[OSM-talk] [Tagging] - RFC - Motorway_linkimpliesoneway=??

Dermot McNally dermotm at gmail.com
Wed Oct 8 14:40:33 BST 2008


2008/10/8 Philip Homburg <pch-osm-talk at u-1.phicoh.com>:

> I don't think this is about local interpretations. It is about having
> safe defaults.

Agreed 100%

> Of course, adding oneway=no to all trunk ways that do not have a oneway tag
> can be done by a script.

Clearly it could be - but it certainly _shouldn't_ be, unless that
script can be confined to an area where it is certain that all trunks
really are dual in accordance with local tagging norms. But I prefer
this...

> Another approach, that may also work for trunk roads is to write a consistency
> checker that tries to detect this situation.
>
> A first pass tries to find roads with the same names, or unnamed roads that
> are roughly parallel. Then report any road in that set that doesn't have an
> explicit oneway tag.

This is a lot safer IMHO, since (arguably) if we have mapped only one
carriageway of a dualled road, it's more useful for routing software
to be allowed to route both ways over it. With incomplete mapping,
insisting on correct setting of oneway isn't all that useful.

> I don't think that a relation should be used to imply oneway=yes. It's just
> too risky.

A dual-carriageway relation should. In a world where not even
roundabouts are guaranteed to be one-way, we can at least trust a
dual-carriageway to be so.

> In a country where just all trunk roads are dual-carriage ways, defaulting
> to oneway=no is just too risky.
>
> But you seem to care more about the burden of retagging some existing trunk
> roads than about having safe defaults.

Not at all. I don't care about the burden of retagging trunk roads.
But I don't want the tail to wag the dog. The trunk tag was conceived
for roads that are not inherently dual-carriageway. Established
practice is to explicitly tag the special (and recognisable) case of
dual-carriageways with oneway=yes.

What I'm saying is that dualled section of trunk highways that are not
yet explicitly tagged oneway should now be so tagged. Alternatively,
we can introduce highway=gelbe_autobahn that implies oneway and bulk
retag the German trunks.

> On the other hand, given that localization is likely to happen eventually
> anyhow, it may at some point become just a local decision.

If we could reliably determine the nationality of a section of road
I'd be a lot more relaxed about this matter (and others, like
maxspeed). And once that day arrives, as it surely will, we can
happily zap tags deemed on a regional level to be implicit. But we're
not there yet.

Dermot

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