What you describe is a mini-roundabout.
That has a different geometry as the center of that one is traversable.
https://wiki.openstreetmap.org/wiki/Tag:highway%3Dmini_roundabout

a)
I don't see a node as anything you "are on" at any time. Only segments.
At most nodes are considered for calculating the metric of making certain turns
between segments.
b)
Routing algorithms that don't know or deal with roundabouts would still work
perfectly well with a circle of segments and give proper instructions.
c)
In reality this is a circle of road-segments. So segments represent reality more closely. So for the purpose of the map as a representation of real world geometry, this is simply a much better approximation. This is not only for routing but also for map-rendering to scale the size of the roundabout correctly. (There are vast differences in possible sizes.)
d)
These segments have a significantly different metric then an intersection (much slower traffic
in the roundabout then the surrounding roads).
They have an angle to the entering and exiting road that can be used in a metric because you need to slow down to make such hard turns, limiting your average speed in the segments before and
after the roundabout (lookahead).
There may be traffic jams or construction sites blocking part of a roundabout but still allowing certain turns to be made. This can not be described with a simple node.


On 2018-02-14 15:40, Dave F wrote:
Hi
Could anyone give me an explanation for this line from
https://wiki.openstreetmap.org/wiki/Tag:junction=roundabout

"Each road has to be connected with the roundabout in a separate
node—that is, between these nodes a segment of the roundabout is
required."

I see no requirement for a separate segment:

        * When a entering road shares a node with a roundabout then the
router knows it's entered that roundabout by reading the tags on the
circular way.
        * Whilst on that node, the router checks to see if there are any
suitable exits. If there are, then it leaves the roundabout.
        * If not, it continues going around until it finds an appropriate
exit.

 Cheers
DaveF
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