[Talk-GB] maxspeed changes

[Andrew M. Bishop]

SomeoneElse <lists at mail.atownsend.org.uk> writes:

> Presumably the argument for tagging "maxspeed=60mph" where it's actually signed
> as "national" is that it's too hard for routers to figure out whether
> something's a single or dual carriageway?  I'm not sure why we have to depart
> from the "on-the-ground" rule in this case - if it's not obvious that
> something's a dual carriageway surely that's a tag that should be added, not
> some curious code value stored against a tag (source:maxspeed) that's usually
> used for something else entirely?

As the author of an OSM data consumer (the router "Routino") I think
that distinguishing between single and dual carriageways is mostly
irrelevant in the argument for preferring numeric values for maxspeed
tags.  The real reason which I see, and which is much more difficult
to handle, is when you consider that there are ~200 countries in the
world and they each might have their own speed limits.

If everybody tags their local part of the world with numeric limits we
create a database that anybody, anywhere, can use without any
knowledge of foreign legislation.  If we tag with local conventions
then we end up with something suitable only for a tiny fraction of the
potential users.

I agree that extra tags for dual-carriageways are preferable to
infering that information from the speed limit.  Trying to deduce
whether a road is a single or dual-carriageway from the speed limit is
bound to fail, not least because of those dual-carriageways that have
60mph limits.  For the record, Routino doesn't allow you to prefer
dual-carriageways when routing but you can select to prefer multi-lane
roads (those that use the "lanes" tag) which largely has the same
effect.


Perhaps we need some "UK national speed limits" tagging presets for
JOSM to encourage people to use a consistent set of tags (if we can
ever agree).

Andrew.
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Andrew M. Bishop                             amb at gedanken.demon.co.uk
                                              http://www.routino.org/