Monday, 16 May 2011

A-road numbers

I had never actually thought about how the system of numbering roads in Britain works. At least, not until my my brother-in-law, Ron – an amazing and reliable source of interesting information – explained it to me when he and my sister, Brenda, were staying with us recently. Well, at least he explained how the A-numbers work in England and Wales.

We were talking about the A47, which runs south of Norwich on its way to Yarmouth. Ron asked me where it started. I had no idea. But Ron commented that it must start a long way away, because the first digit is a '4'. He went on to explain.

The A-roads are all based on a radial framework of 6 trunk roads all starting in London, and numbered clockwise: the A1 (to Edinburgh), A2 (to Dover), A3 (to Portsmouth), A4 (to Bristol), A5 (to Holyhead) and A6 (to Carlisle). Then, apparently, all the two-digit A-roads beginning with '1' start somewhere between the A1 and the A2; all those beginning with a '2' start somewhere between the A2 and the A3. And so on. I honestly did not know that! Did you?

And, what's more, there's a similar principle for the 3-digit A-roads in relation to the 2-digit roads.

So, the A47 must start somewhere between the A4 and the A5! Checking on a road map, sure enough, there it is starting at Nuneaton (as far as I can tell), which is just beyond the A5! Getting from there to Yarmouth involves crossing the A5, the A6 and the A1.

I tried a few others out of interest. The A11 is the main road from London to Norwich. And, sure enough, it starts in London between the A1 and the A2.

The A38 ends in Mansfield, I think, just north of Nottingham. What's it doing up there? Starting with the digit '3' means that it must have started somewhere down in the south-west between the A3 and the A4; check the map ... and there it goes, down through Birmingham, Worcester, Gloucester, Bristol, Exeter, Plymouth, to Bodmin! What an impressive two-digit road that is, justifying its label (38) by starting between the A3 and the A4, somewhere down in Cornwall, and crossing the A4, the A5 and the A6 on its way to Mansfield.

With roads that cut across the major A-roads like this you do have to decide which is the start and which is the finish. The A14 seemed to be an exception when I first looked at it, since it 'starts' between the A5 and A6, near Rugby, linking the M6 with the port of Felixstowe on the East coast. But if you take Felixstowe as the start of the A14 and the M6 as the end, then it satisfies the rule, lying between the A1 and the A2.

There are, of course, exceptions, because roads get redesignated and renumbered, and the motorway system has been imposed on the top of the existing network of A-roads. And it gets more complicated up near the Scottish border, where the A7, A8 and A9 come into consideration. But, in general, it's a nice illustration of an effective labelling system.

The numbers are being used in a nominal sense (as labels for the road), but they are more significant than, say, the numbers on buses. To some extent they are also being used in an ordinal sense – putting the roads in some kind of order. In this ordering system, A2 comes between A1 and A3, as we would expect. But then, A10, A11, A12, and so on up to A19, come between A1 and A2. Counting 1, 10, 11, 12, 13, 14, 15 16, 17, 18, 19, 2, 20, 21, ... is unconventional, but it is nevertheless an effective ordering system. It works as long as the numbers are simply labels for putting things in order, and do not in any sense represent quantity.

Another example of an unconventional ordinal system like this one is the current system for car registration numbers. I'll save that for my next blog.

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