[Talk-us] Check your turn:lanes

Paul Johnson baloo at ursamundi.org
Thu Aug 25 17:25:10 UTC 2016

On Thu, Aug 25, 2016 at 11:58 AM, David Mease <meased3 at gmail.com> wrote:

> From the wiki:
> The *turn*=* key can be used to specify the *indicated* direction in
> which a way or a lane will lead. It is used on the way segment from the
> first indication via *road markings*, *signposts* or similar indications
> to the junction or completion of merge. If you instead want to specify
> legal turning restrictions please see the article about the restriction
> relation <http://wiki.openstreetmap.org/wiki/Restriction>.
> The turn:lanes schema is for identifying the painted/signed lane marking
> arrows, not for describing where you can legally go from that lane. That's
> what the turn restriction relation is for.

No, turn restriction relations describe restrictions applying for *the
entire movement* defined in the restriction, from *any* lane.

> Putting "through" on a lane means that there is a straight arrow painted
> on it. Putting "none" on a lane means that there is no marking.

I really think this is an overly generous view and essentially renders lane
tagging nearly useless as a result, and is the exact opposite of how it's
actually being used.  Remember, the wiki should reflect how things work in
practice, particularly once something's established enough to get
rendering/routing based on it.  As previously indicated, in statistically
significant parts of the world, turn lanes are implied by context and
position and not by signs and lane markings, unless the usage is
nonstandard.  Additionally, routing engines typically interpret "none" or
blank as "there is no specific restriction on which way you can go from
this lane", so lane usage for a right turn will light up all the none lanes
and the right turn lane.
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