London’s famed “black cab” taxi drivers are known for their knowledge of the best and quickest routes around the capital city, but most visitors to the city simply don’t realize just how much training is involved in taking on such a career. There are nearly 25,000 licensed taxi drivers in the city (licensed, as opposed to private hire “minicab” drivers who often charge less but are not tested or background-checked), and each one of them has had to pass an intensive test called the “Knowledge.” It takes the average cabbie nearly three years to learn the information and pass the necessary exams to earn his or her license and own their independent taxi business.

While drivers who will be concentrating their efforts on a particular suburban area of London (i.e. Croydon or Ealing) are tested primarily on those areas, the “All London” drivers whom most tourists will encounter are well-studied in every street, hotel, restaurant, theater, park, and landmark within a six-mile radius of Charing Cross. That sounds like a small area, but once you’ve started walking during your vacation, you’ll see it most definitely is not. Because cab drivers are available and working 24 hours a day, 365 days a year, they need to also have a solid understanding of traffic patterns, road construction, weather delays, and city events that may affect conditions on the streets - conditions that could delay your journey.

The official name for the “Knowledge” test is the Guide to Learning the Knowledge of London, or the Blue Book, with 320 different scenarios for passenger routes (also known as runs). In addition to knowing the most expeditious way to get from Point A to Point B - which is what every tourist wants! - but they need to be aware of all the famous landmarks in and around the route. If you strike up a conversation with the driver, you find that they can provide detailed information about where you’re going and can answer all sorts of tourists’ questions.

http://golftripsworld.info/2008/06/01/london-vacation-why-do-london-taxi-drivers-know-so-much/