[OSM-talk] turn restriction relations: via

Greg Troxel gdt at ir.bbn.com
Tue Mar 31 13:55:54 BST 2009

  As you can see there is a roundabout, but there is also a dual
  carriageway through the middle with the flow controlled by traffic
  lights. If you are in the lanes which go through as the dual
  carriageway you can't turn onto the roundabout, and if you are in
  the lanes that lead onto the roundabout you can't turn onto the dual
  carriageway lanes. I think for the relations to work I might have to
  split the roundabout from a closed way into a number of sections,
  but I've always drawn roundabouts as closed ways before. Will
  splitting it cause issues with anything else? Although I might not
  have to split as the via (node) will indicate where the restriction
  actually applies, even when the dual carriageway lane crosses the
  roundabout at two nodes. Or perhaps I could just split the dual
  carriageway lanes somewhere in the middle of the roundabout to make
  things easier for me to work out what restrictions I need to add.

I haven't thought this through, but I wonder if you could add relations
to describe road groups, putting all the dual carriageway ways in one
relation, and all the roundabout and link ways in another, and then
adding a turns_prohibited restriction between those two relations.  This
seems pleasing since there is logically one extra rule to explain to
humans - no turning between the roundabout and the dual carriageways
that cross it.  Of course this nested relation/restriction complicates
routing software but perhaps not too badly.

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