So write it :-).
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This seems to be a good idea to me, although it could use some simpification. The top used values heavy_truck_less_than_20mt and 4wd_less_than_3.5mt have me imagining a mess of future values that no data consumer would touch with a ten foot pole. downhill_mtb_tires_wider_than_6cm, mule_with_steel_shoes … 
The wiki page presents the beginnings of a simpler schema though: Key:practicability - OpenStreetMap Wiki
I’d have made the prefix simply practical: for brevity. practical:bicycle=no, practical:foot=yes reads just fine to me.
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This seems like a reasonable compromise, but perhaps make it
practical:road_bike=yes, practical:mtb=yes, etc. The issue remains that what is “practical” to one person with one set of skills may not be “practical” for another with a different set of skills, but nothing is perfect.
,
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There also used to be bicycle:practical.
Na whoao, I have missed this one … 
… I agree, this looks simpler for sure but the users seem to prefer the tag practicability=*.
17K uses of practicability=* against some 400 uses of practicability:*=* shows a clear favourite.
If you look for brevity, there is already prac=* using the same values like practicability=*.
How does practicability=* even make sense? You’d need semicolon-separated values to specify different vehicles, like practicability=motorcycle;horse;mtb
A prefixed schema like `practical:*= would be much easier to maintain.
But honestly, lots of mappers take shortcuts, which is why they misuse the simpler legal access tags. Shortening the prefix won’t help much—a proper input form in iD would make a real difference.
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How does
practicability=*even make sense? You’d need semicolon-separated values to specify different vehicles, likepracticability=motorcycle;horse;mtb
serms you didn’t look at the values that are in use:
https://taginfo.openstreetmap.org/keys/practicability#values
I usually balk at namespaces and prefixes as they seem daunting to me. I wonder if I am the only one.
I did and none of them make sense to me. What does practicability=yes/no/limited even mean? Does practicability=motorcycle means its only practical on motorcycle?
I did and none of them make sense to me. What does practicability=yes/no/limited even mean?
I don’t know about these values, that are clearly in the long tail every tags has, that’s why I mentioned actual usage…
Only the mapper having tagged these will know. These are individually created values not really in line with the description on the wiki page.
I’d say using the key practicability=* by including what a track may be usable for is more simple than excluding what a track may not be usable for by using the key practicability:*=*.
Let’s say there is a single track (path) which according to the mappers estimation could be usable by a skilled mtb or dirt bike rider. A track usable for a dirt bike can be assumed to be useable by mtb or foot as well, so one could simply tag it as
practicability=dirt_bike
If you don’t trust everybody will understand that “dirt_bike” includes “mtb” and “foot” you would go for
practicability=dirt_bike;mtb;foot
How would you get the same result by excluding values?
practicability:motorcycle=no would also exclude dirt bikes which are also motorcycles of a special kind. So you would need another key to make clear that “normal” motorcycles are excluded but dirt bikes could be used.
Besides that replacing the key practicabilit=* with 17K uses now by a new key practical=* or practical*=* for the sake of brevity would not make much sense imo.
Anyhow I still believe the best way to specify any track or path is by using the existing tags for surface, smoothness, width and sac/mtb scale while practicability could just be a compromise for those who do not want to get into the details.
[OT]
All such schemes make values far less discoverable, a simple list with a fixed key (instead of essentially random ones is in general always the best solution.
[/OT]
There is no “simple” scheme, smoothness fits somewhat your definition for similar purpose yet very few mappers use it or do it consistently.
Schemes that move values in to key space, see SimonPoole's Diary | More moving values in to keys madness | OpenStreetMap
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I suspect that the most challenging part of this task will be actually surveying the ways involved. Some of the results of that can go in well-known OSM keys (perhaps width). Some may be more difficult, but let’s not drown in a Lilliputian tagging argument. If there’s nothing obvious, just make up new keys and values. They can always be changed later.
No. A dirt bike is going to have a lot more power than a mountain bike, even an e-mountain bike, and that power is going to be accessible by a simple twist of the throttle control. On a mountain bike you are going to have to pedal to get power, and pedaling constrains body position, which is critical for maneuvering and controlling the bike in rough terrain, and may not be possible at all as the terrain may prevent a full pedal stroke (e.g. because there is a big rock under your bottom bracket shell). Often on a mountain bike timing the pedal stroke is critical to getting through an obstacle, and of course one doesn’t need to do that on a dirt bike. Also, a typical mountain bike is going to weight between 25 and 35 pounds, while a dirt bike may weigh several hundred pounds. Dirt bikes tend to have wider wheels and tires and the tires have deeper lugs.
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Thanks for your detailed explanations. I am riding mtb since more than 30 years and also had a good time on dirt bikes, most of the time on tracks, trails and pathless ground in Africa. The issue here is not to distinguish dirt bike trails made for the purpose from mtb trails, but to tag normal pathways through the bush and in such cases one can normally say if the path is usable by foot and by dirt bike, it can also be used by mtb, even if one might have to carry the bike on the shoulder for a very bad section.
Again it is simple enough to go for practicability=dirt_bike;mtb;foot or the like to avoid any misinterpretation.
smoothness is already about practicability: it is defined as “whether the way is usable by the vehicles mentioned”. smoothness=excellent means practicability=roller_blade;skateboard, smoothness=very_bad means practicability=car_with-high_clearance, etc.
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