Despite its size, by being a three-wheeler with an official mass below 450 kilograms , the Robin could traditionally be driven by holders of a B1 category driving licence The B1 category used to cover motor tricycles and quadricycles, weighing up to 550 kg (1,200 lb) unladen (see driving licence in the UK) in the United Kingdom, and registered and taxed at motorcycle rates, which gave a saving of £55 a year over a conventional car.