Need to know: The damage: Quentin Fottrell unearths the alarming cost of building a motorway

The average cost of building one mile of motorway is pounds 18m, according to the Highways Agency. But the final price varies, depending on bridges and interchanges, and how much land needs purchasing - Britain's first toll motorway, linking the M42 and M6, which is 27 miles long, has a projected pounds 500m price tag.

Environmental clean-up costs can sky-rocket. For instance, the Government unsuccessfully tried to sue road protesters of the Twyford Down bypass. Instead, it ended up spending pounds 4m - about double the usual amount - to mitigate harm to local wildlife.

As if to compensate for increasingly contentious environmental impact, roadside art is popping up more frequently. Commissions for 40ft high shopping trolleys are not unheard of. As John Kormeling, Dutch artist and proponent of public art, says: "A hammer is beautiful, a flower is beautiful and a shopping trolley can be beautiful, too." He didn't mention the M25.

Construction costs per mile Earth-moving, pavement, drainage, traffic signs, lighting, safety fencing, diversion of other roads, pounds 7.2m

Land acquisition and compensation costs per mile, pounds 600,000

Structure costs per mile Bridges for other roads, interchanges, rail/river crossings, retaining walls, culverts and sign gantries, pounds 4.8m

Preparation and supervision costs per mile Feasibility studies, public inquiries, site investigations/surveys, design and construction supervision, pounds 2.6m

Communication costs per mile Telephones, electronic signs, control centres, pounds 200,000

Security costs per project Includes private detectives to spy on protesters, helicopters for aerial photographs, thermal imaging and live-link cameras, police "sting" operations to remove eco-warriors, up to pounds 500,000 a pop, average pounds 11.5m

Afterthoughts Annual maintenance per mile, pounds 7,500; wildlife revival pounds 2m to pounds 4m; roadside sculpture, pounds 100,000

Total for a 20-mile stretch of motorway, including pounds 2.6m VAT per mile, pounds 377,750,000