[Talk-nz] Turn restrictions rabbithole

[Kiel Hurley]

I added the no u-turns you've linked, and likely the ones you've removed.
Honestly, I hadn't read the turn restriction wiki page as I'd only ever
done them within iD.

I think you've missed a more important section from the introduction of the
wiki page:

> Don't map turn restrictions that are the default for a given jurisdiction
> and are not signed.
>

So I'm okay with not adding the defaults, and I'll endeavour to clean up
the unnecessary restrictions which I have added.

The only thing I'm a bit puzzled about is u-turns at intersections
with/without islands, such as
https://www.openstreetmap.org/#map=19/-43.55042/172.63678 (just north of
your original links). Technically, you're allowed to perform a u-turn at
any intersection as long as there's an island, and as long as there's no
no-u-turn sign (such as Milton Street heading east).
Unfortunately, there's no differentiation between the roads to indicate
that it's separated by an island, and there's no "u-turn allowed" tag
(which would break the "don't add defaults" rule in this case anyway), so
routers would incorrectly assume the u-turn is not allowed.
Is the preferred approach here to split the road at the island?

On Tue, 4 Aug 2020 at 10:38, Eliot Blennerhassett <
eliot at blennerhassett.gen.nz> wrote:

> Now I've seen these restrictions visualizers...
>
>
> https://ahorn.lima-city.de/tr/?zoom=18&lat=-43.553894&lon=172.635211&layer=Grayscale&overlays=TTT
>
> https://restrictions.morbz.de/#17/-43.55386/172.63474
>
> I'm seeing a huge proliferation of labelling every way of a junction
> with what I call a "Self No U-turn" i.e. the from and to members are the
> same segment.
>
> See for example this area:
>
>
> https://ahorn.lima-city.de/tr/?zoom=18&lat=-43.553894&lon=172.635211&layer=Grayscale&overlays=TTT
>
> https://restrictions.morbz.de/#17/-43.55386/172.63474
>
> If you look at
> https://wiki.openstreetmap.org/wiki/Relation:restriction#Tags, it says
> "*to add a no_u_turn restriction relation seems only useful if
> from-member and to-member are different way objects"
>
> So I think all these no-U that I'm seeing are useless, and in some cases
> actually erroneous e.g. when the T junction is between a driveway and a
> road it is not prohibited to do a U turn on the road.  And they make it
> harder to review actual turn restrictions.
>
>
> I've deleted a bunch, but I'm interested if anyone else has any opinion
> on this.
>
>
> No-U-turn where the From and To members are different ways is of course
> still valid e.g.
> https://www.openstreetmap.org/relation/9027919
> https://www.openstreetmap.org/relation/5391722
>
> --
> Eliot
>
>
>
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