[Tagging] Clarification on the role link in route relations

[António Madeira]

Some months ago, I was surprised to see some changes in Portugal
regarding this with motorway links being included in route relations. I
contacted the mapper who did them and his answer included a link to this
wiki...

¯\_(ツ)_/¯


Às 21:02 de 08/01/2022, Minh Nguyen escreveu:
> The wiki page for route relations lists an optional role "link" for
> highway=*_link ways leading to and from the route. [1] Under what
> circumstances should this role be used? Should we deprecate it?
>
> The "link" role was originally documented in 2009. [2] At the same
> time, the highway=motorway_link documentation was edited to state that
> all link roads should be members of route relations as "link" members.
> [3] I can't find any discussion about the changes. The requirement to
> add link roads to route relations was removed the following year [4],
> but the role remained documented on the route relation page.
>
> Since then, there have been occasional references to this role [5][6],
> but it seems to have gone mostly unnoticed. It goes unmentioned in
> other parts of the wiki that describe route relations. As of writing,
> the "link" role is used on 26,157 route relation member ways (0.06% of
> all route relation member ways). [7] This is a high enough number that
> it can't be dismissed as a mere tagging mistake, yet it falls far
> short of the 766,473 highway=motorway_link ways, let alone other
> highway=*_link ways. [8]
>
> Is there a particular route type, region, route network, or scenario
> in which the "link" role is more appropriate? Or is this is just a
> misunderstanding based on the residual description on the route
> relation page? Perhaps some mappers have misunderstood that this role
> is required in order for a router to announce "Take on the ramp toward
> ABC 123", even though that's what the destination:ref key and
> destination_sign relation type are for.
>
> At the current usage level, data consumers probably would be unable to
> depend on the "link" role for any practical purpose. There just isn't
> nearly enough coverage of it. Routers such as Valhalla are already
> capable of automatically determining connecting routes anyways.
>
> Meanwhile, the inclusion of "link" members causes problems for any
> editor, QA tool, or data consumer that lacks support for this role
> when determining the linear progression of a route. For example, iD
> has a known issue incorrectly ordering members of a nonlinear route
> relation when splitting one of its ways. [9] Renderers such as OsmAnd
> and openstreetmap-americana understand route relations but don't
> understand the "link" role, so they end up showing redundant route
> shields that can easily confuse the user. [10]
>
> I think we should qualify the documentation for the "link" role so
> that mappers don't misunderstand its purpose and feel obligated to use
> them for optimizing router guidance instructions. Perhaps we should
> even deprecate the role, since it seems redundant to the "connection"
> role for recreational route relations. [11]
>
> (There's also a note on the German E54 route relation [12] justifying
> a slightly different usage, apparently for keeping the route
> connected, but the "link" roles were removed last year. [13])
>
> [1]
> https://wiki.openstreetmap.org/wiki/Special:PermanentLink/2207268#Members
> [2] https://wiki.openstreetmap.org/wiki/Special:Diff/270172
> [3] https://wiki.openstreetmap.org/wiki/Special:Diff/270169
> [4]
> https://wiki.openstreetmap.org/wiki/Talk:Tag:highway%3Dmotorway_link#.22Link_roads_should_be_members_of_all_related_route_relations.21.22
> [5]
> https://lists.openstreetmap.org/pipermail/tagging/2010-June/002512.html
> [6]
> https://lists.openstreetmap.org/pipermail/tagging/2010-June/002512.html
> [7] https://taginfo.openstreetmap.org/relations/route#roles
> [8] https://taginfo.openstreetmap.org/tags/highway=motorway_link
> [9] https://github.com/openstreetmap/iD/issues/8578
> [10] https://github.com/ZeLonewolf/openstreetmap-americana/issues/79
> [11]
> https://wiki.openstreetmap.org/wiki/Special:PermanentLink/2185644#Tagging
> [12] https://www.openstreetmap.org/relation/6244657
> [13] https://www.openstreetmap.org/changeset/87072348
>