It uses three digit road numbers preceded by the letter S (for stad, the dutch word for city).
The system was first developed in Amsterdam in 1977, as part of an urban planning effort in which traffic signs used numbers, similar to postal codes, for neighbourhoods.
[3] An inner ring road around the city center was defined as a hub and given the number S 100.
The city routes are the spokes that connect this hub with the outer ring road, in the case of Amsterdam the A10 motorway.
Each city route is in principle numbered according to the exit it connects with at the outer ring road.