"All licensed London taxi drivers need to pass a special test before they can drive one of the Capital's famous black cabs. This test is called the Knowledge. How long it takes to become a licensed taxi driver depends on whether you want to be an All-London driver or a Suburban driver. London or the suburbs? All-London drivers - also known as Green Badge drivers - need a detailed knowledge of London within a six mile radius of Charing Cross. All-London drivers have to learn 320 routes (or runs). They also need to know all the landmarks and places of interest along the runs. It takes between two and four years to pass the All-London Knowledge. Once you are licensed you can work anywhere in the Greater London area." Ref:www.tfl.gov.uk