[OSM-talk] Advanced relationships

[Nic Roets]

On 6/13/07, Frederik Ramm <frederik at remote.org> wrote:
> You are the first one who suggests to abuse the data model to describe
> routing information.

My thinking : If you are allowed to make a U-turn immediately after
the intersection, the autorouter may consider the possibility of
crossing the intersection, immediately making a U-turn, then making a
left (from the other side). This is only slightly longer than making
the illegal right turn.

And if your are not allowed to make a U-turn immediately after the
intersection, then according to
http://wiki.openstreetmap.org/index.php/Editing_Standards_and_Conventions
 (The general rule is that if it is not possible to turn across the
street at any junction then it should be drawn as two paths) you need
2 different ways and they are definately oneway.

Let me just add that I hate dual carriageways, because it takes some
effort take to map them properly and they are usually rendered poorly
if they are too close together. Perhaps a solution would be to allow
them to be modelled as a single way provided they are tagged with
"uturn=no" or something. We can also introduce a tag for ways like
"forward_may_turn_right_at_intersections=no" and
"backward_may_turn_right_at_intersections=no". "Forward" indicate that
the user is moving from the "from" node to the "to" node and
"backward" the opposite, i.e. if "oneway=yes" then the value of the
backward_may_turn_right_at_intersections tag will be ignored.

We may also need a tag for nodes that state that sharp turns aren't
allowed, e.g. where a slipway enters a busy street. Or good routing
software can look at the angle and when it exceeds say 145 degrees it
will qualify as a U-turn.