From OpenStreetMap Wiki
Proposal for tagging of highways
The official classification of regional roads is available as Decreto Legislativo Regional n.º 1/2013/M.
I have used the information from the Decreto and started to systematically tag roads as follows:
|Kind of road||Number||Key||Value||Element||Rendering|
|Via rápida||VR 1, VR 2||highway||trunk|
|Via expresso||VE x||highway||primary|
|Estrada regional principal||ER 1xx||highway||secondary|
|Estrada regional complementar||ER 2xx||highway||tertiary|
|Smaller connecting roads, including old regional roads that have lost their ER classification||highway||unclassified|
- All segments of a regional road are tagged with its official number using ref=*, e.g. ref=ER 215.
- All segments of one regional road become members of a Relation:route. The end points or other information about the route can be helpful and should be tagged as a note=* or description=*. (Example: Road is member of a relation with the properties type=route, route=road, ref=ER 215, description=Meia Serra.)
- The vias rápidas and vias expresso are "on top" of estradas regionais. Therefore, for these parts of a estrada regional the ref=VR x (or ref=VE x) is set, and the ER number is not tagged on the way. However, the way should become a member of two relations, one relation for the VR (or VE) route and another relation for the ER route.
- Old regional roads may be tagged with old_ref=* to store their old number. This might prevent other mappers from adding ref=* tags with outdated road numbers to these roads.