In capital cities across Europe, taxi drivers took to the streets without passengers Wednesday afternoon. They slowed to a snail's pace in what Parisians called "Operation Escargot." Horns blared around Trafalgar Square in London. In Berlin, taxis massed at the Central Station. All to protest the smartphone app Uber.

"We've opened Frankfurt last week, we've opened Lille in France, which is our third city this week. We opened Barcelona a couple weeks ago, and there's many more cities to go," Uber's Pierre-Dimitri Gore-Coty says.

Gore-Coty is Uber's general manager for Western and Northern Europe. He estimates that the company's European fleet is doubling in size every six months, with a presence in 20 European cities already.

"Finally seeing some sort of competition coming to the market is something that is new," he says. "And even on the protest today, what I'm seeing is taxis are trying to bring cities to a standstill, while Uber is focused on helping as many people as possible move around cities."

For drivers of London's iconic black taxi cabs, Uber seems to pose an existential threat. Eddie Tresida spent two years studying for "The Knowledge," the famously difficult test that requires prospective drivers to memorize every street in London before they can drive a black cab.

"Other drivers it takes three, four years. All depends how hard you work at it," says Tresida. "If you want to be a taxi driver, then do the same as what I've done. It's hard for two years. You go without things. You have to sacrifice things in your life. But if you want to be a taxi driver, this is the best taxi service in the world."

Twelve-thousand drivers were expected to participate in the London protest. In Berlin, the demonstration was smaller, with only some 1,000 drivers expected. But the sentiment was just as intense.

"These apps don't offer proper, decent, quality transportation!" says 64-year-old driver Barbara Novak. "Climb into one of those, and you might as well say 'Mug me!' "

Uber insists that it is safer than traditional taxi services, since passengers can immediately evaluate drivers and provide feedback via the app.

In Madrid, the complaint from many drivers had more to do with finances.

"After three years, I bought my license for 137,000 euros ($185,400)," says taxi driver Maria Eugenia Hernaz. "It's more than my house! So I need work, and I have to work 16 hours a day to do it."

Many cab drivers argue that Uber should be forced to follow the same licensing and fare rules as taxis. Uber believes those rules should not apply, because the company says it is more like a car-hire service.

"You know, it happens all the time that new innovations take opportunities that had been the realm of incumbents," says Ben Edelman of Harvard Business School. But he argues that Uber is taking some important shortcuts.

"If there's a medallion to be purchased, they're not purchasing it. If there's commercial driver's insurance, not that either. If there's a commercial driver's license — you got it — no," says Edelman.

Whether that is legal or not is a matter of dispute. In London, the high court has agreed to rule on the matter.

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