[Tagging] when should highway=centreline be used?

[Tom Pfeifer]

We need to distinguish two questions, which were mixed up in the
previous discussion, namely

a) should a centre line be mapped at all,
b) if so, how should it be drawn and tagged.

As for a), mapping certainly provides a value on a road with
bidirectional traffic. We can map if it is a solid or dashed line,
or if it is missing.

For a dual carriageway, which is physically separated and therefore
mapped as two separate ways in OSM, I see no value. However these
were examples cited before, where highway=centreline was added to
a physical barrier.

So for b), the centre line needs to be an attribute of a linear highway,
thus tagging highway=centreline does not work. It is also semantically
wrong, since the line is not a highway, and adds a new feature to the
historic elements in the highway valuespace that are not highways
either (highway=traffic_light etc).

Thus we would need a new key as follows:
<new_centreline_key>=[single|double|dashed|etc]

As the line might not always be in the centre, a better name would be
welcome.

The following existing tags need to be considered in this context:
- turn:lanes[:forward|backward]
- change:lanes[:forward|backward]
- overtaking=*

The proposal for Key:road_marking scares me, if we start mapping every
dash of a line we will end with complete clutter.