If two ways enter a roundabout at the same point, you can turn from
road A into road B instantly,
but going from B to A will require going around the entire roundabout.
For a router to detect this, it would have to check (for every
encountered node):
1) is this a part of a roundabout?
2) if yes, which of the turns I can make here would require going
around the roundabout?

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Note that 1) is a query for all ways using that node and 2) requires
geometry data (instead of just network)
While this is doable (or could be solved with turn restrictions),
it makes roundabouts a much more of a special case.
The current tagging of a one-way circular way is much easier for
routing software to deal with
and is conceptually "cleaner".
Also, having the intersection directly on the roundabout
is potentially also a valid way of drawing a regular intersection
directly next to the roundabout.
(I would not be surprised to see such a crazy construction somewhere)
On 14 February 2018 at 23:04, Mateusz Konieczny <matkoni...@gmail.com>
wrote:
> On Wed, 14 Feb 2018 19:23:39 +0100
> Johan C <osm...@gmail.com> wrote:
>
> > The present tagging (used since 2005 or so, and all around the globe)
> > is fine.
>
> I agree, I see no problem with this recommendation.
>
> _______________________________________________
> talk mailing list
> talk@openstreetmap.org
> https://lists.openstreetmap.org/listinfo/talk
>

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