The Knowledge has a very important secondary effect - it creates a huge sunk cost, vastly increasing the downside of losing your license. Getting a green badge

The Knowledge has a very important secondary effect - it creates a huge sunk cost, vastly increasing the downside of losing your license. Getting a green badge takes three to four years of full-time study. You can lose it for relatively minor offences like short-changing a passenger or taking a tourist on the scenic route. Hackney cab drivers with a green badge have a far stronger incentive to do business honestly than a minicab or Uber driver.