[Tagging] Roundtrip and closed loop in relations

[Volker Schmidt]

On Sat, 21 Dec 2019 at 14:43, Phake Nick <c933103 at gmail.com> wrote:

> Reminder 1: There are loops within bus route doesn't mean the route is a
> circular or round trip route.
>
Fully agreed. That's why I am saying we need to alok at this with a bit of
calm. There plenty of diferent route toplogies

Reminder 2: The roundtrip=* key is designed to use in combination with
> hiking routes or bicycle routes. A hiking/bicyle route that goes A→B→A
> which come back with the same start point with exact same alignment as the
> other direction doesn't really mean anything so I don't think a special
> value would be needed for such case. As for bus routes, whether or not it
> goes back along same road doesn't really mean anything either.
>

Fully agreed.
We never looked at this and tagged different route types (bus and bicycle
for example) independently folowuìing essentially different topological
approaches.
Bus routes are (always?) tagged as unidirectional routes. The same bus line
that connects A <> B is represented by two bus routes A > B and B > A.
But, by tradition, or whatever reason, a topologically equivalent bicycle
route A <> B is represented by a single, mostly bidirectional route A <> B,
where the few pieces where the A > B route differs from the B > A route
(for example for roundabouts or cycle paths on both sides of a road) are
handled by route segments whose ways are tagged with role=forward|backward
within the relation.

By the way, these loop segments are, at least within the route network in
Italy, tagged with role=forward|backward differently from the definition in
the cycle routes wiki page
<https://wiki.openstreetmap.org/wiki/Cycle_routes>
(" *Relation role*: Cycle routes sometimes have different paths depending
on the direction you are travelling. In this case, ways in the relation
should have a role of *forward* or *backward* as described in
Relation:route#Members
<https://wiki.openstreetmap.org/wiki/Relation:route#Members>. The direction
is rendered on the cycle map (example
<https://www.openstreetmap.org/?lat=51.88885&lon=0.8972&zoom=16&layers=C>
).")
in the sense that loop sections that are to be followed in the A > B
direction are marked with role=forward  and loop sections that are to be
ridden in the B > A direction are marked with role=backward.
But that is a different story that needs sorting out as well.
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