LONDON (CNN) -- If you have a business meeting in a strange city, you may have to rely on a taxi to get you where you need to go. In London, you can rest assured that your cab driver knows how to take you anywhere in the city -- the driver's been tested for it.

All taxi drivers authorized to work in London first have to prove to authorities that they know the fastest route between any two sites in the city. For example -- between the Tower Bridge and Westminster Cathedral.

The exam to become a cab driver is so tough, it's called "The Knowledge."

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It can be learned at schools set up just to prepare future taxi drivers. On average, students spend 3 hours a day studying the London map, and another 3 hours riding a motorcycle to get acquainted with the routes and streets of the city.

It takes most students more than a year to prepare for "The Knowledge" -- if they study full-time.

"It's an intimate knowledge of the streets of London -- that's really all it is," said examiner Rodney Stentiford. "Getting to know all the streets and everything of interest in London. Anywhere where a passenger might want to be taken to."

The candidates for cab driver also go through a rigorous check of their driving and police records.

Dave Devanon graduated the same day he took his exam.

"I've proved to them that I'm capable of providing a service that's renowned throughout the world," he said.

Now he will join the 20,000 cab drivers in London -- with another 3,000 candidates trying to pass "The Knowledge" as soon as possible.

Based on a report from CNN's Business and Travel and Beyond. The segment appears weekdays on Early Edition at 7 AM (ET) and on Morning News at 10 AM (ET).