A way to tag “there is no speed limit sign on this section of a way, but I’m not sure in which speed limit zone it belongs” is useful. The information value is small, but I see no reason not to have such a tag. The information could be combined with information collected by other mappers such as way sections where the speed limit zone is known, location of speed limit and city limit signs, etc. to logically conclude by an armchair mapper (or even a router) what the speed limit of that “no speed limit sign” way must be. If a certain way without speed limit signs can only be reached through ways with a know speed limit zone, then that way must also be in that speed limit zone. A maxspeed overlay in StreetComplete would nicely visualise it.
Honestly, I feel like a MapRoulette challenge or similar would be a cleaner solution. The information “The mapper didn’t find anything here”, I’d argue, isn’t that useful apart from mapping itself. There are few instances where we tag the non-existence of things - hence MapRoulette.
The point of tagging speed limits on ways is exactly to avoid that and have the information locally. And if a router does something like this, the difference between “There is no sign here” and “We don’t know if there is a sign here” is quite small.
In summary, while this info could be useful in some instances, I don’t think it belongs in OSM. Having that tag shows the coverage OSM really has and may provide increased confidence in its correctness - though I’d rather see it as an add-on rather than in OSM itself.
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I have been mapping maxspeed in the Netherlands using a database with all traffic signs, 99.4% of all roads have maxspeed mapped, see also NL:Maximumsnelheid in Nederland.
Even with all traffic signs available not everything is 100% clear, I am pretty sure I have often a better overview than local civil servants.
For roads where I could not deduct the maxspeed I just did not map it. Sometimes the intention was crystal clear on the implementation not 100% correct, in that case I used source:maxspeed=interpreted intention.
Instead of a new tag speed_limit_implied I would go for source:maxspeed=implied.
oh, definitely
while surveying max speed info I already found and reported to a local government several cases of bogus signs - that in meantime got fixed