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

[Dermot McNally]

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 :)

For me, there are very few cases where oneway should safely be
implied, and I generally tag these explicitly regardless:

motorway (almost universally so, but we have the option to set
oneway=no where not the case).
motorway_link and other *_link (though we know that there are many
countries where two-way stretches of these are common)
anything with junction=roundabout

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.

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.

So we can solve this problem right now by deciding that a better list
of implied-one-way highway ways is:

those with junction=roundabout
those that are members of a dual_carriageway relation

That will cover any stretches of trunk or motorway (or anything else)
that happen to be dualled.

Dermot

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