# Re: [Routing] Roundabouts - why is a separate segment required?

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On 14/02/2018 15:46, Philip Barnes wrote:
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If an entering way shares a node with an exiting way there is no need to pass through a roundabout way
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If that shared node is also part of junction=roundabout, then it does "need to pass through".
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```It also messes up the exit count in navigation instructions.
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How? It has the same number of exits/entrances, no matter if they share nodes. They're all still countable.
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DaveF.

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Phil (trigpoint)

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On 14 February 2018 15:38:01 GMT+00:00, Dave F <davefoxfa...@btinternet.com> wrote:
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To be doubly clear, this is an example of a road entering a roundabout &
sharing a node with it:

https://www.openstreetmap.org/node/19091900

Dave F.

On 14/02/2018 15:21, Dave F wrote:

On 14/02/2018 15:02, Marcus Wolschon wrote:

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What you describe is a mini-roundabout.
```        No it wasn't. It was perfectly clear as I posted the
'junction=roundabout ' page. Much of the following is
incoherent to me. The rest is irrelevant to my point. DaveF

That has a different geometry as the center of that one is
traversable.
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
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
traffic jams or construction sites blocking part of a
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
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
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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