[Tagging] Turn lane indications vs turn restrictions

[Minh Nguyen]

Vào lúc 18:26 2022-06-12, Paul Johnson đã viết:
> On Sun, May 29, 2022 at 10:32 AM Mateusz Konieczny via Tagging 
> <tagging at openstreetmap.org 
> <mailto:tagging at openstreetmap.org>> wrote:
> 
>     (1)
>     Let say that we have crossing with lane with following signage:
> 
>     - left arrow on left lane
>     - straight arrow forward on center lane
>     - straight arrow forward on right lane
> 
>     You are nevertheless allowed to also turn right from the right lane.
>     And make also u-turns from the left lane.
> 
>     How turn:lanes should be tagged in this case?
>     I am asking as https://wiki.openstreetmap.org/wiki/Key:turn
>     <https://wiki.openstreetmap.org/wiki/Key:turn> seems to
>     be suggesting that actual allowed turns should be ignored
>     and only what is signed matters and want to confirm this.
> 
> 
> Use best judgement.  Tag what the lanes actually do.  There's decent 
> major chunks of pretty much every country's highway system where there 
> are lanes but they're not signed and while the lanes are painted, what 
> they explicitly do isn't.  I'd still explicitly tag what humans can see 
> implicitly within context, for the same reasons why we don't abbreviate 
> names; computers tend to need the context spelled out for them to give 
> effective results.

The turn:lanes=* syntax provides for a "none" keyword (equivalent to the 
empty string) for a lane that lacks a turn indication in the form of a 
road marking, sign, or signal. Some routers are capable of resolving 
these "none" indications to turn indications based on the intersection's 
outlet ways. [1]

Sometimes these heuristics are inadequate due to unusual intersection 
geometry. This is particularly true with roundabouts: in the U.S., 
people don't count roundabout exits; instead, the exits are associated 
with standard left/straight/right turn indications. [2] Five-way 
intersections or more can also be tricky.

It would be tempting to tag unmarked turn indications, similar to the 
"implicit" turn restrictions that some mappers add. However, some 
routers ignore turn indications that disagree with the maneuver 
classifications they've already calculated based on intersection 
geometry, irrespective of turn:lanes. [3] Another challenge is that, 
when these unmarked turn indications are tagged too pedantically, 
engineers like me quickly exhaust our graphic design chops trying to 
squeeze arrows into a tiny box. [4]

[1] https://github.com/valhalla/valhalla/pull/1859
[2] https://mutcd.fhwa.dot.gov/htm/2009r1r2/part3/fig3c_10_longdesc.htm
[3] https://github.com/Project-OSRM/osrm-backend/issues/4965
[4] 
https://github.com/mapbox/navigation-ui-resources/pull/26#issuecomment-924307448

-- 
minh at nguyen.cincinnati.oh.us