[talk-ph] tips for mapping roundabouts

[Ronny Ager-Wick]

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After doing some routing tests, I found I had been mapping roundabouts in a
way that is confusing to routing software.
The problem is that when you have a barrier separating the entry and exit
lanes of a roundabout, but nothing between an exit lane and the next entry
lane (the first exit), like on the west side of the example roundabout, it's
very easy to make those two lanes go to and from the same node in the
roundabout. This Seems to be confusing for routing purposes, as that node is
not counted as an exit.

This is an example:
https://www.openstreetmap.org/#map=19/15.15286/120.59208
If you come from the bridge north of the roundabout, going south on MacArthur,
Poinsettia Avenue (which name I will double check today) is the first exit,
but if the MacArthur Highway entry lane happens to share a node in the
roundabout with the Poinsettia Avenue exit, routing software will tend to call
that a "right turn", not "first exit in the roundabout". More confusingly,
continuing on MacArthur southbound is reported as "Second exit in the
roundabout", which may have the unintended effect of directing drivers to Jake
Gonzales Blvd.
I've already fixed this roundabout, but I'm sure there are many more with the
same problem.

In a way it makes sense. If the exit and entry nodes are the same, you don't
actually enter the roundabout, you just touch it as you pass by.

In order to avoid this issue, always use separate entry and exit nodes in
roundabouts.

Ronny.
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