OpenStreetMap has a few assumptions about road surfaces, based on the fact most western cities have paved roads.
For roads for motor vehicles there there is normally an assumption that the surface is surface=paved unless otherwise stated. Paved in OpenStreetMap is non-specific and may cover sealed, tarmac, asphalt, bitumen. surface=unpaved is treated as the opposite of paved. More specific tags can used used for surfaces which are normally classified into paved or unpaved for routing purposes. Navigation software should assume that roads-that-are-not-paved will have slower driving speed (and therefore longer driving time) and may be impassable in some weather conditions.
In South America (Rural), Rural Australia, Africa, Haiti and more this is not the default. During activities like a HOT activation, quite a lot of road data gets added without specific surface tags.
In places like South America, Strava route building a big thing - but knowing if you can go on a road bike or a mountain bike is not something routing engines can answer with much certainty.
Alternatively, knowing if you are likely to need a 4WD vehicle in disaster relief is another use case which can't be easily answered by a routing engine.
In an idea world, it'd be great if:
- ID had some notion of 'rural' vs 'metropolitan' mapping profiles; and allowed you to create a number of roads with the same surface properties as the last few you've traced.
- QA tools (maproulette? similar?) existed that looked at specific countries, finding highway=unclassified and highway=residential without explicit surface tags; plus a relationship to any bounding residential landuse to suggest a value (Big city? Its more likely to be paved residential roads)
- Something like DeepOSM that could guess the difference between "light, sandy coloured road" and "Dark, asphalt road" to suggest an appropriate classification.
Even if these tools weren't used to write out lots of surface data back into OSM, it would be interesting to see if they could be used to generate a likely profile of an area for routing engines - ie; better answering if a certain section of the planet is likely to be unpaved roads.