Almost unnoticed last month, the government paved the way for one of the biggest civil engineering projects of recent times: the widening of a 50-mile stretch of the M6 from just north of Birmingham into Cheshire.

Plans to build a new toll motorway, or "Expressway", had been abandoned on the grounds that it would take more land, would be too difficult, and couldn't be completed earlier than the widening (2017). The transport minister, Stephen Ladyman, declared: "The decision will be welcomed by many local residents and environmental campaigners who prefer the widening option."

Not so. In fact, in the government's own survey two years ago, 98 per cent of residents came out against both the widening and the Expressway, and in favour of managing the existing traffic better. But this third option was withdrawn from the list of options by the Department of Transport (even though English Nature and the Environment Agency said it should be on the table).

The M6 widening decision is the final nail in the coffin of new Labour's aspiration to get people out of their cars and on to public transport - and it's not even going to ease congestion.

This is partly because most of the congestion is caused by accidents, and the extra lanes won't stop those. But worse, though the widened stretch is described as the "Birmingham-Manchester" stretch, it won't reach all the way and will simply create a new bottleneck near Manchester.

To achieve all this, houses will be torn down, schools will be drawn closer to the noise and pollution of the motorway, and more land will disappear under tarmac. During the six-year construction period, disruption will be huge. Already the Group Against Motorway Expansion (Game), formed to oppose the government's plan, reports that houses on the route are harder to sell.

How this fits with the government's climate-change strategy is hard to fathom. Transport is the fastest-growing sector for emissions, and this new stretch of motorway is as bad for people living on low-lying islands in the Pacific as it is for M6 locals.

Game will continue to campaign, arguing that alternative measures have been insufficiently explored. These include selective use of the hard shoulder, better motorway driving instruction, lower maximum and new minimum speeds. In the longer term, however, people and things have to travel less, so the government should encourage teleworking and flexitime to flatten out peak traffic periods.

Above all, it should provide incentives to keep food production and consumption more local. One-third of the HGVs on our motorways are carrying food, sometimes taking it on 100-mile journeys before it is returned to be sold next to where it was grown.

Opposition to this road is likely to be fierce.Work won't start until 2011 and as the local, regional and global impacts become clearer, the M6 is sure to follow the Newbury bypass into the annals of road protest history. Any volunteers for this generation's Swampy?

Andrew Dobson is professor of politics at Keele University