OpenStreetMap
After I found answers to my questions, I started mapping the international cycle route "Euro Route R1" in Poland.
The signposts there are maintained by PTTK, you cycle on the normal road.
I did not take any notes which means I used highway=road when I was not sure. I used a tag not saying anything.
While writing (and thinking) about it, I think it makes more sense to tag highway=tertiary even if it may be residential or secondary sometimes.
It happened a few times that there was no pavement but sand. Still it was the only connection between some smaller villages. What would be the appropriate tag if I remembered the sections?
Discussion
Comment from eAi on 8 November 2008 at 20:56
highway=track, surface=sand. If you're feeling obsessive, you could use smoothness=[something] too. In my experience, cycling on sand is asking for trouble.
Comment from petdr on 9 November 2008 at 23:21
I've used surface=unpaved for similar sorts of tracks.
Note you can have highway=primary and surface=unpaved if required. There are some major roads in central Australia which are dirt roads, but still reasonably busy with tourist traffic. So the highway and surface tags are orthogonal.
Comment from emka on 10 November 2008 at 19:45
Thank you!