It’s a tricky one for sure. I am almost done with “Runnymede” in Surrey in the UK. Yesterday on the last few to do was a single dual carriageway 40 mph road Windsor Road - CityStrides with no sidewalk immediately adjacent to the road but as it goes through an area very popular for walkers (where the Magna Carta was signed) there are lots of paths set back a bit from the road and one which is at least sort of “parallel”. I did however think, especially in Hard Mode that I use, that it may well leave me with isolated nodes to complete when say the path is further set back. In the attached Strava screenshot I did the “road” on way out and kept to the path on way. Clearly the path was more pleasant and actually I got shouted twice that I should be on the path - once by a cyclist ironically on the road going the other way and once by a passing motorist who think was annoyed that my presence meant had to slow down momentarily even though only 40 mph anyway.
Here is StreetView at a point I would say that the path is possibly too far setup back to “count” Google Maps
Ironically I can see a cyclist in the image so it is “OK” for them and yet this runner (even in a bright pink top) was not at all “welcome” - partly for my perceived “safety” but really I think a not untypical irate motorist losing a few seconds that am sure they soon caught up by the next traffic lights further up.
Now this is neither a “trunk” road, nor a dual carriageway nor has an overly high max speed and yet it “probably” should not be one we should be doing. Conversely I have run on single carriageway supposedly 60 mph rural A “primary” roads with no sidewalks but have felt perfectly safe - largely as clear of trees, bends and a fairly wide road.
So my main point is that unless you really “know” a road, it can be hard to be sure if it is one you feel OK to run/walk down.
I feel that a “good” solution could be the foot=discouraged setting. The big problem with that is that the notes on OSM say it is itself discouraged - think largely as the real world examples of such a sign “on the ground” are minimal and then, I suppose, it becomes a subjective setting.
Another thought is like a segment on Strava that you can suggest is unsafe/unsuitable and it gets downgraded accordingly. So maybe that is another possible suggestion albeit then needing some extra data layer over OSM and again subjective to opinion. I guess like the Sydney example running past/through “wastelands” may not be pleasant but if you are Strider you basically have to accept that no every run would be one you would choose to do for its nice surroundings…
