I have only been to Madrid a couple of times, relatively short visits, but was immediately taken with the place.
I took a copy of Rick Steve’s ‘Spain’ and it led me to this discovery. It is one thing I would not have noticed just wandering aimlessly.
The original radial road network in Spain was established by King Carlos III June 10th 1761, the idea being to construct solid and straight roads to facilitate commerce between regions and provinces, these roads radiating from Madrid.
The original radial roads were the N-I (Madrid to Irún), N-II (Madrid to La Junqyera), N-III (Madrid to Valencia), N-IV (Madrid to Cádiz), N-V (Madrid to the Portuguese border near Badajoz) and the N-VI (Madrid to La Coruña).
Point zero, from where all distances are measured, is in Puerta del Sol, Madrid, and a plaque marks the spot. It is almost the centre of the Iberian peninsular.
The six main radial highways, also radiating from Madrid, are the A-! to A-6.
The old Roman numerals are now only used for administrative purposes