A special cab line
The 21,000 black cabs that roam the streets of the British capital are the modern version of "Hackney carriage" that existed in London since the early seventeenth century. They occupy a special position in the law: unlike other taxis that must be booked in advance, black cabs can be hailed in the streets. Drivers must pass an exam called "The Knowledge to demonstrate their knowledge of the city's thousands of streets". You have to use one while in London, and try to find an old-style one, with rear-facing doors!