[OSM-talk] Turn restrictions ambiguity

[Shaun McDonald]

On 23 Apr 2009, at 22:56, andrzej zaborowski wrote:

> 2009/4/23 SteveC <steve at asklater.com>:
>>
>> On 23 Apr 2009, at 12:32, Teemu Koskinen wrote:
>>
>>> On Thu, 23 Apr 2009 22:25:36 +0300, SteveC <steve at asklater.com>  
>>> wrote:
>>>
>>>>
>>>> On 23 Apr 2009, at 12:17, Teemu Koskinen wrote:
>>>>
>>>>> On Thu, 23 Apr 2009 21:34:05 +0300, SteveC <steve at asklater.com>
>>>>> wrote:
>>>>>
>>>>>> I don't see a clear explanation as to why there is ambiguity if  
>>>>>> you
>>>>>> don't do turn restrictions at the end of ways on the wiki.  
>>>>>> There is
>>>>>> some stuff in the talk page
>>>>>>
>>>>>>    http://wiki.openstreetmap.org/wiki/Talk:Relation:restriction
>>>>>>
>>>>>> Anyone care to provide an explanation?
>>>>>>
>>>>>> The reason I ask is that I've come across some roads where there
>>>>>> is a
>>>>>> restriction every  other turn in both directions... and  
>>>>>> splitting a
>>>>>> mile long road in to 30 pieces seems nuts. As a follow up, I can
>>>>>> guess, but what will the renderer do in that situation? I'm
>>>>>> guessing
>>>>>> mapnik will give up trying to put 30 names on a one mile road and
>>>>>> won't notice they're the same name?
>>>>>
>>>>>
>>>>> If both from and to ways continue after the via point and neither
>>>>> is one-way, there's two possible ways to interpret it: the
>>>>> restriction could apply when coming from either of the ends of the
>>>>> from-way. This of course doesn't matter if there is similar
>>>>> restriction coming from both directions, but that's not nearly
>>>>> always the case. And even if there is symmetry in the real life
>>>>> restrictions, it's not appropriate in my opinion to map those with
>>>>> just one restriction.
>>>>
>>>> eh? don't you assign direction by saying 'from' and 'to' ?
>>>>
>>>
>>> Yes in the sense of which of the two ways you are coming from, but
>>> if the way is not one-way and it doesn't end at the via-node,
>>> there's two possible directions from where you can come to the via-
>>> node using the way.
>>
>> Um... no.
>>
>> The restriction has handedness - left or right... and the way coming
>> off it has an angle.. lets try some ascii
>>
>> B
>> |
>> |
>> |----------C
>> |
>> |
>> |
>> A
>>
>> I am going from A to B. There is no 'right_turn' restriction on the
>> corner that stops me turning to C.
>>
>> That cannot be interpreted as a restriction from B to A as it would  
>> be
>> a left turn, not a right turn. To figure that out you just need to
>> compute the angle it makes with your direction of travel to see if
>> it's left or right?
>
> The no_left_turn, no_right_turn is only to indicate the type of
> streetsign to show AFAIU.
>
> Practically, adding angles to the specification will be a hell to
> implementers, and there are few use cases that would benefit from
> this.  Sometimes you will have a way splitting off to C that first
> turns slightly left, enters a tunnel or viaduct and then goes on the
> other side of AB, something that at low zoom level looks as in your
> drawing, and the streetsign might stilll be no_right_turn.
>
> Or something like this is common:
>
> B  C
> \  |
>  \ |
>   \|
>    |
>    |
>    A
>
> where the straight line is considered a turn even though it's
> straight, and the turn from A to B is considered straight even though
> it's an arc :P
>

This is yet another kettle of fish of how do you get the routing  
engine to tell you when the general flow of traffic is from A - B,  
even so the road name of A is the same a C, but different to B. I have  
come across a lot of these on my travels, and still haven't come up  
with a way to tag it.

Shaun