[OSM-talk] turn restriction relations: via

[marcus.wolschon at googlemail.com]

On Tue, 31 Mar 2009 08:55:54 -0400, Greg Troxel <gdt at ir.bbn.com> wrote:
> 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.

There is no need for any additional relations to describe
these turn-restrictions.
It's just a bunch of "only_ahead".

However a relation to group the left and right lane of a dual-carriageway
is a very welcome thing for many algorithms.
What about:
type=road to collect the ways of a long street and if it's a
dual-carriageway
some may have a role of "left" or "right" while for single carriageways
and sections where the ways are joined temporarily have some other role.

Marcus