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[1] : there should be advice how to do it better.

It is interesting that verifiability lists sac_scale as an example - this is the ultimate proof that such keys are needed whether or not some people think they are verifiable. RicoZ (talk) 21:19, 9 April 2016 (UTC)

The problem with a key like maxspeed:practical is that the value is nearly meaningless. If you are a 75 year old pensioner who is driving an old RV (motorhome) down a curvy gravel road, you are likely to determine a much lower practical maxspeed than a young man riding an off-road sport motorcycle on that same road. Should the maxspeed:practical be set for motorcycles, for caravans, for mid-sized passenger sedans, for SUVs? How uncomfortable is still practical? If you are driving alone you can go much faster than when you have young, car-sick children in the back, on a curving road.
The reasonable way to determine the expected speed is to use statistics, based on many GPS records. This is what commercial routing software does to get realistic trip time estimates, and such data can also be adjusted for each user. We aren't going to successfully tag every highway with a speed estimate. That's why this tag was rejected. --Jeisenbe (talk) 13:58, 30 June 2019 (UTC)
Of course it is encouraged to set maxspeed:practical for motorcycles and other vehicles separately, see Key:maxspeed:practical#Use_with_conditions. In principle there is nothing to prevent you from using a condition like (driver_age>=75 AND pensioner=yes) - it would fit under the "user group" conditions. Not sure if "driver_age" has been used/defined previously but I would not be surprised if it will be needed anyway for other purposes. An alternative would be to use "daredevil", "normal", "cautious" and "anxious" as abstractions instead. Further, since daredevils probably don't care about maxspeed:practical and the values may be above legal limits and what is considered reasonable by OSM I think daredevil data should not be entered into OSM. So "normal" would come as default, leaving perhaps cautious, anxious, disabled and female as extra conditions.
The idea that statistics based on GPS records would be better is remote for the reasons you have explained yourself - unless the statistics are stratified according to age, pensioner,female,disabled,vehicle type, time of day, season, weather conditions and other variables. Also, this key is expected be used predominantly in less frequented areas where it would be even more difficult to gather all necessary data and as far as I can see all projects gathering those stats seem dead. RicoZ (talk) 20:17, 2 July 2019 (UTC)

"no legal speed limits"

Is there any case where there are no speed limits at all outside motorways? Many roads have no signed max speed but in such situation default max speed limits apply. And is it actually useful to tag maxspeed:practical=* on motorways, except cases where traffic jam are standard and speed is lower than expected from a motorway? Mateusz Konieczny (talk) 07:13, 3 July 2019 (UTC)

I think desert and wilderness tracks and similar don't have any implicit/default speed limits in many countries. Maybe legally the speed limits of regular highways would apply to them but don't think that would be terribly useful to consider. RicoZ (talk) 17:36, 11 July 2019 (UTC)