[Talk-GB] No U-turn on a long stretch of road

David Woolley forums at david-woolley.me.uk
Sat Dec 5 12:54:52 UTC 2020

On 05/12/2020 12:39, Edward Bainton wrote:
> Any established tagging system?
> The turn restriction wiki 
> <https://wiki.openstreetmap.org/wiki/Relation:restriction> envisages 
> turn restrictions at junctions only; my case is along the length of a 
> major road (~3km). There's no barrier to prevent it, but presumably 
> routing engines ought to know that route correction after a wrong turn 
> will have to wait until the next roundabout.

That's a subjective judgement, so would be tagging for the renderer. 
The renderer (in this case a router) should be using some sort of 
heuristic like only  permitting U turns on residential or service roads, 
and giving a heavy weighting to the use of formal junctions at the the 
expense of distance travelled.  A real human would consider actual 
traffic levels and sight distances but they would be difficult to 
capture on the map and time and season dependent.

I think No U Turn signs tend only to be used where traffic volumes or 
junction structures, might otherwise suggest U turns were acceptable.

I don't think any driver (or autonomous vehicle) should be making U 
Turns based solely on the instructions of an automated router.

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