Quad bikes are motorised sit-astride vehicles that travel on four low-pressure tyres on unpaved surfaces. They have a seat designed to be straddled by the operator, with handlebars for steering control. These vehicles are intended to be used by a single operator - without a passenger. While Quad bikes are designed to cope with a wide variety of terrain types, including steep slopes, they can't go everywhere and do everything. If used outside their safe operating limits they can very rapidly become unstable.