I assume you are talking mainly about lorries. Basically these are governed by the Construstion and Use Regulations and the Plating Regulations. All HGV's (which are actually LGV's now, but we'll use HGV as everyone knows what it means) must carry a plate in the cab showing their permissble weights. This shows maximum axle load and gross vehicle weight (GVW). For example, I drive an 8 wheel tipper. The maximum allowable is 32 T GVW. On my truck the front 2 axles have a max weight of 7.9T each and the rears 9.7T each. Now a quick bit of maths shows that the total axle weights are 35.2 T. This gives you a bit of leeway, so you don't have to be loaded perfectly. The maximum 'normal' weight in the UK is 44T for a 6 axle articulated vehicle on 'road friendly' (this means air) suspension. You are allowed to be 10% over this maximum weight to allow for discrepencies in weigh bridges. These weights apply to all roads unless there is a weight restriction on the particular road indicated by round weight limit signs, and in most cases allow for access by vehicles delivering/collecting within the weight limit. There are heavier lorries about (lowloaders, oversize loads etc) and these come under the 'Special Types' rules. You may have seen lorries with black and white plates saying 'STGO CAT 1' on them. STGO =Special Types General Order. From memory, Cat 1 is up to 75 T GVW, Cat 2 is 75 - 100 T, and Cat 3 is 100+ T. As they are oversize, they normally require a Movement Order and as such must follow a specific route agreed by all the Councils on the route and the Police. Therefore they will use motorways and A roads unless they really have to go to smaller roads (usually at pick up and destination).