Usually, I would say “highway=track” as it is the closest I find : narrowed, unpaved, not named. + “tracktype=gradeX” and “surface=X” for the car accessibility (forget ‘smoothness’).
Dangerousness is difficult to tag as it is very subjective (e.g. may also depend if you are local or foreigner etc).
But all of this may depend on local rules definitions for highway tag usage. You might have a look on this page:
http://wiki.openstreetmap.org/wiki/Highway#International_equivalence
or here:
http://wiki.openstreetmap.org/wiki/Tagging
to find out how it is done in other similar countries.