A TfL rendering showing a West London road covered over with new trees and construction. TfL

The city ponders its own version of the Big Dig.

A new plan for London suggests a bold solution to the city’s congestion problems—huge tunnels running beneath its core.

Launched by Mayor Boris Johnson, the proposal would see London dig two east-west tunnels underneath the city center, sucking car traffic off the streets and tidying it into two subterranean highways with a combined length of 43 kilometers (26 miles). To clear the streets of yet more vehicles, the tunnels would be complimented by nine additional “fly-unders”—as excavated, sunken highways are being called in Britain—that would bury major feeder roads in other parts of London as well. With no cost as yet confirmed for the plan, the mayor’s office has suggested that funding could come in part from constructing housing on space no longer needed for roads.

London isn’t the only European city looking at burying its road traffic. Taking some inspiration from Boston’s Big Dig, Stockholm is building an almost entirely sunken north-south highway across the city. Paris has long been considering covering its inner beltway, the Boulevard Périphérique, a project that is now partly underway.

The advantages for London in aping plans like these are potentially great. Not only could the new road system speed transit across the city, it could give much-needed relief to central streets that are clogged with cars and mired with pollution. Space could be freed for public transit and whole neighborhoods torn apart by highways could be stitched back together.

Possible locations of the two tunnels. (TfL)

That’s if the plan works. In reality, its success is no foregone conclusion. Certainly the suggested tunnel routes (still entirely provisional) make sense on first glance. While bisecting the city center laterally, they would be positioned to take a lot of north-to-south through-traffic as well. The northern tunnel’s western exit would connect it well to the main highway for Birmingham, Manchester, and Liverpool, while the southern tunnel’s eastern section would flow towards key cross-Thames routes for traffic heading south to Dover.

There’s a big catch, however. Building all these streamlined subterranean roads could just bring in more cars. As any resident of Los Angeles can testify, the most congested cities are typically ones with the most capacious road networks. London already has a good system for diverting through-traffic away from the city center—the M25 orbital beltway, which circles the city just outside its official perimeter. Adding a slicker route through town could divert drivers from using the beltway, unloading their particulate-laden fug in built-up areas as they pass. Meanwhile, drivers on shorter distances who have avoided much of inner London might decide to take their cars into Central London after all—why shouldn’t they, now all the trucks are supposedly out of the way?

That Mayor Johnson’s infrastructural swan song (he steps down in May) is a tunnel plan should come as no surprise. During his time in office, London has already fielded another proposal for an inner-ring road tunnel, though it never made it beyond the doodle-on-a-map stage. There’s much that is promising in the plan, not least the idea of reducing surface road space to build more housing. But if these proposed tunnels were to avoid sucking yet more cars into the city, London would have to proceed very carefully.

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