[OSM-talk] Turn restrictions

David Stevenson david at avoncliff.com
Thu Aug 16 11:35:19 BST 2007

Jon Bright wrote:
> Steve Coast wrote:
>> Given a road direction, and roads 'Baker Street' and 'Marylebone  
>> Road' you tag the way 'Baker Street' with
>> no_turn_left:'Marylebone Road'
> As an alternative, what about instead splitting the ways appropriately 
> for making turning restrictions?  So if I have
>                     |  /
>                     C E
>                     |/
>                     v
> ----A--->+----A--->+----A--->+----A--->
>           ^         ^         ^\
>           |         |         | \
>           B         D         F  G
>           |         |         |   \
> Then I could split way A into four parts, A1, A2, A3 and A4 (left to 
> right).  Assuming no turns at all from A into the side roads are 
> allowed, I could then mark
> A1: no_right_turn
> A2: no_right_turn, no_left_turn
> A3: no_right_turn
> This covers travel in the direction of A's segments.  Assuming A is 
> two-way and no turns are allowed from A to the side roads in the other 
> direction either, you could additionally add
> A4: opposite:no_left_turn
> A3: opposite:no_left_turn, opposite:no_right_turn
> A2: opposite:no_left_turn
> The only problem remaining is how to represent the case that it's 
> permitted to turn into C but not into E (ditto F/G).  But I've no idea 
> how this would be represented in street signs either.  Does anyone have 
> an example of C/E or F/G-like things with turning restrictions?
> The advantages:
>   - No chance of misspelling (or becoming out of date when someone 
> changes/corrects a name)
>   - No changes to JOSM needed.
>   - A route planner doesn't need to make an additional query in order to 
> work out which way it may not turn into.  (I have a working route finder 
> - as yet without descriptive capabilities - based directly on the OSM DB 
> - the number of queries is already quite enough :-)
+1 for me this is easy and efficient.

For full route instructions  we will also need road priorities. Recent
experience with Tom Tom shows how on many junctions they have got this
wrong, giving instructions for minor junctions and ignoring Stop signs.


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