Puzzled by traffic sign mapping

Last night I was following up a discussion on IRC and adding lane, sidewalk and, where appropriate verge, tags to some highway=trunk where they were missing (lanes is relatively poorly used in the UK).

At one roundabout junction I could see a couple of destination signs in the aerial imagery and it was useful to map them to more accurately check a change of speed limit. I don’t normally map traffic signs and a few things surprised me both in support within iD and on the wiki:

  • highway=traffic_sign is deprecated. My understanding is that originally all “street furniture” was tagged using the highway key (street_lamp, bus_stop etc.) so I presume the original intended form was highway=traffic_sign with traffic_sign=*. It looks as if this pattern didn’t survive very long and traffic_sign itself became a standalone ‘top-level’ key: something which often happens with heavily used values. So not something I’m worried about.
  • traffic_sign itself predominantly contains values which refer to a given reference for a specific sign in a given national sign directory. I would have expected it to provide a more human readable indication of the nature of the sign: precise for very common values (e.g., give_way, stop, city_limits, maxspeed, …) and more generic for signs with variable content (destination) or ones used less frequently (hazard, instructional …). It is this latter approach which is best documented on the wiki, but usage does not really reflect this (hazard: 2135, destination: 1507, stop: 31,907, but NL:G11: 57,080). iD offers about 5 of the documented textual values and not the ones most widely used.
  • There is little or no documentation about mapping actual destination signs compared with providing destination information for turn-by-turn routing. Users of the relation type=destination_sign don’t seem to generally map the sign, although there is a role sign for this relation.
  • There are often different kinds of destination signs at a junction, at least in the UK (primary destination signs, local destinations, other local amenity destinations), and tourist information destinations (e.g. hotels) in other countries. I can’t see a way of tagging this for the sign at present.

My general thoughts are:

  • Many traffic_sign values of the form “XX:123”, where “XX” is a country code and “123” a reference to an official sign manual, might be better held in a separate key, say traffic_sign:ref=“XX:123”. traffic_sign would then have more human readable text for mappers. This would make validation in some editors easier (i.e., those without explicit support for many country-specific codes).
  • Current practice is poorly structured for mapping though incremental steps, as the primary way information if captured is via very specific sign codes or through relations.
  • Not including the actual sign in the relation also makes validation difficult: I checked relations at my local junction and it is very difficult to see which signage a given relation applies to, and I suspect at least one of them is wrong (it looks as though the mapper assumed destinations from the S would be replicated for traffic coming from the W, but they are not.
  • A vast number of relations would be required to map all signed destinations at my local junction: “Ring Road” (two directions), Mansfield, Grantham, Birmingham, Melton Mowbray, Derby, “City Centre”, “The North”, “The South”, Hospital Emergency dept, football & cricket traffic, University of Nottingham, East Midlands Conference Centre, Multiplex Cinema, several suburbs.
  • Mapping signs as street furniture is potentially important for some applications: for instance routing and/or hazard warning for people with limited vision.

In closing I’m also impressed at just how much information has been captured. Presumably this is used by various routing applications.

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Ce sujet de discussion accompagne la publication sur https://community.openstreetmap.org/t/puzzled-by-traffic-sign-mapping/100126