[Talk-GB] Spurious No U turns in GB

[SK53]

Andy (SomeoneElse) has just pointed out some odd mapping of No U-turn
restrictions in Notts. These seem to originate in the ImproveOSM QA tool
developed by Telenav. A quick inspection of No U-turns in the local area
shows that very many have been added in the past year, but the vast
majority of these have no actual legal restriction. Many fall into one of
these categories:


   - Where an road approaching a roundabout is split just before the
   junction. Restrictions have been added to prevent routers selecting u-turns
   at the point where the approaches merge again.
   - Junctions with islands which also cause a single carriageway to split
   and merge around the junction.
   - Similarly in more complex junctions where lanes join to form a single
   carriageway.

In most of these cases executing a U-turn would always be difficult, not to
say foolhardy. However, the only signed restrictions which I am aware of
are usually at traffic signal controlled junctions. In the improveosm
dataset I have only found one junction suggested of this type. Although
this is signed as no U turn if approached from the other side, it is not
restricted in the other direction.

The approach to u-turn restrictions would appear to vary by highway
authority. In Leicester many of signal controlled junctions on the A6 and
A607 are signed "no u turn".

In most cases these mapped restrictions are harmless in that they mainly
stop a nonsensical action. Where they are misleading is that there large
number now means that actual no U turns where a violation would be an MTA
are now no longer accessible on OSM.

Jerry
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