One London trainee logged more than 50,000 miles in three years, on a motorbike and on foot. That’s about twice around the Earth, and nearly every mile of it stayed inside the dozen boroughs of inner London.

He was studying for the Knowledge, the exam every black cab driver in London has to pass. Candidates learn some 25,000 streets within six miles of Charing Cross, plus the landmarks on them. They have to know which roads run one way and which corner a passenger’s favorite restaurant sits on. Most need three or four years. Transport for London tells them up front that nobody learns the Knowledge from a book or a map. So they get on mopeds and ride it, street by street.