It's a lot more than just a "fancy-pants test". So much more. They know pretty much everything about London. Try entering "Half-way down that road opposite the old Tesco's in Essex Road" into your GPS and see where that leads you. Cabbies know London intimately. They know how the traffic at a given location is, and how it will be in 30 minutes. They know which roads have temporary closures, which legal-for-them-but-not-for-non-cabbies short-cuts they can take, and so on. GPS is truly a wonderful invention - I absolutely love it. Having used black cabs for years, it's clear they have nothing in a black cabby.