What is below is supposed to document existing consensus rather than introduce new rule. I am aware that there is significant majority convinced that some road types universally guarantee or imply certain values (like widespread usage of highway=track to mark all unpaved roads and only unpaved roads, popular in some regions).
I am also aware that in some regions "dual carriageways are usually at least highway=tertiary", "paved roads are at least highway=unclassified", "highway=primary has at least 2 lanes in each direction" or "paved roads passable for the entire year are highway=trunk" are good approximations - but it does not make it universally true or necessary requirement to use these tags. proposed description to be added on http://wiki.openstreetmap.org/wiki/Key:highway - maybe also on pages about individual road types. Only highway=motorway/motorway_link implies anything about quality. Other road types, from highway=trunk through highway=tertiary to highway=residential=residential/service or highway=path/footway/cycleway/track do not imply anything about road quality. In area with poor infrastructure road forming main road network, of the highest importance in region should be tagged highway=trunk - no matter whatever is is high-quality asphalt road or unsurfaced tract unusable after major rains. In absence of surface, tracktype and other tags describing quality of road one may try to extrapolate this information from value of highway tag. Note that this needs tuning for every region. Typical highway=primary may be drastically different in various places across the planet. It is highly recommended to add and use tags describing road quality like surface. In one region it may be obvious that all highway=tertiary are paved and highway=track unpaved but no assumption like that will work worldwide. _______________________________________________ Tagging mailing list Tagging@openstreetmap.org https://lists.openstreetmap.org/listinfo/tagging
