If I remember correctly, the idea of using source:maxspeed for urban
limits came from somewhere on continental Europe where there's a rule
that if you're within the boundary of an urban place, the speed limit is
automatically the urban limit rather than the national one, making it
quite feasible to set the speed limit on urban roads (without a survey)
and adding "source:maxspeed" to indicate that the maxspeed hadn't been
surveyed.
In the UK it's not so straightforward - there are implicit rules about
"what creates an urban limit", but they're not as simple as "slow down
at the village sign and speed up when you see the village crossed out
one". Also, the plethora of slow-down and speed-up zones, and the large
number of rural non-national limits and urban non-30 ones make surveys
essential. Finally, the same "national speed limit" sign can mean
different things on different roads (and different things to different
sorts of traffic).
Hence the need for "source:maxspeed" in the UK to mean "I've actually
surveyed this rather than just guessed" and the use of "maxspeed:type"
to say e.g. "this isn't a 60mph sign, it's a national speed limit one"
and all that that implies.
Cheers,
Andy
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