This requires them to know every street within an eight mile radius of the centre of London, and can take two years of dedicated study to pass. Around London, you will see candidates for the Knowledge studying street signs while riding motor scooters with a list of street names attached to a clipboard between the handlebars. Some people link this to developments in cabbies' brains caused by studying for the Knowledge. Black cabs are obliged by law to accept all fares within an eight mile radius of London. Although they are not supposed to, many cabs will turn their light off as an excuse to pick and choose which fares to accept.
Black cabs are one of the safest places to be in London.
You will rarely be ripped off in a black cab. If you seem to be taking an odd route, the cabbie is probably avoiding traffic which can be horrendous on the main roads that you will be familiar with. Tipping is not necessary, however rounding up to the nearest pound will be appreciated if the cabbie has provided a good service. Anyone, driving any car, can be a mini-cab driver. Mini-cabs are unregulated and some reports suggest that most of them are illegal in some way or other.