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

[Philip Homburg]

In your letter dated Tue, 7 Oct 2008 14:34:48 +0100 you wrote:
>2008/10/7 Philip Homburg <pch-osm-talk at u-1.phicoh.com>:
>> I'm a bit worried about routing software sending people the wrong way up
>> a dual-carriage way. I very much prefer to default to a safe state. And that
>> means either requiring explicit yes/no oneway tags for both motorway and
>> trunk or implying oneway for those roads.
>
>Only the explicit tagging is a valid choice, then. I'm certainly not
>retagging the bulk of my national road network to oneway=no just
>because of shifts in local interpretations of what a trunk road is.

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

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

>And it still doesn't solve the problem of dual-carriageway primary,
>secondary or tertiary roads, of which there are plenty.

That's true. 

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.

>> I think this is risky: if one way or another the dual_carriageway relation
>> is not there, then routing software will default to an unsafe configuration.
>
>I think you're misunderstanding me here - nothing will protect us from
>broken tagging. But I'm saying that the mapper should have a choice.
>Either explicitly tag your carriageways as oneway or, if you value
>tidiness, provide a relation. It's my counter-offer to those who say
>"all trunks in my country are dual, therefore I want to reduce clutter
>by implying oneway". I'm saying "No, if you want to reduce clutter, do
>so by using the relation. This way, we solve the problem of clutter
>for all road types, but we don't invalidate existing trunks.".

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

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.

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