[OSM-newbies] Transitions between divided to undivided roads

[Murry McEntire]

I am looking for best practice on handling transitions between divided and
undivided segments of a road.

In the area I live (Colorado, United States) there are a large number of
roads that transition between divided and undivided segments along their
length. Typically, the divided segments are two lanes each direction with a
physical barrier between them (OSM convention seems to be to represent
these as two separate two lane one-way roads) and the undivided segments
are five lanes total with two lanes each direction and a center turn lane.
The segments are the same width where they transition, the physical barrier
being a lane in width at the transition though it may be much wider or
narrower elsewhere on the divided segments. From my view, the divided roads
are best handled as two one-ways as the barrier can be rather wide at times
(10 meters (yards) or more) and the handling of intersecting roads that can
only turn right on one one-way is better handled. A transition may occur
either at an intersection with another road or at some point along the road
away from an intersection.

Following convention, the undivided road segments are plotted along the
middle of the road (middle of the center turn lane) and the divided roads
are each plotted down their own center (the center line divider between the
two lanes).

For transitions away from an intersection, the existing practice I
typically see is a line segment from each divided section angle from the
center of their road to the center of the undivided road (forming a "Y").
Rendering suggests the angle between the two top segments of the "Y" should
not be too great or the one ways appear to turn at the transition when they
do not, whereas physically this angle should be 180 degrees (making the "Y"
a "T")  since the transition is abrupt and the segments forming the top of
the T should not be considered a turn or rendered. This "T" representation
also better matches with the change in number of lanes of a road in OSM
which is tagged by splitting  the road at a node, using different lane
counts for each way as opposed to handling such a change over some short
length of the way.

Is there any tag that could be given to the way segments forming the top of
the "Y" essentially telling renderers to ignore them?

Without such a tag is there a recommended interior angle (30, 60, 90
degrees) for plotting the ways forming the top of the "Y" ?

For transitions at intersections, I typically see each divided one-way road
connected to the cross road at a node that would be the true physical
center of the one ways, and the undivided road connected to the crossing
road at it's (the undivided road's) physical center resulting in three
connections to the cross road but no way labeled with the road's name and
other info connecting the (divided) one ways to the undivided road of the
same name. This looks good in the renderer, but when tracing the ways you
would appear to enter the intersection on one road, turn onto the crossing
road, then turn again to leave the intersection on the same name road which
is logically strange (if not outright inaccurate) and must be hard for
routers to handle.

Is there some sort of tagging that should be used on the cross road ways
between the three nodes of the transitioning road to indicate they are
connected, no use of the crossing road is actually done ?

If not, would this be better handled by putting a "Y" just one side or the
other of the intersection so the same number of ways leave the crossing
road as entered it for the same named road? (I see problems with traffic
circles and other complex intersections where this might not work well.)

If such does not exist. it seems like it would be useful for a tagged
single segment way connecting the divided one-way roads to the undivided
road that says "do not consider me a turn, do not render me, but I can be
used for routing along the same named road" or an intersection tagged node
that says the equivalent about immediate single segments of connecting
ways, or a intersection way (line, box or other shape including nodes at
all accurately placed entering ways) that would be similar to the effect of
a roundabout without the oneway and ordering of nodes (i.e.,
junction=intersection) ?

Murry
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openstreetmap.org/pipermail/newbies/attachments/20130418/bdc1fd01/attachment.html>