It takes an average London cabbie 34 months of preparation and 12 attempts to pass the test to get their license:
http://en.wikipedia.org/wiki/Taxicabs_of_the_United_Kingdom#...
There was a shortcut on a route that I used to regularly take in London, so i'd tell them "Don't go to the lights, just turn down here". I think I had fare meters stopped out of appreciation on 3 occasions after sharing that tip. Showing a London cabbie a new shortcut totally blows their minds.