While debating surface=clay and tracktype, I realized that transnational agreement on how to detail unpaved roads and how to handle them in applications (rendering, routing penalties) has been difficult probably due to differences in the composition of local soil.
In regions where the soil is clay-rich, less-developed unpaved surfaces vary dramatically with weather, from firm when dry to muddy and slippery when wet. Soil firmness then becomes the dominant variable determining usability, much less than maintenance (which I believe means keeping the road clear of ingrown vegetation and the surface smooth).
In regions where the soil is clay-poor, these surfaces tend to remain more stable across seasons. In this case, development/construction and maintenance becomes the more evident characteristic to differentiate between them.
On well-developed roads with added material (gravel, crushed stone), firmness and maintenance level tend to correlate. But on natural soil surfaces, they decouple due to differences in the local geology.
This suggests that:
tracktype, while consistent locally, is fundamentally problematic globally- firmness and maintenance level should be decoupled using different tags (and to avoid conflicts, probably using new tags for both)
- routing penalties and rendering styles for unpaved roads (particularly those tagged with natural surface values) should ideally be soil-type- and climate-aware (these can vary within the same country, especially in larger countries)
What are your ideas on this? Has anyone observed similar environmental patterns affecting these or other tagging practices?