Talk:Tag:dual carriageway=yes

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Edge Cases / Criterias

I have recently started to use this tagging in some districts in Berlin and from time to time I have noticed edge cases that lead me to the question of what criteria dual_carriageway roads must/should match and at what point a divided carriageway is no longer tagged with dual_carriageway. Here are some example cases where I did not use dual_carriageway. Are there other opinions?

  • Case 1 – Dual_carriageways to the west and east, but in this section the street curves around a church on both sides (consider: a street around a square would usually also not be a dual_carriageway).
  • Case 2 – Somewhat similar: road around a village green.
  • Case 3 – A larger village green area, more like an elongated town square with single buildings.
  • Case 4 – Two streets running in opposite directions at a relatively wide distance, separated by a green area, different street names on both sides.

Here is another case where I have used dual_carriageway=yes, but where I am not sure if it really fits (because the distance between the carriageways is quite large and there is a promenade and playgrounds in the middle):

What criterias could be derived from this that should apply to a dual_carriageway? A certain minimum distance? No buildings on the separating area? Same street name on both sides? Some specific parking regulations? Is anyone aware of any traffic engineering definitions? --Supaplex030 (talk) 18:29, 30 October 2021 (UTC)

I don't like using this tag for the physical attribute, arbitrarily decided by anyone with different standards. I prefer carriageway=*, similarly used in carriageway_ref=* and width:carriageway=* by some.
I'm of the opinion dual_carriageway=* should only be used when it is signposted to be verifiable, or at least have a relevant speed limit. My view is similar on expressway=* (although the US community recommendation is more relaxed on it), since the meaning definition can vary. This requirement is to be consistent with the legal-based motorroad=*.
  • US Expressway (only for ending) [W19-2]

  • UK "Dual carriageway ahead" [818]

  • UK "Dual carriageway" for certain miles of distance [818.1]

  • UK "Dual carriageway" supplementary plate below give-way and stop signs (and perhaps others) [608]

  • ---- Kovposch (talk) 04:56, 31 October 2021 (UTC)
    This is especially troublesome when it comes to conflict between OSM practice and authorities' definition. For one, real life requires continuous, fixed, semi-permanent separation, eg walls, grass, jersey barrier. It is related to the specification of the cross-section built. OSM drawing style allows the mere presence bollards/delineators and kerbstones to split a road up as 2 lines. This would still be considered a "single carriageway".
  • Https://

    Flexible deflector posts in the center-line, often seen in Commonwealth countries.

  • Japan provisional 2-lane "Expressway" (in the "motorway" sense)

  • ---- Kovposch (talk) 05:20, 31 October 2021 (UTC)
    Moreover in UK and Commonwealth countries, Single Carriageway roads can receive a "single-lane dualling" treatment (as opposed to "ghost island") at T-intersections to become divided for a short distance to provide a left-driving right-turn lane. This isn't really a Dual Carriageway.
    TSM 7-8
    ---- Kovposch (talk) 05:27, 31 October 2021 (UTC)
    "I prefer carriageway=*" - what is its meaning? Note that I started to use dual_carriageway=* tag in place where carriageway_ref=* is not assigned, dual carriageway status is not signposted and typically has no relevant speed limit impact (and in many places road splits only for tiny segment, sometimes just meters before crossing so it would be unlikely to cover by anything you mentioned). What you would propose to record that info? And yes, if would be represented as separate OSM geometries with oneway roads I would use dual_carriageway=yes there Mateusz Konieczny (talk) 08:35, 31 October 2021 (UTC)
    The problem with this is "dual carriageway" is used for legal purposes such as speed limit. Eg maxspeed:type=GB:nsl_dual and Default speed limits. In fact, according to TSRGD, it seems this is not a "dual carriageway", because "central reservation" explicitly excludes "traffic island".
    Some discussions from other forums:
    Although only describes shorter ones (like a traffic_calming=island) and roundabout splitter islands, it would be difficult to differentiate longer traffic islands from a section of dual carriageway. They can be mixed up, as illustrated in DMRB CD 123. --- Kovposch (talk) 13:25, 29 July 2022 (UTC)
    @Supaplex030: I would consider 2, 4, 5 of that - so except the church case, Richardplatz - as covered by dual_carriageway=yes and I used such tagging for similar objects. And to give one more example, for me is not dual_carriageway=yes but are Mateusz Konieczny (talk) 08:40, 31 October 2021 (UTC)