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

[Philip Homburg]

In your letter dated Tue, 7 Oct 2008 11:04:44 +0100 you wrote:
>2008/10/7 Philip Homburg <pch-osm-talk at u-1.phicoh.com>:
>
>> For trunk roads, it might be just a safe default to assume that the way is
>> oneway, unless tagged explicitly as single-carriage (oneway=no).
>
>People can keep saying that, but it won't make it true :)
>
>Trunk roads shouldn't be implied oneway, simply because of the
>established usage of this tag to represent normal single-carriageway
>roads. (National primary routes in Ireland, primary A roads in UK,
>possibly others). My contention is that implicit tagging is only valid
>where we have a global norm. Local norms aren't enough.

I think local norms are fine. However that requires a lot of localization work.

But a global norm is better than a local one. 

Localization is likely to happen anyway when people start displaying speed
limits. Or do you want to tag even the smallest country road with the 
appropriate speed limit for all types of vehicles?

Maybe technical solutions are an option: defining administrative areas that
contain the defaults that apply. 

>Based on this reasoning, if you choose to accept it, it ISTM that only
>motorway mainlines and roundabouts should be assumed oneway.
>
>BTW, there may be an alternative resolution to this matter. The
>problem here is that we can't agree, country-to-country, which highway
>tags should generally imply _dual-carriageway_. However, we do have
>ways of recognising a dual-carriageway from other clues - either the
>existence of parallel ways with the same ref and different directions
>or (better) the existence of a dual_carriageway relation.

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.

>those that are members of a dual_carriageway relation

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.