Thanks, Minh. Yes, there is no way to indicate an order of precedence
between relations. But I also do not understand yet why this should be
needed. It would be sufficient to have one relation per 'turn', i.e. for
any combination of members with role from/via/to there would be one
relation. And for a given transportation mode we could check the
restriction value for this transportation mode for each turn. An order
of precedence would only be needed if there were multiple (restriction)
relations using the same from/via/to members, but I don't see why this
would be necessary?
Of course there could still be contradicting relations, like an
'only_left_turn' relation paired with an 'only_right_turn' relation (and
different to-members according to these directions). But that has
nothing to do with the transportation modes.
So do you think there should or there should not be mixed restriction
relations?
Regarding https://www.openstreetmap.org/relation/10650926:
```
restriction=no_right_turn
except=bus;bicycle
except:network=Q1140138;Q7407040
```
This would be a 'conditional except' and for it seems like another
indication that the 'except' tag should be replaced by the conditional
restriction tag? This would be more in line with what we do for
'access', maybe something like:
```
restriction=no_right_turn
restriction:bus=unrestricted
restriction:bicycle=unrestricted
restriction:conditional=unrestricted @ network=Q1140138;Q7407040
```
On 29.10.22 20:59, Minh Nguyen wrote:
Vào lúc 01:07 2022-10-29, Tobias Knerr đã viết:
On 29.10.22 07:13 easbar.m...@posteo.net wrote:
I like your idea of not using the except tag but rather something
like restriction:value=unrestricted. Actually that would be the first
useful combination of restriction and restriction:vehicle that I have
heard of. But unfortunately this is neither mentioned in the wiki nor
does it seem to be used that way.
Yes, this is something that would require a proposal to introduce,
it's not established practice at this point. The reason I mention it
in this discussion is indeed that it would be one example where
restriction and restriction:vehicle on the same relation could
meaningfully coexist. So I wonder if a wording could be used that
would leave this door open for the future, e.g. by discouraging
"different types of restriction on the same relation" (or some better
wording to the same effect).
I'm still wondering: Do we ever need different restriction values for
different vehicles, or for different conditions, for the same relation?
No, with the exception of the "unrestricted" concept I mentioned, this
should never be necessary. Even if there is an obscure real-world
situation with different restrictions on the same turn (same
from-via-to) for different vehicles, it could be modelled with
separate relations.
For context, this question came up the other day in the iD bug tracker.
[1] It was unclear how an editor should depict a mixed-restriction
relation. If it's unclear for editors, it's certainly unclear for routers.
In a sense, this is actually a common phenomenon: trucks must exit to a
weigh station but cars may not. Or cyclists must turn right from a bike
lane to a bike path but cars may not. However, these restrictions are
already represented by access restrictions on the ways themselves -- no
need for relations. Normally, way-based restriction tagging breaks down
when the restriction depends on the direction of approach, but that
would require separate relations anyhow.
One potential issue with relying on separate relations is that there's
no way to indicate an order of precedence explicitly. For example, at
one of San Francisco's most prominent intersections, an "except Muni"
sign modifies a no left turn sign, exempting one bus operator that comes
from the west, but not AC Transit. [2] At the same intersection, "no
right turn except Muni and SamTrans" exempts both bus operators that
routinely come from the south. [3]
Most of the bus routes in this part of town follow strict paths, so I
just coined an obscure except:network=* tag and called it a day. [4] But
this experience leads me to suspect there may be places where the
real-world restrictions turn our usual access key hierarchy [5] on its
head. Unfortunately, there's nothing to say that one relation takes
precedence over another relation with the same members. (Incidentally,
this is why I've shied away from proposing that we use relations to
track overlapping parking lane restrictions.)
[1] https://github.com/openstreetmap/iD/issues/9337#issuecomment-1283492156
[2]
https://commons.wikimedia.org/wiki/File:City_of_San_Francisco_(Unsplash).jpg
[3] https://www.mapillary.com/app/?pKey=4265238293573050
[4] https://www.openstreetmap.org/relation/10650926
[5]
https://wiki.openstreetmap.org/wiki/Key:access#Land-based_transportation
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