A special road is the official classification of motorways in the United Kingdom. For a road to become a special road, it must have a Statutory Instrument sanctioned under the Highways Act 1980. A road which is not a special road is termed an all-purpose road. The vast majority of the roads in the UK are all-purpose roads. Most special roads are motorways.
Special road Wikipedia
The passing of the Special Roads Act 1949 through Parliament allowed the UK to construct roads that were not automatically rights of way for certain types of user. All previous roads were automatically rights of way for all road users, including pedestrians, so it was not possible to build roads designated only for motorized traffic. The Act therefore allowed the construction of motorways.
The Special Roads Act was merged in with the Highways Act 1959, and later the Highways Act 1980
The Special Roads Act was first used in late-1950s to designate the Preston By-pass in Preston, Lancashire, now largely part of the M6 motorway, as a special road.
Although the majority of special roads in the UK are also motorways, there are a number of special roads that are not motorways. Quite a number of these are toll bridges, and several others are former motorways which have since been downgraded. In order to charge a toll on a newly built road, special road regulations are usually necessary.
In addition, a small number of non-motorway special roads are relatively newly built dual-carriageway roads, such as the A1 Dual Carriageway east of Edinburgh and parts of the A720 Edinburgh bypass, or parts of the A55 in North Wales. These particular roads have many of the same regulations as motorways, apart from the speed limit regulations, since that only applies to special roads which are also motorways. The usual speed limit regulations that apply to all-purpose roads do not apply to special roads, so a non-motorway special road must define a speed limit as part of its Statutory Instrument.