They explain that:
The allocation of numbers is based on a hub-and-spoke system, with the country divided into nine zones that radiate from a central point.
Because mainland Britain is long and narrow, and because things are always a bit different north of the border, there are actually two hubs, London and Edinburgh.
Radiating from these hubs are the nine principal A-roads. There have been a few changes since the 1920s, and today they can be broadly described like this:
- 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 zones. Each one takes its number from the A-road on its anticlockwise boundary – so, for example, zone 5 lies between the A5 and the A6. 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. As a result the A2 doesn’t form a zone boundary.
All other A- and B-roads get their number from the zone they lie in, so a road in zone 5 will have a number that starts with a 5. Simple.
Of course, there are also rules to decide 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.
You can read the full article here which has loads more info about how the whole thing works.
More on the Zones & Where do Motorways fit in?
As mentioned above, Britain uses a zonal numbering system centred on London (for England & Wales) and Edinburgh (for Scotland).
The zones are defined by the country’s biggest radiating roads. The system applies most clearly to A roads, with B roads following the same rules and motorways (M roads) loosely inspired by them.
A Roads (Primary Numbering System)
The country is divided into 9 numbering zones
Each zone is bounded by major single-digit A-roads:
| Zone | Bounded between | First digit of A-road numbers |
|---|---|---|
| 1 | A1 and A2 | A1xx and A2xx etc. |
| 2 | A2 and A3 | A2xx |
| 3 | A3 and A4 | A3xx |
| 4 | A4 and A5 | A4xx |
| 5 | A5 and A6 | A5xx |
| 6 | A6 and A7 | A6xx |
| 7 | A7 and A8 | A7xx |
| 8 | A8 and A9 | A8xx |
| 9 | A9 and A1 (in Scotland) | A9xx |
So:
- A road numbers begin with the zone number (except for 1-digit ones).
- Example: A34 lies in Zone 3 (between the A3 and A4).
- Example: A47 lies in Zone 4.
Single-digit A roads define the zones
These are the most important early roads: A1 through A9 (see above)
Two-digit and three-digit A roads
- 2-digit A roads: major regional routes (e.g., A40, A82).
- 3-digit A roads: medium importance, still follow zonal numbering.
- 4-digit A roads: local / lower importance, still follow their zone first digit.
B Roads
B roads follow the same zonal rules as A roads:
- B-roads use three or four digits.
- The first digit indicates the zone.
- Example: B4031 is in Zone 4.
- B-roads are typically local or connecting routes that didn’t qualify as A roads.
Motorways (M Roads)
Motorway numbering is loosely based on the A-road zones but with more exceptions.
Key rules:
- M1 roughly parallels the A1 (Zone 1).
- M2, M3, M4, M5, M6 correspond roughly to A2–A6 zones.
- M roads do not strictly follow zone boundaries, especially newer ones.
- 3-digit motorways (e.g., M602, M621) usually:
- Are short spurs or bypasses,
- Take their first digit from the parent motorway zone (M6 → M60x, etc.).
Examples:
- M25 is the London Orbital, ignoring traditional zones entirely.
- M62 crosses multiple zones; its number was chosen to match the A62 corridor, not zone rules.
Why some numbers appear “wrong”
Historical renumbering, bypasses, and reorganisations have caused “violations,” such as:
- A roads that cross multiple zones but keep a single number for continuity.
- Motorways taking convenient or symbolic numbers rather than strictly zonal ones.
Example: A38 runs from Cornwall to Nottingham, crossing several zones, but retains its number because it predates strict enforcement.
Can you think of any other “wrong” examples?
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