The numbering system

Have you ever wondered why roads are numbered as they are? If not, maybe you should have done - there's a nice system for doing it, and understanding it even has some practical benefit (e.g. preventing you from getting lost).

We'll restrict our attention to the roads in the United Kingdom, except those in Northern Ireland, which have a separate numbering system (but not separate enough: their numbers overlap with the numbers used in Great Britain, and even with each other), and those on the Isle of Man. It hardly seems worth having a different numbering system for an area as small as the Isle of Man, but there is one, and again, it overlaps with the numbering system used for roads on the mainland (although there are no motorways).

The basic idea is that most roads in England and Wales begin with numbers 1, 2, 3, 4, 5 and 6, with London as the hub, and most roads in Scotland begin with numbers 7, 8 and 9, with Edinburgh as the hub. There are three basic types of numbered road: motorways (beginning with an M), A-roads and B-roads. There is actually a fairly rigorous definition of ``motorway'', although I'm not sure exactly what it is - perhaps I should consult the highway code? There are also sections of A-road, such as the A1(M), which have motorway restrictions on them, although these do not need to have numbered junctions (which motorways do). These restrictions ban, among other things, pedestrians, animals, small motorcycles, domus, humus and rus. [Surely some mistake - Ed.]

One might have thought that the first-built road would be numbered 1, the second numbered 2, and so on, but it doesn't work like that, although admittedly the A1 is one of the oldest roads, being reputedly two thousand years old! (Legend has it that it started with someone walking from Edinburgh to London.) No attempt is made to use all the numbers: for example, there is no M7, although after reading this, you'll be able to guess where it would be if it existed. Instead, the number of the road should be thought of as a sequence of formal symbols (such as letters) which just happen to look like numbers. So, for example, the roads which begin with 1 are more closely related to each other than to roads which begin with 2, just as words beginning with A are grouped together in a dictionary.

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  • A brief history of the numbering system

    By James Bufford

    With the rise in road traffic at the beginning of the 20th Century and the need to provide government funding to improve them, the British government started work on classifying the road system. The First World War interrupted work but it resumed afterwards and in 1919 an Act of Parliament was passed which gave the OK for the Government to fund roads and, in order to do so, they decided to number them.

    It was fairly quickly realised that a numbering system would also be useful for navigation. The newly created Ministry of Transport (MOT) set about identifying which roads needed funding, and numbering them. There were to be two classes of roads: A roads, more important with more funding, and B roads of lesser importance.

    By 1921 the MOT had identified the system for England and classified 97 main A roads, with one or two digit numbers - some of these were published on Michelin and Bartholomews maps, much to the Ministry's irritation - and to Bartholomew's irritation as some of the numbers then changed.

    The formal classification was not published until 1923, in a little booklet published by HMSO. The Ordnance Survey published a series of maps - the MOT series and several other mapmakers published revised road maps. Road numbers also went up on signs.

    Numbers changed quite a bit in the 1920s:

  • with more money for A roads than B roads, there was an increase in A roads
  • the 1920s saw a large programme of building
  • the MOT had got the main routes wrong in some cases - for example the A1 ran through Northallerton and Thirsk whereas all the traffic took the route up via Scotch Corner

    The numbering system works as follows.

    In England 6 main radial routes out of London were identified:

    • 1: London - Newcastle - Berwick (and on to Edinburgh)
    • 2: London - Dover
    • 3: London - Portsmouth
    • 4: London - Bath (London - Bristol, writes Christopher Larsen)
    • 5: London - Holyhead
    • 6: London - Leicester - Manchester - Carlisle

    Working in an clockwise direction roads starting in the zone east of road 1 (A1) were given numbers beginning with 1; south of the A2 numbers beginning with 2, west of the A3 with 3 and so on. There were exceptions - roads starting in North Kent began with 2, and the A40 between the city and marble arch marked the southern boundary of roads beginning with 5.

    In Scotland 3 main radial routes out of Edinburgh were identified:

    • 7: Edinburgh - Carlisle
    • 8: Edinburgh - Glasgow - Greenock
    • 9: Edinburgh - Inverness

    Going clockwise again, roads between the A1 and the coast started with 1, roads between A1 and A7 started with 6 (continuation of the 6 zone) between A7 and A8 with 7, A8 and A9 with 8 and A9 and the coast with 9.

    Roads which began in one zone but continued across several others maintained their initial number - thus the A46 begins in zone 4 in Bath, but keeps the same number despite crossing into zones 5,6 and 1 on its way to Grimsby.

    The A roads were given two, three or four figure numbers broadly in terms of their importance. All 2 digit numbers, barring 79,89 and 99 were used. Not all 3 digit numbers were used (289-299, 598-599 for example). Some numbers were allocated to roads yet to be built The four digit numbers were mainly given to link roads.

    In another twist, some central London roads were given numbers in the sequences 12XX, 22XX, 32XX, 42XX, 52XX.

    For B roads the first digit of the number was given according to the same system as A roads. But there were no one or two digit B roads. Also, numbers 100-149, 200-249, 300-349, 400-449 and 500-549 were reserved for use in central London while 150-199, 250-299, 350-399, 450-499 and 550-599 were used for roads starting in outer London. There is one exception - the B259 ran between Brighton and Eastbourne. Because the MOT expected more roads to be built in London B numbers in the series BX90-BX99 weren't used. Once these numbers were used up B roads were all given 4 digit numbers.

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  • Readers' comments on the numbering system

    Chris Hillcoat writes:

    The A329(M) at Reading should be the M329, as it runs parallel to the A329. Unfortunately I think I'm the only one who's noticed this despite the fact that it has been around for ages.

    [As I recall, the A3(M) from Havant to Waterlooville is another of these: it runs parallel to the A3 - Ed.]

    Stephen Holt has a theory about motorway numbering anomalies (which, judging from
    this, is correct). He writes:

    I think that a different system operates for the motorways in England and Wales with two hubs in the vague areas of London and Birmimgham. The zones are thus:

    All motorways east of the M1 but north of the M2
    (eg M11, M180)
    All motorways south of the M2 but east of the M3
    (eg M20, M27)
    All motorways west of the M3 but south of the M4
    (eg M32 in Bristol)
    All motorways north of the M4, east of the M5, south of the M6 and west of the M1
    (eg M40, M42)
    All motorways to the west of the M5, and to the west of the M6
    (eg M50, M54, M55)
    All motorways east and north of the M6, but west of the M1
    (eg M69, M61, M62)

    Chris Boots writes:

    Peter Harding sets some parameters for motorways as follows:

    Christopher Larsen adds:

    A semi-anonymous hotmail user comments:

    Tom Sutch writes:

    Tim Lidbetter asks:
    Chris Marshall writes:

    Fraser Smith writes:

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  • Cones

    The basic criterion determining the first digit in a road's number is which ``cone'' (or zone) the road begins in. The cones are numbered 1-6 (for England and Wales) and 7-9 (for Scotland), and are basically defined in terms of the nine key roads A1 to A9 (which do all exist). Roads in England are generally considered to begin from the London end, and roads in Scotland from the Edinburgh end. When the road has numbered junctions (usually because it is a motorway, although there are exceptions, like the relatively new A14), the low-numbered junctions are at the beginning of the road.

    As I mentioned, the roads A1 to A6 are considered to start from London. They emerge clockwise starting from the north: the A1 goes to Edinburgh, the A2 to Dover, the A3 to Portsmouth, the A4 goes west to Bristol, the A5 to Holyhead and the A6 to Carlisle. (Note that some of these roads are no longer continuous, since sections of them have been replaced by more major roads: for example, one section of the A5 near London was renumbered, presumably to confuse people into using the M1 instead.) The 1-cone, where the roads begin with 1, lies to the north of London between the A1 and the A2, and so on, ending with the 5-cone between the A5 and the A6, and the 6-cone between the A6 and the A1.

    Numbering in Scotland works analogously. The key roads, which emerge clockwise from Edinburgh, are the A7, which goes southwest to Carlisle, the A8, admittedly largely eaten up by the M8, which goes west to Greenock, and the A9 which goes north to Thurso.

    This basic plan applies to all the types of road, not just A-roads. However, there are some exceptions, some of them irritating. One of the most striking is the M5. It is usually the case that if a motorway and an A-road have the same number (e.g. the A3 and the M3), then they go approximately in the same direction. However, the M5 is a blatant exception: it goes from Birmingham to Exeter and is clearly very friendly with the A38, which it is essentially replacing throughout its length, so it should be called the M38. The A5, in contrast, is almost at right-angles to the M5!

    Charles Walkden's numbering game, and other numerological pursuits

    Charles Walkden writes:
    Stephen Holt answers: Matt Hales adds: Toby Speight observes: Christopher Larsen adds: Fraser Smith writes:

    Cone anomalies

    Alasdair Kemp writes:

    Gareth Leyshon writes:

    Bob Wingrove writes:

    Chris Marshall asks
    Andrew Smith points out

    Roads entirely in the wrong cone

    Aidan Westwood asks
    Paul Berry replies
    Aidan Westwood adds
    Aidan Westwood adds
    Steven Jukes confirms
    Guy Barry gives a frighteningly comprehensive answer
    M4 Man got in touch with Swansea City Countil about the B5444. The National Assembly of Wales got back to him:

    I understand you have been in touch with Swansea City Council about the route number of the above mentioned road.

    The classificaton of highways and route numbering, where appropriate, in Wales is the responsibility of the National Assembly for Wales, and formerly the Welsh Office. Pentrechwyth/Jersey Road was allocated a classified road status in 1991 and given the number B5444 due to the construction of the Pentrechwyth bypass and cross valley link.

    Unfortunately the route number used was not appropritate for use in South Wales, all roads in this area would normally start with the number 4. Also, as you have pointed out, the number was already in use on road in the Mold area. We will be discussing an amendment to the route number with the City Council and once agreed we will inform the map production companies. The City Council will then amend the necessary traffic signs.

    Thank you for making us aware of the error.

    I [M4 Man] was quite impressed with this reposnse - an admission of guilt from government!

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  • The rest of the number

    So, that's the first digit of the number explained? What about the rest of the number, you may wonder. The rules for determining the subsequent numbers are necessarily more vague, since not all roads lead to Rome (respectively, London or Edinburgh), but the idea is that the closer to the hub the road starts, the lower the second number should be. The secondary numbers also tend to sweep clockwise round the cone in the same order as the primary numbers. This is well-illustrated by the roads A20 to A24, which essentially start in London. The A20 goes south-east to Folkestone, and the Channel tunnel. The A21 goes to Hastings, which is further to the west; the other roads occur in sequence passing to the west: the A22 goes to Eastbourne, the A23 to Brighton and the A24 to Worthing.

    Motorways and B-roads are qualified in a vaguely similar way. The main difference is the number of extra digits which may be added. Motorways usually only have one or two digits, but a few (like the M180) have three. A-roads may have one, two, three or four digits, even if they have motorway restrictions (like the A6144(M) near Manchester). B-roads have three or four digits. The length of a road's number tends to be in inverse proportion to its importance.

    Jeremy Marshall writes:
    Gareth Leyshon writes:
    Matthew Garrett writes:
    Theo Markettos writes:
    Steve Barnard writes:

    Alasdair Kemp writes:
    Brad Emerson writes:
    Tony Priest writes:
    Toby Speight writes:
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