Talk:Tag:dual carriageway=yes

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):

• Case 5 (Mapillary)

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://www.barriersdirect.co.uk/assets/0001/9922/290B0477.jpg

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 https://wiki.openstreetmap.org/wiki/File:Traffic-signs-manual-chapter-5-2004-figure-7x08.svg 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". https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/914149/traffic-sign-drawing-schedule-1-definitions-document.pdf

Some discussions from other forums:

https://www.pistonheads.com/gassing/topic.asp?h=0&f=10&t=1786866

https://showmeasign.online/2017/03/22/dual-carriageway-signing/

https://www.sabre-roads.org.uk/forum/viewtopic.php?t=35640

Although https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/773421/traffic-signs-manual-chapter-05.pdf 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)

In extension:

1. Boulevards with raised separation for frontage roadway ( side_road=* not frontage_road=* , with its own problem) , but a single undivided two-way main roadway (eg Dunhua Rd, Taipei); ditto for perhaps less uncommon one-way case.
2. One-way divided roads,
3. one-way roads with frontage roadway on either or both sides (may again have narrower removable and discontinuous raised separation, eg coincidentally File:Frontage lane on South 11th Street before San Fernando Street, San Jose, California.jpg in Key:side_road#Examples )
4. two-way undivided road with frontage roadway on one side.

All these may constitute as "a road" having multiple physical roadways that won't be considered a "dual carriageway".
The distinguishing of a parallel highway=service from the "public" road will also be important. There may even be ones that serves similar functions but are inside the "public" road corridor / land RoW somehow, before the sidewalk.
—— Kovposch (talk) 00:07, 3 December 2023 (UTC)

What is "one-way divided road"? Mateusz Konieczny (talk) 07:23, 3 December 2023 (UTC)

One-way road with multiple carriageways. Usually a two-way divided road changed to one-way, for accommodating a bridge, landscaping, preventing "jaywalking", or a dedicated center transitway. Eg Xinyi Rd Section 2 to 4 in Taipei if the busway is not counted as a main carriageway.
—— Kovposch (talk) 22:35, 3 December 2023 (UTC)

For the opposite case, a road may be improved or dualled by expanding to a new alignment. Different directions can then be widely spaced, or even switch sides. This makes them appear to be a pair of single carriageways functioning as a dual carriageway. Eg (though this may be new construction planned together) 24376329 24376329 (reminded me to add this from Discord) divided to two sides of a river in a gorge.
Not related to the term "one-way dual carriageway" sometimes used to describe one-way corridors / couplets, or gyratory junction systems.
—— Kovposch (talk) 04:47, 4 December 2023 (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, https://www.openstreetmap.org/?mlat=50.37556&mlon=19.77176#map=19/50.37556/19.77176 for me is not dual_carriageway=yes but https://www.openstreetmap.org/?mlat=50.05902&mlon=19.92823#map=18/50.05902/19.92823 https://www.openstreetmap.org/#map=17/50.05302/19.94271 are Mateusz Konieczny (talk) 08:40, 31 October 2021 (UTC)

Roads with separate mapped direction but without physical barrier

What about roads without physically separated directional lanes, but which are still mapped with two separate lines? (Occurs even if it is not recommended). Are they also to be mapped with dual_carriageway=yes or not? The description explicitly excludes this, but the aim of explaining the "one-way regulation" would also be necessary in these cases. --Langläufer (talk) 17:41, 24 November 2023 (UTC)

In such case map data is broken and should be mapped correctly as a single lane. Adding dual_carriageway=yes would make it worse by claiming that there is dual carriageway in place where there is a single carriageway Mateusz Konieczny (talk) 18:35, 24 November 2023 (UTC)

I would map them as dual_carriageway=yes. At the moment I regard this tag as a general indication that a carriageway is mapped in two divided ways (including, for example, segments around traffic islands). Otherwise we would need a second tag with a similar meaning to distinguish between "real" and " non real" dual carrigeways in the narrower sense (to keep the advantages of this tagging, e.g. when calculating road length or in detailed rendering). Supaplex030 (talk) 13:49, 25 November 2023 (UTC)

This shouldn't be considered lightly. As mentioned above, "dual carriageway" can have legal implication in speed limits.
—— Kovposch (talk) 15:57, 25 November 2023 (UTC)

" we would need a second tag with a similar meaning to distinguish between "real" and " non real" dual carrigeways" we already have notes and fixme=* to point out broken tagging. Roads where there are no dual carriageways mapped as dual carriageways are broken, badly mapped and should be edited to fix them. It is not valid mapping. Mateusz Konieczny (talk) 19:00, 26 November 2023 (UTC)