Maybe they’re not just for airports.

International architecture firm NBBJ recently unveiled a radical new concept for the London Underground’s Circle Line: a three-lane moving walkway. Commuters at each station would enter the slow lane at 3 miles per hour and then gradually pick up the pace by stepping into the adjacent lanes, at 6 and 9 mph. All three lanes would accelerate between stations, reaching a top walking speed of 15 mph in the outermost lane, before dialing back to slower boarding speeds at the next platform.

As imagined, this travelator (the British term for “moving walkway”) would replace the Circle Line’s entire 17-mile loop. NBBJ claims it would actually reduce travel times on the congested railway, since the sidewalk wouldn’t have to stop periodically like a train does.

Rendering of the Notting Hill Circle Line station. (NBBJ)

While the Evening Standard called the plan “outlandish,” Londonist points out that the same concept was considered for the Underground as early as 1894. And, according to Gabriel Lodewijks, a transport engineering professor at the Delft University of Technology, the speculative thought experiment is not necessarily as far-fetched as it seems.

Lodewijks has been studying the potential of so-called continuous transport systems for years. These people movers offer a number of advantages over discontinuous systems like trains and buses: They’re quiet. They don’t add to traffic. They don’t directly generate air pollution. And, crucially, there’s no waiting. “Whenever you arrive, off you go,” he says.

That said, there are plenty of reasons why moving walkways are now almost exclusively relegated to airports. Lodewijks explained some of the design considerations to CityLab:

Accessibility. If a moving walkway is being used for public transportation, it has to accommodate everyone—including elderly people, wheelchair users, and the blind. Lodewijks says that accelerated walkways should also have handrails or poles between lanes so that riders can keep their balance.

Transport time/length. Most people can stand for five or 10 minutes on a travelator, but a standing commute of, say, 30 to 45 minutes is bound to get tiring—health benefits notwithstanding.

Speed. No matter what peak speed the system reaches, riders need to enter and exit at walking pace and change speeds gradually. Paris’ Montparnasse metro station had to reduce the speed on its experimental moving walkway because riders kept falling over as it accelerated.

Turns. Moving sidewalks can be built to maneuver around curves—think of a trailer hitch, which allows the front vehicle to move in a different direction from the trailer. But this can’t be done with multiple lanes at once.

Maintenance. Moving walkways are almost always indoors because exposure to the elements would compromise the reliability of the system. If they were placed outside, they’d still have to be covered.

Transit alternatives. Another reason moving walkways are rarely used outdoors: “If they don't work, there's no alternative,” Lodewijks says. “You have to walk all the way; there's no backup system. If a bus doesn't work, you get another bus. With a moving sidewalk, if you have a stretch of a kilometer or a mile and it doesn't work, then there's no public transport.”


The NBBJ Circle Line design may be purely speculative, but Lodewijks says it fits into larger trends in accelerated walkway development. Just last year, the Swiss Transport Research Conference investigated ways to optimize the design of an urban network of moving walkways, “starting from the hypothesis of a world without private cars.”

The walkable Circle Line may look wacky, but it’s not impossible. Judging by the history of world’s fairs past, one era’s science fiction often turns out to be the next era’s reality.

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