Transitions between divided to undivided roads

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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) ?


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