In the United Kingdom the maximum permitted gross weight of a semi-trailer truck without the use of a Special Type General Order (STGO) is . In order for a 44,000 kg semi-trailer truck to be permitted on UK roads the tractor and semi-trailer must have three or more axles each. Lower weight semi-trailer trucks can mean some tractors and trailer having fewer axles. In practice, as with double decker buses and coaches in the UK, there is no legal height limit for semi-trailer trucks; however, bridges over do not have the height marked on them. Semi-trailer trucks in continental Europe have a height limit of . Vehicles heavier than 44,000 kg are permitted on UK roads but are indivisible loads, which would be classed as abnormal (or oversize). Such vehicles are required to display an STGO (Special Types General Order) plate on the front of the tractor unit and, under certain circumstances, are required to travel by an authorized route and have an escort.