Most trucks on UK roads live inside the Construction and Use and Authorised Weight regulations. But some loads simply cannot be broken down - a transformer, a crane section, a bridge beam - and the law makes room for them through the Road Vehicles (Authorisation of Special Types) (General) Order 2003, universally known as STGO. The official enforcement guide sets out how the system works, and at its heart sit three weight categories that determine everything from axle limits to notice periods to motorway speed.
Special types vehicles are those which do not meet the C&U and Authorised Weight regulations but can be used outside those rules under STGO’s authority. Vehicles that do not comply with an STGO order can still go on the road if a Special Order has been issued - by National Highways for abnormal loads outside C&U and STGO, or by the Vehicle Certification Agency for special vehicles and divisible loads such as crane ballast outside that scope. Two disciplines sit underneath. First, loads that comply with C&U and the weight regulations cannot be carried on an abnormal indivisible load vehicle, even as a backload - unless the vehicle is dual-plated for both C&U and STGO use, in which case normal loads ride at the weights on the ministry plate. Second, even when carrying an abnormal load, a normal-sized vehicle must be used where possible and safe, overhangs and all; only where that is unsafe does a larger vehicle or combination come into play, subject to STGO’s conditions.
Category 1 covers vehicles not exceeding 46,000kg on a minimum of five axles, or 50,000kg on a minimum of six. The Authorised Weight regulations’ maximums still apply to axle and vehicle gross weights - only the train weight can exceed them. The drawing vehicle displays an “STGO Cat 1” plate to the front. Two working days’ notice must go to highway and bridge authorities about the weight, and dimensions may need to be given to the police. Speed limits are 60mph on motorways, 50mph on dual carriageways and 40mph on other roads.
Category 2 runs to 80,000kg on a minimum of six axles, with a maximum axle weight of 12,500kg. The drawing vehicle displays an “STGO Cat 2” plate. The notice requirement mirrors Cat 1: two working days to highway and bridge authorities for the weight, dimensions potentially to the police. Speeds drop sharply: 40mph on motorways, 35mph on dual carriageways and 30mph on other roads. The vehicle must also carry a plate marked “Special Types Use” showing the manufacturer’s recommended maximum weights - gross, train and axle - when travelling at particular maximum speeds.
Category 3 covers the heaviest STGO movements, up to 150,000kg, again on a minimum of six axles but with a maximum axle weight of 16,500kg. The drawing vehicle displays an “STGO Cat 3” plate. Notice lengthens: five working days to highway and bridge authorities, with dimensions potentially required by the police. Speed limits match Cat 2 - 40, 35 and 30mph - and the “Special Types Use” plate requirement applies equally.
The categories reward early thought. Axle count and spacing drive the category as much as gross weight does, so trailer choice is a compliance decision, not just a lifting one. Notice periods are working days, not calendar days, and they run before the movement - a Friday afternoon decision does not move a Cat 3 load on Monday. And the speed limits are part of the legal authority to be on the road at all: a Cat 2 vehicle running at ordinary HGV speeds is outside its authorisation. Beyond 150 tonnes or outside the category conditions entirely, the movement leaves STGO and needs a Special Order - a different process with its own lead time. Get the category right, plate the vehicle, serve the notices and keep to the speeds, and STGO does exactly what it was written for: moving the immovable, legally.
Two operational edges of the STGO regime deserve special attention because they catch experienced hauliers. The first is the backload rule: a vehicle running under STGO authority for an outward abnormal load cannot simply pick up a normal return load - loads that comply with C&U and the weight regulations cannot be carried on an AIL vehicle, even as a backload, unless the vehicle is dual-plated, in which case the ministry plate weights govern. An empty-looking revenue opportunity can therefore be a regulatory breach. The second is the “normal-sized vehicle first” principle: the system expects operators to use the smallest vehicle that can safely do the job, overhangs included, before reaching for the big gear - a bias built into STGO that enforcement applies when movements are examined. Both edges share a theme: STGO is an authority with conditions, and the conditions are part of the movement plan, not fine print for afterwards.