Reason for incomplete tags for maxspeed quest?

[NathanARF]

I've noticed when marking a single carriageway road segment in the UK as the National Speed Limit, that only the tag maxspeed:type=GB:nsl_single is applied and not maxspeed=60 mph as well. Why is this?

[mnalis]

See #3812 (comment).

Short answer: because StreetComplete only tags what the user sees. It requires no knowledge of local legislation from users (after all, non-drivers, children, and people just visiting UK might be using StreetComplete in the UK).

There are tools like https://westnordost.github.io/osm-legal-default-speeds/ to infer the actual speed limit from what is tagged (and it is much more complex then just maxspeed=60mph, see example)

Also, think what happens when law changes (ref: #4282 (comment))

[NathanARF]

But the maxspeed:type=* tag implies a speed limit, either 30, 60 or 70 mph, and this accompanying tag is specified in the wiki page: "maxspeed:type=* describes how a speed limit has been set into force. It should be accompanied by a maxspeed=* tag." Is the wiki incorrect with respect to this? Additionally, if the national speed limit were to change, then would it not be easy to change tags across the country, as has been done nationally with the change of name of the road operator 'Highways England' to 'National Highways'?

[mnalis]

"maxspeed:type=* describes how a speed limit has been set into force. It should be accompanied by a maxspeed=* tag."

Well, I just link you to previous explanations, have you read them too? I do not really hold hard opinion on it (I don't drive car, so I find a maxspeed tagging useless either way for my cycling and walking 😄).
So it would seem to me that wiki "should be accompanied" is at least disputed (I've added a note to its talk page now, linking here).
As a person coming from database world, I however do see how having such duplicated (and potentially conflicting) sources of implicit/explicit values (and thus breaking 2NF) sounds like a bad idea.

Additionally, if the national speed limit were to change

As far as I understand, problem with that part is (if you read those discussions) that since SC doesn't ask mapper to manually input speed value (as it doesn't expect them to know it, and only asks them to tag what they see), only alternative would be if SC had a world-wide curated database of implied speed limits (like osm-legal-default-speeds does). The problem with that is, even when law changes and someone does automated edit to update existing values, any existing non-updated version of StreetComplete would continue to put wrong values on the map for a long time (until they are upgraded - if ever), which I can imagine would be quite annoying and problematic (and thus make StreetComplete unpopular).

then would it not be easy to change tags across the country

Well, if it is easy and safe to do with automated edits, then couldn't they just be setup to automatically add not only maxspeed=60mph, but also:

maxspeed=60 mph
maxspeed:bus=50 mph
maxspeed:conditional=50 mph @ (trailer); 20 mph @ (trailers>=2)
maxspeed:hgv=50 mph
maxspeed:motorhome:conditional=50 mph @ (emptyweight>3.05)

on all roads tagged with maxspeed:type=GB:nsl_single. Because, bot periodically running such automated edit would have same result, and less potential problems, then if StreetComplete were to do it (as noted above).

Because, there is another issue if you add only maxspeed=60mph, you're making map worse -- it (without other maxspeed subtags) would mean that all vehicles (including buses, HGV, trailers etc.) can also go 60 mph, which is incorrect. So, one should either add all implied maxspeed tags (if one thinks that having them explicitly tagged is beneficial for ease of use) or none of them (if one thinks that implied defaults, and data-consumer-side processing of them is preferable).

But then there is also a question is explicitly adding all those implied tags actually desirable? It would certainly bloat the map, making processing slower and maps taking more space, introducing updating problem (as noted above) and QA issues (what to do when implicit and explicit data is out of sync), while not really bringing any new information on the table that isn't already implicitly known. Such defaults better belong in data consumers instead of being explicitly tagged, IMHO. Much like adding currency:gbp=yes to every shop in GB would be useless, IMHO.