(This feature is more or less taken verbatim from the sheets and diagrams provided by the Department of Transport. Since I have no scanner, I have painfully redrawn the two diagrams so the copyright is mine!! I have however, included a section of my own at the end, and it is indicated where this begins.)
In 1919, the Ministry of Transport was empowered to classify highways and introduced a classification system of Class I roads (more important roads connecting large centres of population, and of outstanding importance from the point of view of through traffic), and Class II roads (roads forming important links between Class I roads and smaller centres of population). More recently the classification system was reviewed and Class I roads were given 'A' numbers while some Class II roads were given 'B' numbers. Class III roads were later introduced and defined as those which had more than local interest but did not justify inclusion in any other group. No official numbers were allocated to these roads, but some were given 'C' numbers by the highway authorities.
[Editors note: You can find a 'C' road in action if ever you have occasion to drive into the centre of Scunthorpe, where you might find yourself driving on the C271 if you are near the railway station...]
The administration of the highway system continues to be based on its function and is organised into the following categories:
The road numbering system is based on the radiating zone principal (see diagram 1 and 2). The six single number 'A' routes in England (A1, A2, etc.) mark the boundaries of these zones. Zone 1 is between the A1 and the A2 (moving clockwise); Zone 2 is between the A2 and the A3, and so on. Scotland has three further zones; Zones 7, 8 and 9. Motorway markings follow a similar pattern. Roads in Zone 1 have numbers beginning with the figure 1, roads in Zone 2 have numbers beginning with the figure 2, and so on. Roads are allocated in a random number in the zone in which they commence e.g. a principal road between Zones 2 and 3 could be A272 or a classified non-principal numbered B2224. Roads keep their number irrespective of the zones they cross or in which they terminate. When a new road number is allocated the road is given the next free number within that zone.
The status of each road is decided by its function rather than the standard of construction or traffic volume using it and there can be no guarantee that a road will retain its number. If the function of a road alters then the highway authority can apply to the Department to amend the status and this may result in a change in the road number.
The left hand diagram is the Numbering Zones for Trunk, Principal and Classified Roads.
The right hand diagram is the Numbering Zones for Motorways. Click to enlarge. Copyright Me 1999.