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.