See restriction=only_u_turn | Tags | OpenStreetMap Taginfo
This seems to only be documented on the RU page and is a low use tagging. I can see that it might make sense for dedicated u-turn lanes that exist now and then, but modelling them would likely require a separately mapped from way.
Is there a rule that from and to must not be identical?
And in the end whenever you have “only_*” the suffix doesn not matter for anything. Be it only_left_turn or only_u_turn - processing is the same.
So it also could be “only_this_way” and it would/should be processed the same.
Flo
If they are identical (in this case) the results are likely nonsensical:
to via
---------------------------X-----------> road continues on
from
In other words the road after the via would not be accessible, the only case in which this might exist in the real world is if the part of the road after the via is one way in the opposite direction, but then at least for routing purposes the restriction is not needed.
to via
---------------------------X-----------<
from--------------/
------/-------------------------------->
Is the case that I’ve seen now and then IRL, but as said that would require a separate way for the from.
Not encountered ones, wondering if those are on a regular bi-directional road if there is some kind of facilitation in place to make that turn, JOSM having a turning_loop on offer for a street end, but see no reason why such could not be middle of a road, mapped as dummy on the only forced U-Turn I know but routers needing no instructions here… pick your lane and go.
A sample found near Rome got me headscratching as to what it’s function is supposed to be… exit the gate as from, the node of the driveway connecting to the street as via and then back ‘to’ the gate. Certainly viewing this in ID Editor no fault signaled for having both to/from on the same way segment.
https://safety.fhwa.dot.gov/intersection/rltci/fhwasa14069.pdf has a lot of examples, but that wasn’t the question, which was if the only_u_turn restriction is useful in modelling them.
No, and now I know the one I snapscreened is called a median u-turn close resemblance to the one pictured in your PDF.
There’s a standard sign for “U-turn only” in some parts of the U.S., but although it can theoretically be used in a restriction=u_turn_only situation, I’ve only seen it on turn lanes that can be modeled differently. For example, this no left turn restriction in Cupertino is technically a U-turn only restriction according to the signs, but we aren’t representing the lane as a separate way, so it would be inaccurate to say that you can only U-turn, as opposed to going straight or turning right.
As far as U-turn only restrictions from and to the same way, I’ve seen situations that come close. For example, a street dead-ends with a turning circle, and a one-way street empties into that turning circle. However, the oneway=yes tag on the latter street makes a turn restriction relation unnecessary. Still, this is close enough that I’m not confident about completely ruling out something out there that needs it. Maybe a situation where restriction:conditional=u_turn_only @ … would be necessary?
