# [Tagging] Turn Restriction usage

Georg Feddern osm at bavarianmallet.de
Thu Apr 12 08:38:51 BST 2012

```Am 11.04.2012 15:42, schrieb Simone Saviolo:
> 2012/4/11 Ross Scanlon<info at 4x4falcon.com>:
>>>> No.  The router should know not to do this. Likewise as below the router
>>>> should not make u turns at traffic lights.
>>>
>>> Based on what? How does the router know that the two ways are two
>>> carriageways of a single road? Couldn't they be a straight road, that
>>> becomes a oneway street at a certain point, and at that point a
>>> junction brings to a oneway secondary road?
>>
>> The name of the way, the fact that you are turning>  180 degrees on the same
>> way.
> I don't agree.
>
> First, if you're on the same way, you're not turning, but going
> straight and following the road. In the case of the OP, I expect to
> see three ways, two of which tagged oneway=yes.
>
> Second, if you turn more than 180 degrees, you're hopefully going on a
> bridge ;-)
>
> Third, think of a situation like this: http://osm.org/go/0CKuMhs89-
>

in your example the angle at the considered point is far from dangerous
or considerable.
Your angle is in the range up to 45° and doesn't even reach 90° (a
typical crossing / turn situation).

Points of no u-turn - that have to be considered - have angles
- beyond 90° (may be a sharp turn or a crossing at a combine situation)
- mostly beyond 120° and often beyond 150°
This is the range to consider - and that is possible for a router.
The angles depend a bit on the mapper's 'style' and should be considered
by the mapper - but in most cases the angle will reach those range
anyway - a split and combine would not be 'rectangle'.
(U-turns at dual-carriage ways with two 90° turns because of a via-way
have to be considered separately.
If not allowed - even only by law - a turn restriction may be helpful
there.)

Greetings
Georg

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