[Tagging] Micro mapping traffic signals

[Pieren]

Hi all,

If you did not notice, the OSM routing fans [1] are just pushing the
sub-tag "traffic_signals:direction" in the wiki:

http://wiki.openstreetmap.org/wiki/Traffic_signals
http://wiki.openstreetmap.org/wiki/Key:traffic_signals:direction

This is trying to fix one old issue in OSM. The tag
"highway=traffic_signals" has been created to say "this intersection
is controlled by traffic lights" and was placed on the intersection
node itself. It wasn't intended to tag each individual spotlight.
But since we have hi-res images, some contributors started the
micro-mapping of each individual traffic light and used the original
tag to see them on mapnik.
It looks nice on high mapnik zooms but this creates some issues for
data consumers because the same tag and the same feature can have
different amount of elements e.g, routine engine where routes with
traffic signals are penalized. And on complex intersections, we cannot
rely only on complex topological analysis. Note that we have a similar
issue with the "stop" sign, for instance.

The current solution with an additional tag
"traffic_signals:direction", pushed without a public discussion, is
solving most of the routing issues. But it is not sure to help other
applications where it is hard to determin which spotlight controls
which intersection and how it is synchronised , e.g. the rendering
issue where one, two, four or eight icons could be drawn depending on
the zoom level, icon size and distance between nodes; e.g. complex
intersections where more than one traffic light can be synchronized
(and should be counted only once for penalties); etc.
At low zoom levels, it's interresting to see only one symbol for the
whole intersection. At high zoom levels, each individual traffic light
can be painted. The current tagging solution does not help in this
regard. It's just working more or less by accident on mapnik when
icons are overlapping (if not, the rendering will fail).

My proposal:
- when micro-mapping the traffic lights, use a different tag than
"highway=traffic_signals" which should be reserved for the "simple,
old" fashion way of mapping the intersection itself.
- create a new tag for the micro-mapping of each individual traffic
signal e.g.: "highway=traffic_light" (any other suggestion is welcome)
- create a new relation type e.g. "type=traffic_lights" (or reuse the
proposal "type=junction" [2]) with a role for each "traffic_light"
node and one role on the "intersection" node(s)
- advantages : for routers, they just collect the relations where the
intersection node is member to calculate penalties. For renderers,
either they draw an icon for each individual traffic light at high
zoom levels or a centroid of the junction nodes member of the relation
at lower zoom levels. In principle, the old fashion is compatible with
the new one but once the relation is widely supported, the old
"highway=traffic_signals" could be removed where the relation is
present. Common traffic signals attributs like the "button_operated"
could be generalized into the relation.
- disadvantage : contributors going to micro-map the traffic signals
will have to use relations. But I don't see this as a big issue since
micromappers should be considered as advanced users. If they cannot
handle relations, they can fall back on the previous simple old manner
with a tag on the intersection node. It's also a good opportunity to
learn relations which can be used later for many other things (e.g.
turn restrictions ;-)

Your comments ?

Pieren

[1] https://github.com/DennisOSRM/Project-OSRM/issues/31
[2] http://wiki.openstreetmap.org/wiki/Relations/Proposed/Junctions