[Tagging] source:maxspeed vs. maxspeed:type

[Martin Koppenhoefer]

2013/2/26 SomeoneElse <lists at mail.atownsend.org.uk>:
> 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).


Also in Germany or Italy it is not possible to add source:maxspeed
without a survey, for several reasons, like the ones mentioned:
- there can be differing limits inside the urban boundary (e.g. 30, 40
or sometimes 60 or 70 instead of the default 50 km/h).
- the limit of the urban area (legally, trafficwise) has to be
obtained by survey
- there can be an explicit maxspeed=50 signposted inside the urban
area (so in both cases the limit is 50, but the reason is different
and in case of a change of the law only the implicit limits would
change)


> 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.


I don't agree that you have to use "source:maxspeed", you could as
well use "maxspeed:source" to indicate the way the information was
collected (or better put it into the changeset comment).
"maxspeed:type" isn't more intuitive or unambiguous anyway, you can
see this by looking at the actual values, it's chaotic. Stuff like
"numeric", "national" doesn't convey information (IMHO), there is no
proposal which tags to use for "context" and there isn't even
consensus whether to use UK or GB.
The definitions from this page:
http://wiki.openstreetmap.org/wiki/Key%3Amaxspeed%3Atype are not
describing what the actual values look like (e.g. there is no "sign"
value at all, but "signed" is used a poor 10 times).

In the end it doesn't matter, we can invent variants for everything
that already has established and defined tagging, the system allows
for it, and if you're particularly interested in maxspeeds you will
evaluate also the typos and variants, I'd only question the utility.

cheers,
Martin