100 years ago Britain's main roads weren't numbered because they didn't need to be. Cars were new-fangled inventions that had yet to take over the world, so most important roads still had names from the era of the horse and cart. Most were named after the town at the other end (Cambridge Road) or the direction in which they travelled (Great West Road) or perhaps the Roman road that they followed (Watling Street). But then motor transport started to get popular and in 1921 the newly-formed Department of Transport decided that all of Britain's major roads should be numbered.
Civil servants selected six particularly important roads leading out of London (most starting close to the Bank of England) and numbered them A1 to A6 (clockwise from the north). Then they took three important roads leading out of Edinburgh (starting at Princes Street) and numbered them clockwise A7 to A9. These nine key trunk routes were as follows:
A1 London to Edinburgh (409 miles, originally the Great North Road) A2 London to Dover (77 miles, originally Watling Street) A3 London to Portsmouth (74 miles) A4 London to Bath (103 miles, originally the Great West Road) A5 London to Holyhead (270 miles, originally Watling Street) A6 London to Carlisle (282 miles, actually started in Barnet) A7 Edinburgh to Carlisle (101 miles) A8 Edinburgh to Gourock (67 miles, via Glasgow) A9 Edinburgh to Wick (273 miles, via Inverness)
You might have thought that the remainder of Britain's roads were numbered fairly haphazardly, but there is in fact a clinical logic to the fact that the A366 is in Somerset, the A666 is in Lancashire and the A966 is in the Orkney Islands. And it works like this:
The Numbering of Roads (Michelin Guide, 1921) For the purpose of numbering the roads, Great Britain has been divided into nine sectors, six of which radiate in clockwise order from London, and the remaining three similarly from Edinburgh. Sector I includes all the roads situated between roads A1 and A2, and so on clockwise for the remaining sectors. Note: an exception occurs between road A2 and the estuary of the Thames which is part of sector II and not sector I. All roads take their initial number from the sector in which they start, eg A12 and A17 start in Sector I, A36 and A310 start in sector III. A road does not necessarily terminate in the same sector in which it begins. The commencement of a road is determined by the end of it which would be reached first by the hands of a clock radiating from London.
It may help to imagine the country divided up into nine 'cones', six emerging from London and three from Edinburgh (there's a helpful diagram here). For example, all the roads in Devon start with a 3 because they lie between the A3 and the A4. And the A57 from Liverpool to Lincoln starts with a 5 because Liverpool is the first of the two towns reached clockwise from London, and because Liverpool lies between the A5 and the A6. Erm, simple. There are exceptions, of course, but that's roughly how the system still works.
In honour of the fact that the country's first five arterial roads all start in central London, I've decided to take a walk along the first mile of each of them. Join me every day this week to discover where and how these five Great British Roads begin. I'll meet you tomorrow morning on the A1 just north of St Paul's Cathedral...