Indeed. excellent would be something like this File:Moscow State University, Corridors, Moscow, Russia.jpg - Wikimedia Commons
Are these thin gaps an obstacle to stuff like roller blades or skate boards?
If yes, then good
I was comparing the roughness of the wooden surfaces with those of a newly paved surface=paving_stones (smoothness=good almost by default) or a newly paved surface=sett (smoothness=intermediate almost by default).
Letâs use the âbeaver campâ picture for smoothness=good (though itâs a bit towards the bad end of that category in my opinion) (the âZiaratâ picture has sunny and shady parts, making it less suitable), âPlitviceâ for smoothness=intermediate and âTrebevicâ for smoothness=bad
can you upload it to Wikimedia Commons?
image is at File:Loose sand surface.jpg - Wikimedia Commons
but is it really horrible? can âSUVs and other cars with higher clearanceâ really not pass there relatively easily?
I dare say that smoothness doesnât really apply to sand. What would smooth vs. non-smooth sand look like to you?
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whether it is loose and impassable on say bicycle or more compacted/mixed with dirt and passable
I agree: smoothness is about the physical shape of the surface, not how firm it is. With a loose surface, such as sand, mud and loose gravel, car tires will smooth them out while a narrow bicycle tyre will sink into it.
I think thatâs what we have track_type for (firmness of the surface).
in case of sand firmness and shape are deeply correlated (compare loose sand with sandy soil)
nearly all (or all?) surface=sand sits in tracktype=grade5, smoothness= may allow finer distinction
Iâll try to remember to upload later.)
From cycling perspective some sand is firm enough to ride on pretty much any kind of bike, while some is too loose for normal mountain bikes (like in image below).
Some types of âgroundâ holds it shape more than others. Like in the previously commented photo, those horse hoofprints are solid enough to make it bumpy ride on a bicycle, but probably not very noticeable to any kind of car. On muddy forestry roads, heavy tracked vehicles imprint the road in a way that would be uncomfortable for any normal car, but isnât necessarily any worse for a bicycle than the hoofprints.
But yeah, maybe more thought is needed for how smoothness applies to the types of surface that donât really hold a shape when driven over (at the scale smoothness covers, e.g. a sand dune is not a surface feature and a single hole or a trench is an obstacle, not a definining feature of a wayâs surface).
Firmness of sand also correlates with wetness, but I wouldnât factor that into smoothness. Same for âdunesâ, or just heaps of sand: they are too much dependent on weather, and time of day.
Also, sandy soil, for me, isnât sand. Itâs more ground than sand, sometimes, in case of loam, even more so.
A (perhaps silly) question for the experts: I chose very_bad for the gravel road in picture below. Not really able to argue for that though. What do you think?
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It looks âbadâ to me, not very_bad. A normal passenger car could drive on that
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If I told you, the rocks are mostly railway ballast size, one to two inches, with some bigger ones and a some grit in between, but not of the kind that would glue the rocks, would you say the same?
I guess now, I used very bad, because bad includes bicycle. At least surface=gravel was easy to tell 
Looking at the documentation and estimating scale from just the photo, I would echo very_bad. I would not change my estimate for bicycle use. A road bike would be impossible, a gravel bike would struggle, a XC bike could manage, wider tires being necessary and a fat bike would have no problem.
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This doesnât need âhigh_clearanceâ, though it may be particularly difficult for bicycles because of the loose gravel.
Descriptions of use with motorbikes was recently added to the smoothness wiki. It would be great if some bicycle experts would get together and add descriptions for bikes too (though in such a way that the evaluation of smoothness wonât depend on the vehicle youâre using). Bicycles generally have narrow tires which have difficulty with loose surfaces that they can sink into (gravel, sand, mud) that other vehicles with broad tires donât have.
Maybe if tracktype=grade5 (mud, loose sand, loose gravel), then itâs useless to specify smoothness=* because it wil strongly depend on how the wheels of your vehicle affect the surface. A car with broad tires will smooth it out, a bicycle with narrow tires will sink into it. Maybe we should recommend in the wiki that in such cases, donât tag smoothness.
Indeed, I think it is generally understood that tracktype=grade5 will be more difficult to pass when the weather has been wet, but that only applies to clay surfaces (that become surface=mud when very wet, and can be rock-hard when dry. However, sand becomes loose when dry, and more firm when wet⌠and loose gravel is not affected by humidity.
The number of upvotes for the post that graded the scene as bad received makes me a little wonder, how much cyclicsts can learn from smoothness. After all, the piste in my picture will be âpush the cycleâ for I guess 95+ % of cyclists, including touring cycles with robust tires. While I fully understand, that with an ordinary car this can be driven, at least if you do not care about the underside of your car - as it is steep there too.
PS: Actually, the tires there do no sink into the surface, like within fine-gravel or sand or mud, but the wheels constantly get pushed in random directions by the rocks, erm gravel. The wider the tires, the less so, certainly.
EDIT: I put the photo above here File:Schotterpiste.jpg - Wikimedia Commons
PS: Regarding firmness, I am certain this can carry a 20 ton lorry without a problem â so it is a grade2 track-type? I hold my breathâŚ