[Openstreetmap] Re: key values to represent restricted turns

[Simon Hewison]

Gabriel Ebner wrote:
> On Mon, Jan 23, 2006 at 08:27:54PM +0000, Tom Carden wrote:
>> How about something like a property on a node like
>> no-turn=a.b,c.e,e.d; where a/b/c/d/e are segment ids for segments
>> connected to that node, and order matters?  All other combinations are
>> fair game for routing.
> 
> Can we have that on segments too?
> 
> E.g. there are some junctions (in Austria, at least) where you can drive from
> A to B but not from C to B:
> 
>           |
>           C
>           |
>      --D--+--B--
>           |
>           A
>           |

Restriction on the approach segment could work, but I don't like the 
idea of using segment IDs, for a start, it makes offline editing 
impossible, since the segment ID would not be available until it's been 
committed. Segment names are easy for everyone to keep up with.

If we reffered to segment names instead, that could work, so in the 
above example, both B and D could be named "Billy Street", and A and C 
could be named "Alfred Street"

given the above example, y

On segment C, you could have "no-left-turn=Billy Street", and that's the 
only place where the restriction needs to be. Obviously, you could turn 
right onto Billy Street (segment D), and all other turns permitted.

even on a more complex situation, this could work

         |
         C
         |
-----D--+--B----
         |\
         | E
         A

with the above rule, C-B is forbidden, but C-E is permitted, because 
despite it being a left turn, the name of segment E is not listed. If it 
was also forbidden, you could define segment E as "Edgar Street" and then:

no-left-turn=Billy Street,Edgar Street

but C-D is permitted (even though it is Billy Street) because it's not a 
left turn, it's a right turn.

This also makes the tags easy for both humans and machines to read.
-- 
Simon Hewison