[Talk-ca] Mapping intersections properly - the impact on routing and illegal routing maneuvres

[Richard Weait]

On Thu, Apr 8, 2010 at 4:33 PM, Marc Provencher <marc at marcprovencher.com> wrote:
> I recently posted with an open question asking how others map junction nodes
> at complex intersections.
>
> Today I was trying to reproduce certain routing errors which I observed
> while driving, and was able to cause Mapsource to route me through an
> illegal U-turn, because of a misplaced junction node where there shouldn't
> be one.
>
> This picture shows the U-turn:
> http://provencher.smugmug.com/Other/Misc/IllegalUturn/831296827_sYc5e-X3.jpg
>
> The problem lies with the access ramp,which has a junction node on both ways
> of the divided residential road.  And for some reason the routing algorithm
> doesn' t consider this a U-turn, even thought the "avoid U-turns"  setting
> is enabled.
>
> Actually, the real problem is that the routing algorithm is using these
> "connecting ways"  between the two sides of a divided road, to make a
> U-turn. I'm not sure if this problem can be resolved easily.
>
> In my attached picture, removing the junction node which I thought was the
> problem simply moved the problem to another intersection.

I see this fault from navigation systems too.  I'm not sure that the
commercial vendors worry about this error.  Their systems seem to
suggest those illegal turns often enough.

We can do better, and eliminate these routing errors with turn
restriction relations.
http://wiki.openstreetmap.org/wiki/Turn_restrictions

See what you think.