Money pit: the Big Dig project in Boston is said to have gone 10 times over budget and became a byword for mismanagement

It has been done in Sydney and Seoul, Oslo and Madrid, and now Boris Johnson wants it done in London. But is the mayor’s plan to drive the capital’s traffic underground anything more than a publicity stunt?

On his trip to the US this week, the London mayor stopped in Boston to survey the Big Dig, a $15bn project in which an elevated six-lane traffic nightmare was replaced by an eight-lane tunnel. On the surface, the newly available space was transformed into a series of urban parks.

This was Mr Johnson’s cue to announce that five sites in London had been identified as suitable for feasibility work on tunnelling. Perhaps the most eye-catching was the idea of demolishing the creaking Hammersmith viaduct and building a “flyunder”, and redeveloping the surface to reconnect the town centre with the Thames.

But the five sites, comprising tunnels, flyunders and decking, or covering existing routes, are only an hors d’oeuvre. The longer-term vision is for a “replacement ring road, in the form of an inner orbital tunnel or two cross-city tunnels”, an idea Mr Johnson set out in the London Infrastructure Plan 2050, released last year.

The concept of a subterranean ring-road has attracted praise for its goals of reducing congestion and its environmental benefits on the surface.

But it has also been criticised as expensive infrastructure when Transport for London statistics show car use in the capital is falling and Londoners are taking to buses, trains and bikes in increasing numbers.

Christian Wolmar, a transport expert who is seeking the Labour candidacy for the 2016 mayoral elections, called the idea of tunnelling “unworkable and expensive”.

“We’re talking billions — and it’s interesting that the mayor went to Boston, because the Big Dig, while it was ultimately a success, was built at an insane cost,” he said.

The Boston project is believed to have cost at least 10 times its original budget and became a byword in the city for mismanagement.

But the idea of going underground raises important questions about how we will get around our cities in several decades from now.

“The most important thing is we don’t want to do this in transport terms,” Mr Wolmar said. “[With the] congestion charge, bus lanes and cycle lanes, we are shifting away from the notion that you can jump in your car and drive into the centre of London.”

London Mayor Boris Johnson Left Hanging On Zip Wire...LONDON, ENGLAND - AUGUST 01: London Mayor Boris Johnson, dangles from a zipwire in London's Victoria Park, shot by passer by Rebecca Denton on August 01, 2012 in London, England. A publicity stunt at a public viewing area for the Olympic Games went awry leaving Johnson stranded some 15 feet off the ground. The Mayor was left clutching his Union Flags for roughy five minutes before rescuers came to help him down. (Photo by Rebecca Denton/Getty Images)
Stunt man: Boris Johnson dangles from a zipwire

Mr Wolmar doubted the orbital would ever be built. “This is a ‘look at me I’m Boris’ project, dangling from a wire or planning [an estuary] airport. Unfortunately lots of people seem to swallow it.”

Caroline Pidgeon, leader of the Liberal Democrat group in the Greater London Assembly, said Mr Johnson was “adopting transport modelling for the future that assumes there will be a significant growth in car journeys”, and lacked “an alternative vision of significant modal change in how journeys are made”.

However, Martin Knights, global head of tunnelling and earth engineering and sciences at CH2M Hill, an engineering company, said London’s population growth during the next 20 to 30 years left it with no choice but to move underground.

“You cannot avoid traffic use in London no matter how much you cajole people to use public transport,” Mr Knights said. The question was how to do so in a sustainable way.

“There is lots of precedence for doing this work in urban areas,” he said, citing Sydney and Seoul among other places. “In particular, urban planners will be thinking about how to replace postwar infrastructure that is coming up to 70 years old.” The Hammersmith flyover, for instance, was completed in 1961 and is in need of continual repair.

Isabel Dedring, deputy mayor for transport, said London needed to think strategically about improvements to the road network to emulate the long-term rail planning that had produced Crossrail and updates to the Underground network.

“This is not about building a motorway under London to dramatically expand vehicle traffic,” Ms Dedring said. “This is about reducing severance [roads that cut off communities], cleaning up the air and freeing up space for walking, cycling, trees, pedestrian crossings and so on.”

Tunnel supporters also say that taxing the increase in land values on the surface would help cover the cost of large-scale projects: the price tag on an orbital tunnel is an estimated £30bn.

Mr Wolmar also attacked the tunnelling concept for practical reasons. “In London, the first 10 to 15ft [underground] is a nightmare — you’ve got cabling, gas, electricity, cable TV, you’ve got so much there it’s massively expensive [to dig past it].”

But Mr Knights said the problems were surmountable and that phased construction could minimise the impact on the city’s residents.

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