In the Plaza del Sol, in Madrid, capital of Spain more frequently than not since 1561, there is a plaque set in the pavement marked as Kilometre Zero. If Google Maps tells you that it's 395 km to Madrid, that's the point it's measuring to. As a local aside there's a similar point in Murcia in the street called Platería.
There are a series of arterial roads that radiate from Madrid. The arterial roads have numbers prefaced by the letter N for Nacional/National. The six radial roads were given Roman numeral names, NI, NII, etc. As examples, the NIII goes to Valencia from Madrid. The NIV goes from Madrid to Seville and on to Cádiz. The effect is that Spain is divided up, cake-like, into slices or segments. Any main road, any national road, in the slice between the NIII and NIV, in a clockwise direction, will have a number that starts with N3. The NV goes to Badajoz from Madrid. Any road clockwise of the NIV but before the NV will have a name in the style of N420. Lots of us know the Guardia Civil Facebook page N332, for instance. It's called that because that's the road the site's authors patrol. I think the photo at the top left makes the idea very clear.
Going back to our informative Guardia chums and the N332. We know now why it's N3, but why the second 3? Go back to that point in the Plaza del Sol in Madrid and draw concentric rings from it with a distance between the rings of 100 km. If a road in that segment between the N3 and the N4 starts between 0 and 99 km from Kilometre Zero, then the road number will start with N30. If the road begins more between 100 and 199 km from Madrid, and it's in the segment between the N3 and the N4, then the road number will start with N31. So the N332 starts between 300 km and 400 km from Madrid. The last number comes from whether the road, and remember this only applies to the National roads, is radial (if it were prolonged, it would go towards Madrid) or transversal (it would never get to Madrid no matter how much it were lengthened).
Now we have the basic idea.
It all changed, of course, with the advent of motorways. The National Roads are no longer the reference points they once were. The National Motorways have numbers that begin with A or AP (around Madrid, some begin with R), but the road numbers follow the original model and the same sort of numbering system is applied. The A3 heads out of Madrid to Valencia. The A4 goes to Seville and on to Cádiz.
Oh, and the A7, the one that runs along the coast - somebody just thought to call it that because it didn't fit the scheme. They did the same up on the North Coast with the A8.
Next time we'll have a look at how to spot the types of roads and non-National Roads. Bate that breath!
