Road Numbers:
Numbers for A and B-roads

The former A5 and A6 in St Albans
The former A5 and A6 in St Albans

These days, most countries around the world have road numbers of some description. They have become a basic staple of navigation, one of the things that is expected of a civilised nation with a modern transport network. They work on a variety of principles - some use letter prefixes and some use stylised shields, and they can be laid out in grids or trees or spokes or just at random.

The system we use for A- and B-roads is the first system the UK had and the one that mainland Britain still uses today. This page explains how it works.

Classified details

All-purpose roads in mainland Britain (that is, all public motorable roads) are divided into three types.

The first two types are referred to as classified roads, and they are A-roads and B-roads. A-roads are the major through-routes, forming the basic network of main roads and arteries. B-roads are a lower class of road, often of a poorer physical standard, and forming links within the framework of A-roads. They often serve smaller settlements or form less important through routes within urban areas. All A- and B-roads are given numbers.

The third type of road is, logically enough, unclassified. These are the minor roads that are left over - country lanes and city streets. Unclassified roads are not allocated with numbers in this scheme.

Hubs and spokes

A and B-road numbering zones. Click to enlarge
A and B-road numbering zones. Click to enlarge

The allocation of numbers is based on a hub-and-spoke system. Because mainland Britain is long and narrow (and because Scotland always likes a measure of autonomy from the Sasannachs), there are actually two hubs, London and Edinburgh.

Radiating from these hubs are the nine principal A-roads 1 to 9:

  • A1 London to Edinburgh
  • A2 London to Dover
  • A3 London to Portsmouth
  • A4 London to Avonmouth
  • A5 London to Holyhead
  • A6 London to Carlisle
  • A7 Edinburgh to Carlisle
  • A8 Edinburgh to Greenock
  • A9 Edinburgh to Scrabster*

These roads divide Great Britain into nine distinct areas, shown in the diagram on the right (click it for a larger version). Each zone is numbered, taking its number from the A-road on its anticlockwise boundary. The exception to this rule is the boundary between zones 1 and 2, which is formed by the Thames Estuary and not the A2, in order to prevent a thin sliver of zone 1 being orphaned in the north of Kent.

From this point, the system is remarkably simple: other roads get their number according to which zone they lie in. Any road in, say, zone 5 gets a number with the first digit 5. It's as simple as that.

Of course, there's a system to work out how the rest of the digits are assigned, and we'll come to that shortly. But there's a more pressing concern to examine first.

Crossing the line

An example of a road crossing a zone boundary
An example of a road crossing a zone boundary

In the real world, of course, roads aren't as neat and tidy as you might hope. If you don't keep a close watch on them, they have a tendency to clear off in whichever direction takes their fancy, and just because you have declared that this is a certain zone and that is a different one, it won't stop a few maverick roads from drifting away across the boundaries.

To put those unruly deviants back in their place, there is an extra rule to the system. Just as the system counts upwards around its two hubs in a clockwise direction, roads can only proceed around the country clockwise. Roads that like to get around simply take their number from the furthest anticlockwise zone they enter.

In the example on the left, the road lies across two zones. It has a number prefixed with a 4 because it begins in zone 4. It can proceed eastwards into zones 5, 6 and 1, but it can't go the other way into zone 3. If it does that, it needs a zone 3 number.

Under the rules introduced in 1922, every road had a starting point which was at its furthest anticlockwise terminus. These days it doesn't always hold true - the A41 causes a headache because its start and finish are both in zone 5, and its middle is in zone 4 - but even so, most of the time you'll find that it works.

This clockwise movement means that you should find no zone 3 roads in zone 2, but you'll find plenty of them in zones 4 and 5. (The truth is that you shouldn't find them straying anticlockwise, but sometimes you just do. Life's like that.) Some roads really take advantage of the clockwise rule and clock up some serious mileage; famous travellers include the A38 which leaves its home zone by crossing the A4 in Bristol and proceeds through zones 4 and 5 to end in zone 6 at Mansfield (where, presumably, it has a well-deserved sit down).

Filling the gaps

Now we have a system for allocating the first digit of all road numbers, and for dealing with the many roads that will cross zone boundaries, we need to fill in all the other numbers.

The numbering system is beautifully simple in that it allocates shorter numbers to important roads and longer numbers to minor roads. A-roads have one, two, three or four digits; B-roads only have three or four digits to their numbers.

In each zone, the lowest two-digit numbers are used - where necessary - as further spokes radiating from the hub city. The remaining numbers are used as cross-country routes, with lower numbers near the hub and higher numbers further away. Zone 1 illustrates this nicely, with its two-digit numbers proceeding logically:

  • A10 London to King's Lynn
  • A11 London to Norwich
  • A12 London to Great Yarmouth
  • A13 London to Southend
  • A14 Rugby to Felixstowe**
  • A15 Yaxley (Peterborough) to Hull
  • A16 Stamford to Grimsby
  • A17 Newark to King's Lynn
  • A18 Doncaster to Grimsby
  • A19 Doncaster to Wide Open (Newcastle)

Three-digit road numbers, and B-roads, get their numbers in batches. Again lower numbers are near the hub and higher numbers are further away. So in zone 4, A400 to A409 are in central and west London; A440 to A449 are in Worcestershire; A480 to A489 run across west Wales.

Four-digit A-road numbers work slightly differently: they give you the idea that they were used for filling the gaps where the system's architects had run out of ideas. Unlike the rest of the scheme, where the lowest numbers lie closest to the hub, the Ax2xx series lie within London (including the A2207, below) and the Ax0xx series - with lower numbers - lie further out. One theory to explain this is that three-digit A-roads were allocated first, and a few holes in the network meant a start was made on the four-digit numbers, giving numbers like A6001 somewhere out in the provinces. When four-digit roads arrived in London, they used the Ax2xx series for consistency as the Ax0xx series had already been started.

Sign showing the A2207 number, near London Bridge station
Sign showing the A2207 number, near London Bridge station

More recently, the vast numbers of unassigned numbers for four-digit A-roads mean that new numbers are often hand-picked to sound or look good. A nice example is the southern extension of the M57 near Liverpool, opened in the early 1990s as an all purpose road, which has the snappy number A5300. It's not part of a sequence of road numbers anywhere nearby, and was probably chosen simply because it looks important.

So that's the long and short of route numbers for all-purpose roads, in mainland Britain at least. Offshore, there are all sorts of systems. Northern Ireland subscribes to the system of A- and B-roads, but has assigned them pretty much at random across the province, and a similar situation exists in Jersey and the Isle of Man. All three of the above have one- and two-digit B-roads in their systems.

* Technically, the A9 is Grangemouth to Scrabster these days, but its first section can be assumed to run concurrently with the M9, so Edinburgh is a fair approximation.

** Originally the A14 ran from Royston to Alconbury, a short link between the A10 and A1. Most of this route is now numbered A1198. The modern A14 was created in the early 1990s as a new cross-country route to the port of Felixstowe. It breaks the numbering rules as it starts in zone 5, but apart from this act of rebellion, it works nicely as part of the zone 1 sequence in this new location.

Picture credits

  • Picture showing A2207 courtesy of "Tony"; a full size picture is in the Photo Gallery.

With thanks to Owen Brown for information on this page.

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Published13 February 2017

Last updated11 July 2021