you can see from the 10-year plan is it builds up, there's an increase in the profile,' said the official. 'We'll do the appropriate level of road building afforded in the confines of the budget.'

Fearing this momentum at the heart of government, the opposing Way To Go campaign is not going to let up without a fight.

Representatives have already met officials and ministers are now considering further talks after receiving hundreds of postcards from supporters in a bid to counteract big budget advertising with a show of popular support.

Given the government's anxiousness about the sway of the motoring lobby, the environment movement knows it must prove to politicians that not all voters, not even all drivers, agree with populist opposition to speed cameras, bus lanes, parking attendants and road charges.

More fundamentally though, they argue road building is a waste of money - money much better spent on better bus services and encouraging more people to walk and cycle.

On the one hand repeated studies show congestion is eventually an effective deterrent to more congestion; conversely extra road space generates more traffic, which simply 'fills up' the new capacity.

In the most extreme case reported, on one section of the M25, the extra lanes were as busy as before in less than a year.

'Widening motorways has been tried in the past as a solution and it's failed,' said Tony Bosworth. 'It's a triumph of hope over experience if you think it's going to succeed again.'

On the other hand, work by Transport 2000 suggests 'economic returns' from smaller schemes - from priority lanes to help buses make their way through urban traffic to one-on-one advice about how to cut down car use by even one or two trips a week - are even greater than those for new roads.

Critics complain the last 10-year transport plan in 2000 boasted improving public transport and encouraging walking and cycling would cut congestion - and it failed so badly that congestion is still rising.

Supporters of the ideas say the plan failed because the ideas were not properly implemented as most local councils - with no visible government support - ran shy from introducing urban car charges or other changes to give buses, cycles and walkers priority over cars.

Where these things were done - particularly in London, but also in Brighton, Nottingham and other cities - they have been proved to work, said Professor David Begg, chairman of the government's Commission for Integrated Transport.

'London has had a bigger modal shift from car to public transport than any other country in the world,' said Begg. 'When people say this stuff in the [last] 10 year plan is never going to work, I say: look at London.'

Which takes us back to charging: the potentially momentous policy move which overshadows all debate about transport in this country - and around the world - today.

Last June Alistair Darling, the transport secretary, declared the government was in principal in favour of a national charging strategy and set up a special steering group to examine if and how it could be done.

That group's report is due next month - although it is unlikely to be published until July.

So far the feeling seems to be that charging is possible, but big question marks hang over whether the charges should apply to all roads or only trunk roads, whether to vary them by time of day or levels of traffic, and how the money collected should be spent.

Then, what everybody wants to know is how quickly this can be done. Ten years is the suggested minimum, but many experts - even supporters - believe this is optimistic. In which case there could be support for more localised urban charges and tolls on motorway lanes and bypasses in the meantime.

The new transport strategy will be Labour's last chance to persuade voters they can sort out a system seen as a national joke - and an expensive joke at that. It is sure to include some of these ideas: the question is how many?