From Roader's Digest: The SABRE Wiki
There is sometimes confusion about the separate numbering schemes for A-roads, B-roads and motorways within Great Britain. To the uninitiated, it may seem illogical to have the A275 in Sussex, the B275 in south London and the M275 in Portsmouth. This article attempts to set out the principles on which the three systems of numbering were originally developed, as well as explaining some of the changes to the system that have subsequently taken place.
The overarching principle for all classes of road is the zone system. There are nine numbered zones, and the number of a road begins with the same digit as the zone in which it originates. This rule was rigidly enforced in the early days of road numbering, though many exceptions have been permitted over the years (see Category:Out of Zone roads for a list).
However, it is important to realize that there is one zone system for A-roads and B-roads (all-purpose roads} and a different one for motorways. With one exception, the boundaries of the all-purpose zones are the single-digit A-roads. (The exception is the A2 as the River Thames forms the boundary between zones 1 and 2.) Zones 1 to 6 are numbered in a clockwise direction round the "hub" at London, and zones 7 to 9 are numbered in a clockwise direction round the "hub" at Edinburgh. The A7 forms the boundary between zones 6 and 7. There are two small "fault lines" in the system - the 1 and 9 zones touch to the north of Edinburgh, and the A900 stands as the zone boundary; whilst at Carlisle, the A6 and A7 meet, leaving a small area to the west of the city where the 5 and 7 zones touch - here the River Eden is used as the zone boundary.
With the motorways, the situation is a little more complex. In England and Wales, the M1 to M6 form the basis of the boundaries between zones. However, not all of these motorways start in London, or run all the way to the coast. The result is a number of rather strange-shaped zones which appear on the map here. Scotland is more straightforward, using the same zones for motorways as for all-purpose roads.
Crossing zone boundaries
Under the strict numbering rules adopted in the 1920s, routes were permitted to stray out of their zone and into one or more neighbouring ones, but in a clockwise direction only (or from zone 6 into zone 7). So for example the A38 runs through zones 3, 4 and 5 (and was later extended into zone 6). It was not permitted for a road from zone 3 to enter zone 2, though this principle has later been compromised: for example the new A14 violates the rules by running westwards from zone 1 through zones 6 and 5. Nevertheless the vast majority of route numbers still obey this principle.
A similar rule was adopted when the motorways were numbered, so that for example the M42 runs from zone 4 into zone 6. Again, the rule has not always been enforced, with the M62 being extended to the west of the M6 into zone 5 over the originally planned M52 route.
A-road numbering principles
Two-digit A-road numbers
The original principle was that routes were numbered firstly in a clockwise direction as they radiated from the hub, and then in increasing order of distance from the hub. So for example in zone 1 the numbers A10 to A13 were given to routes radiating clockwise from London, and the numbers A14 to A19 were given to cross-country routes increasing in distance from London. This principle still holds in general, except in the small number of cases where a number has been allocated to a different road from the one where it was originally used. Specifically, the A42 (originally a road from Reading to Birmingham) is now a road in zone 5, taking its number from the adjoining M42.
In the Scottish system, the principles were the same except that the numbers A79, A89 and A99 were not originally allocated. These have subsequently been fitted fairly neatly into the system (except the A89, which runs along former sections of the A8 between Edinburgh and Glasgow).
Three-digit A-road numbers
Three-digit numbers were originally allocated within each zone roughly in increasing distance from the hub. In some zones not all the numbers were used (e.g. zone 2 only went up to A288). Otherwise then the 100 most important roads within the zone were selected, so that the numbers reached the edge of the zone. Most of the unused numbers have subsequently been allocated, although there are still unused numbers in zones 7 and 9.
Changes to the pattern have occurred when a number has fallen into disuse and then been reused elsewhere in the zone. Sometimes these are inconspicuous (e.g. the A369 in Somerset) but sometimes they are more obvious, such as the A176 in Essex, transplanted from its original location on Teesside. The best-known example of one of these "misplaced" numbers is probably the A303, moved from its original location in south-west London in 1933 to create a long-distance route to south-west England.
Four-digit A-road numbers
Outside London, four-digit numbers were given to the less important roads in increasing distance from the hub, filling in the gaps between the three-digit roads. In the initial allocation no zone received more than 100 four-digit numbers, so the only second digit needed was 0. Further numbers were then allocated in sequence as needed. (In zones 7, 8 and 9 no four-digit numbers were originally allocated at all.) In London the pattern was slightly different, with 2 being used as the second digit, to allow for at least 100 additional numbers outside London.
Since nothing like all the four-digit numbers have been used in any zone there is no longer any need to allocate numbers in sequence, and it has become possible to use numbers that are in some way distinctive, such as the A5300 near Liverpool or the A34 being renumbered as the A3400 through Warwickshire. Very few four-digit numbers have been used in zones 7, 8 and 9.
B-road numbering principles
In the original scheme there were no single-digit or two-digit numbers in order to avoid giving the appearance that they were more important than three-digit or four-digit A-roads. This principle still applies (the short-lived B38 notwithstanding).
Unlike with A-roads, the distinction between three-digit and four-digit numbers did not relate to the importance of the road, but to its distance from the hub.
Three-digit B-road numbers
The first 50 numbers in each of zones 1-5 were originally allocated to London, though not all of them were used. (In zone 6, which does not enter London, they were not used at all.) The next 40 numbers in zones 1-6 were then used in increasing distance from the hub, mostly in the Home Counties (although in zone 6 they got as far as Nottinghamshire). The final 10 numbers were reserved for future use. So for example in zone 1 the only three-digit numbers used were B100-B125 (London) and B150-B189 (mainly in Essex). In zones 7, 8 and 9 there were no special allocations and the numbers were simply used in increasing distance from Edinburgh.
A single exception occurred from the original published list, where the B290 followed the B289. This was due to a dispute between between the Ministry of Transport and the local regional division which created the original B259 as a last-minute bodge.
Four-digit B-road numbers
When the three-digit numbering scheme reached its limit, the pattern switched to four-digit numbers; for example the next road in sequence after B389 was B3000. These were used right up to the edge of the zone and subsequently in sequence as needed. Nowadays any available four-digit number may be used, sometimes with no obvious motivation, e.g. B4696.
Motorway numbering principles
There are two separate systems of motorway numbering. One is an extension to the A-road system and follows A-road numbering rules.
"A" prefix, "(M)" suffix
Motorways numbered in this fashion are normally motorway-standard upgrades to a section of A-road, e.g. the A1(M) number is given to various motorway-standard sections of the A1. In some cases the motorway runs parallel to the similarly numbered A-road, e.g. A3(M) north of Portsmouth runs parallel to the A3, and in others the motorway forms a link between the A-road and another motorway, e.g. the A48(M) links the A48 to the M4.
In England and Wales, there are no specific rules governing the second digits of motorway numbers, beyond a preference to match the number of a parallel A-road if the zone system permits it, e.g. the M23 runs parallel to the A23. Sometimes this is not possible because of the separate zone systems: the motorway replacing a section of the A46 between Coventry and Leicester lies entirely in zone 6 and has the number M69, not M46. But the M48 and M49 are fairly recent additions to the system, and do not conform.
The original numbering system did not allow three-digit motorway numbers, but when zone 6 started to run out of numbers they were permitted as short urban spurs, e.g. M602. Since then they have mainly been allocated on a "tree" system, e.g. the M271 is a branch of the M27.
In Scotland, motorways generally take the number of the A-road they replace, e.g. the M90 replaces a section of the A90. A partial exception to this is the M73, which does not replace any A-road but happens to run roughly parallel to a section of the A73.
Other Numbering Systems in Britain
While unsigned in the UK, the E Roads that run through the country follow a Europe-wide grid-system. Odd routes run N-S, even routes run W-E. Routes ending in 5 or 0 are major routes. 3 digit routes are numbered by grid reference: <first digit of major route to the north> <first digit of major route to the east> <number>. No three digit routes exist in the UK, though the E201 runs along the M8 between Port Laoise and Cork. The '2' comes from being south of E20, but north of E30. The '0' comes from being west of the E05. The '1' comes from the UNECE not liking 3 digit routes ending in zero.
(see also Sustrans' own description)
NCN routes roughly follow the A road numbering, but are based on a misconception. They follow a branch-based system, rather than a zonal one. As such a branch of NCNx would be NCNxy, whichever way they branch off. The routes of the single digit routes are sometimes roughly similar to the A road network, other times radically different.
Here's a list of single digit routes:
- NCN1 East Coast (Shetland - Dover)
- NCN2 South Coast (Dover - St Austell)
- NCN3 Bristol - Lands End
- NCN4 London - Fishguard
- NCN5 Reading - Holyhead
- NCN6 London - Carlisle
- NCN7 Sunderland - Carlisle - Glasgow - Inverness
- NCN8 Cardiff to Holyhead
- NCN9 Belfast towards Dublin