In the United Kingdom, the name hackney carriage today refers to a taxicab licensed by the Public Carriage Office, local authority (non-metropolitan district councils, unitary authorities) or the Department of the Environment depending on region of the country.
Historically four-door saloon cars have been highly popular as hackney carriages, but with disability regulations growing in strength and some councils offering free licensing for disabled-friendly vehicles, many operators are now opting for wheelchair-adapted taxis such as the The London Taxi Company (LTI). Other models of specialist taxis include the Peugeot E7 and rivals from Fiat, Volkswagen, Metrocab and Mercedes-Benz. These vehicles normally allow six or seven passengers, although some models can accommodate eight.
In London, hackney-carriage drivers have to pass a test called The Knowledge to demonstrate that they have an intimate knowledge of the geography of London streets, important buildings, etc. Learning The Knowledge allows the driver to become a member of the Worshipful Company of Hackney Carriage Drivers. There are two types of badge, a yellow one for the suburban areas and a green one for all of London. The latter is considered far more difficult. Drivers who own their cabs as opposed to renting from a garage are known as "mushers" and those who have just passed the "knowledge" are known as "butter boys". There are currently around 21,000 black cabs in London, licensed by the Public Carriage Office. (Wikipedia)