From Roader's Digest: The SABRE Wiki
This article attempts to classify the various types of anomaly that exist in the Great Britain road numbering system. It is meant as an overview rather than an exhaustive list of examples. For a description of how the system is meant to work, see Numbering principles.
First we shall look at roads whose number falls either entirely or partly in the wrong zone. A list of such numbers is at Category: Out of Zone roads.
Probably the commonest cause of zone violations is when a road is extended into a neighbouring zone, but in an anticlockwise direction around the hub. For example, the A66's western terminus was originally on the A6 at Penrith, and so it was allocated a number from zone 6. In the 1970s it was extended westwards (mainly along the routes of the A594 and A595) to reach the coast at Workington. Clearly it would have made little sense to give the whole road a new number from zone 5, and no two-digit number was available anyway. A similar thing happened with the M62, which was extended westwards into motorway zone 5 (the route was originally going to be numbered M52). Sometimes this has happened as the result of downgrading, e.g. the A5127 (former A38), which was extended south of the A5 when the A38 was re-routed to bypass Sutton Coldfield.
Sometimes roads end up in the wrong zone by accident when part of the route is reclassified. A good example of this is the A427 in Northamptonshire, which used to start just outside Rugby (in zone 4) but now starts east of Market Harborough on the A6, placing it entirely in zone 6. Another arguable example is the northern section of the A34, which was split in two when the section between Oxford and the M42 junction south of Solihull was renumbered as the A44/A3400. The southern section is correctly numbered but the northern section runs through zones 4 and 5, meaning it should strictly have a zone 4 number.
Move of zone boundary
Problems can arise when the road forming the zone boundary is rerouted, as it can place existing roads in apparently the wrong zone. For example, the number of the A177 in County Durham dates from when the A1 ran through the City of Durham on what is now the A167, rather than along the A1(M) a few miles to the east as it does now.
One school of thought says that zone boundaries should not move when a single-digit road is rerouted and so this is not a genuine zone violation. It could also be argued that if the zone boundary route moves, the boundary should move with it. There are examples of both these things happening when routes forming zone boundaries have changed.
For example, many roads on Tyneside had their numbers changed from zone 1 to zone 6 when the A1 was rerouted through the Tyne Tunnel; the numbers were then reverted back to zone 1 en masse when it was again rerouted along the Western Bypass. Whether this level of disruption is desirable is a matter for debate.
Re-use of number
Occasionally numbers fall into disuse in one zone and are then redeployed in a different one. The best-known example is probably the A42, originally running between Reading and Birmingham but renumbered in the 1930s. When a number was needed for the all-purpose extension to the M42 in Leicestershire it made sense to match the motorway number, even though the road is entirely in zone 5. Less clear is the motivation for the A88, originally running from Inverness to Scrabster but now a three-mile road in Stirlingshire, entirely in zone 9.
Distinctive number required
The A3400 is the best example of this - a renumbered section of the A34 lying entirely in zone 4. A number was presumably required that was reminiscent of the original number. (Sadly, no one spotted that the more appropriate A434 was available!)
No obvious motivation
Some numbers just seem to be in the wrong zone for no good reason, such as the A3023 on Hayling Island in Hampshire (zone 2) or the A6144 southwest of Manchester (zone 5). Both roads are fairly close to zone boundaries so perhaps this is due to the influence of nearby numbers.
This term is used when a number is used in the right zone, but out of the usual numbering sequence. The term is not usually applied to motorway numbers, which are not conventionally allocated in sequence, nor to four-digit numbers, which (after the initial allocation) have generally been used as required without regard to location.
The commonest reason for misplacement is which a number has fallen into disuse in one part of a zone and been re-used in a different part. Examples include the A176, originally on Teesside but now in Essex; the A403, originally in west London but now in Gloucestershire; and the A437, originally in Gloucestershire but now in west London. (It has been suggested that the last two could be swapped!) The best-known example, the A303 long-distance route to south-west England, is slightly unusual in that the number never fell into disuse, but was exchanged with another number, A3036 (now a road in south-west London). More on this in a later section.
Use of unallocated numbers
Less frequently, numbers that did not form part of the initial allocation are used in what appears to be the "wrong" part of the zone. A rare two-digit example is the A89, which along with the A79 and A99 did not form part of the 1920s scheme; it was then used on parts of the former A8 between Edinburgh and Glasgow. In zone 5, the three-digit numbers originally only went up to A597 (in Cumberland); both A598 and A599 were later allocated to Lancashire, though the A598 has subsequently been reused again in north London.
Three-digit B-road numbers are particularly susceptible to this form of treatment as many of them were not used in the initial allocation. For example, the B439 in Warwickshire comes from a range that was initially reserved for London, but not required; the B390 in Wiltshire, like other numbers with a 9 as the second digit, comes from a range that was held in reserve in case more numbers were needed in south-east England. The range B600-B649 is particularly interesting as it was apparently never reserved for any purpose, and a handful of numbers from that range have been used in various locations, e.g. the B600 in Derbyshire and Nottinghamshire. Such numbers appear to be "misplaced" even though there's nowhere that they properly belong!
Inappropriate number length
The principle that the number of digits in an A-road number reflects the importance of the road has been compromised on many occasions since the roads were first numbered. Many current numbers still reflect the traffic priorities of the 1920s even though the network has changed substantially since then.
Perhaps the earliest violation of this principle was when the A1079 was created as a consequence of the 1924 A1 Renumbering. It was previously part of the A66 but became isolated from the rest of the road when the A1 was rerouted via Scotch Corner, so a new number from zone 1 had to be found. As no two-digit or three-digit numbers were available the next four-digit number in sequence had to be used. The section between York and Hull retains the number to this day even though it is an important trunk road. Other important roads from the early days of road numbering were sometimes given the next four-digit number, such as the A4123 Birmingham - Wolverhampton New Road, whilst others (such as the A580 between Liverpool and Salford) were given shorter numbers, with the original numbered route given a new, longer number.
In other cases a road bearing a three-digit number has been given priority over a two-digit road that was considered to take a less appropriate route. For example, the A57 across the Pennines between Manchester and Sheffield runs through the Snake Pass, notorious for being closed in bad weather. The designated Primary Route is in fact through the Woodhead Pass via the A628 and A616, but the numbers have not been changed to reflect the changed priorities. Similarly the A39 via Porlock Hill in Somerset (with an 1 in 4 gradient) has been superseded as the main route to north Devon by the A361. The A56 between Manchester and Chester is another road that has been eclipsed by a three-digit rival, the A556 (though nowadays the M56 relieves both of them).
Perhaps the best-known example, though, is the A303 between Hampshire and Devon, already mentioned in another context. This route dates back to the 19th century when it was constructed as an express turnpike road as an alternative to the traditional coaching route via Salisbury and Yeovil, which became the A30. For some reason it received no coherent number in the 1922 scheme, and the number A303 had to be transferred from London in the 1930s, presumably because of its similarity to A3036, which was already in use on a section of it. It was later designated as a Trunk road, and upgraded to dual carriageway along much of its length. Today the parallel A30 is no longer even a Primary Route.
There are two types of duplicated numbers: those created by the division of a road, and accidental duplications. Some people do not regard the first type as genuine duplications.
Duplications by division
These happen when an intermediate section of a road is renumbered but the sections to each side retain the same number, giving the appearance of two roads with the same number. One example already cited is the A34, which is in two sections, one from Winchester to north of Oxford, and another from south of Solihull to Salford. The intermediate section was renumbered as the A44 and A3400 after the parallel M40 was constructed. A less prominent but more unusual example is the A603, which originally ran between Bedford and Cambridge but is now interrupted by twelve miles of the B1042. The two remaining sections are each shorter than the gap between them.
These happen when a number already in use in one part of the country is unwittingly used elsewhere. In theory this should never happen because the Department for Transport keeps a central register of road numbers for England and Wales (how duplications between England and Scotland are avoided is unclear). Nevertheless mistakes do happen and a list of duplicated road numbers is given here.
One of the most notorious duplications is probably the A594, both the number of a road from Papcastle to Maryport in Cumberland and the Leicester Inner Ring Road. It seems that when most of the former A594 was incorporated into the A66 (see earlier) the authorities were unaware that a small section remained, and reallocated the number. Another is the A601(M) near Carnforth in Lancashire, which according to the usual numbering conventions should be a motorway-standard upgrade to the A601, a number in fact in use on the Derby Inner Ring Road. (The original location of the A601 was in fact in Hertfordshire so it seems that two different authorities laid claim to the disused number.) The lack of coordination between England and Scotland has given rise to a B6374 in Derbyshire and a B6374 in the Scottish Borders.
Multiple numbers on one route
There are a few occasions when a road changes its number partway through its route for no good reason. The most prominent of these is the motorway route between north-west England and Glasgow, which changes its number not once but twice: from M6 to A74(M) as it crosses the Scottish border, and then again to M74 at Abington in south Lanarkshire. The reasons for this are complex (and not fully understood) but originally the M6 stopped a few miles short of the Scottish border just north of Carlisle, while the M74 ran south from Glasgow as far as Blackwood (later extended to Abington). When the decision was made by the Scottish authorities to join the two together by means of an online upgrade to the intermediate A74, the route was given the temporary number A74(M) on the understanding that it would eventually become part of the M6. At some point, however, this decision was reversed. When the Highways Agency in England closed the gap between the M6 at Carlisle and the Scottish border, the existing English number was naturally extended, giving rise to the current confusing state of affairs.
A slightly different type of example occurs when a continuous Primary Route has been created out of parts of two or more differently numbered A-roads, giving the impression of a main route that suddenly changes its number, while the previous number is diverted onto a secondary route. For example, at Pevensey in Sussex, westbound traffic using the primary A259 will find that the primary route continues straight ahead as the A27 towards Brighton, while the non-primary A259 branches off to Eastbourne. (In this case we have the additional anomaly of a two-digit road terminating on a three-digit one - this is apparently because the A27 originally terminated at Brighton and was later extended to meet the A259.) Another baffling example is the change from A419 to A417 near Cirencester in Gloucestershire on a more or less direct route from Swindon to Cheltenham, and of course the A303 (which must win some sort of prize for numbering anomalies!) continuing directly from the A30 near Upottery in east Devon.
It's very rare for the authorities to allocate a number that is not permitted by the rules at all, but at least one example has been recorded: the former B38, which was allocated to a former section of the A38 in Selly Oak, Birmingham on the opening of the bypass. All other B-road numbers in Great Britain have three or four digits. In December 2011, the Department for Transport confirmed they were aware of the abnormal numbering, and in early 2012 the number was changed to B384. This is also highly anomalous, being both an out-of-zone number and a three-digit number in an area that would normally use four-digit numbers, but it seems that the DfT doesn't object to it.