[Tagging] Traffic sign relevant direction: relation type:enforcement vs. direction=* vs. traffic_signals:direction=*

[Jean-Marc Liotier]

(Sent again, this time without all the cc: which probably are the
cause of the previous attempt being held in the listserv's spam filters...
After eleven hours I guess it won't be delivered to the list so I resend)

On Fri, 24 Mar 2017 19:08:13 +0100
yo paseopor <yopaseopor at gmail.com> wrote:
>
> I would start a "definitive thread" with all the options, all the
> possibilities, all the points of view, all the information and then,
> passing all to the wiki with a votting or aproved by list complete
> proposal.

Well, heres is an attempt... At some point one may wish to paste it
into a wiki page. In cc: the participants in the threads recently
discussing this subject.

So, here is a summary of the methods that, after discussion here, seem
to have some traction among participants for the mapping of "stop"
restrictions and similar restrictions such as "give_way" and
"traffic_signals". This is about the restriction, not the sign that
advertises it. It applies to two-ways way, in cases other than an
all-ways restriction. I omitted some methods mentioned which seem too
ambiguous, too heavy on processing, lacking current use or all of the
above.

----------------------------------------------------------------
1- highway=stop+direction=forward node on the way to an intersection

Documented at https://wiki.openstreetmap.org/wiki/Tag:highway%3Dstop
and https://wiki.openstreetmap.org/wiki/Tag:highway%3Dgive_way

Advantages:
- Only one additional tag
- Uses the widely used direction=* tag

Disadvantages:
- To be unambiguous, must be tagged on a non-intersection node
  adjacent to the intersection to which it applies
- Only good for covering simple cases: complex case require
  a more powerful method such as the relation (type:enforcement)
- direction=* is also used for cardinal directions, so two
  semantic fields coexist within the same tag

----------------------------------------------------------------
2- highway=stop+stop:direction=forward node on the way to an
  intersection

Documented at
https://wiki.openstreetmap.org/wiki/Tag:highway%3Dtraffic_signals

Advantages:
- Only one additional tag
- Uses the *:direction=* scheme which is widely used for
  traffic_signals:direction=* and stop:direction=* and might
  therefore be unified later. Also used for railway signals.
- Cognate to the way Kendzi3D models signs

Disadvantages:
- To be unambiguous, must be tagged on a non-intersection node
  adjacent to the intersection to which it applies
- Only good for covering simple cases: complex case require
  a more powerful method such as the relation (type:enforcement)

----------------------------------------------------------------
3- relation (type:enforcement)
Documented at https://wiki.openstreetmap.org/wiki/Relation:enforcement

Advantages:
- The only method to unambiguously covers all cases
- Few additional tagging steps: creating the relation and adding a
  couple members covers the simple case
- Compatible with the node bearing the highway=* restriction being
  either on the way or elsewhere at the actual location of the sign

Disadvantages:
- A couple more additional tagging steps compared to
  the other methods
- It is a relation (relations are perceived as not
  beginner-friendly, and some consumers don't grok them)

My opinion is that method #3 must be promoted but current practice
hints that it might have to coexist with one of the two others (or both
if no convergence consensus emerges).