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

```On 14/02/2018 16:46, Marcus Wolschon wrote:
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We often have roundabouts with dedicated lanes leading only to the (heavily trafficed) exit just off to the right. These are not part of the logical roundabout and the particular traffic rules regarding roundabouts do not apply. Yet they share nodes with the roundabout as you can freely switch lanes in that circle segment.
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I think I understand what you're saying but for clarity could you provide an example.
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Only if you calculate the angle in an euclidian XY-plane for each one and then sort them in clockwise
```or counterclockwise fassion.
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The geometry is irrelevant. Entrances/exits can be determined because they don't contain a junction=roundabout tag.
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Assuming you can find out what side of the road people drive on in this part of your route.
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Any person writing a routing/navigation shouldn't be doing it if they can't determine that. And anyway it's irrelevant to my point - it's the same in either direction.
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Something that can be avoided altogether with oneway segments making up the roundabout.
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All ways with junction=roundabout are one way.

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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

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A nice example of 2 shared nodes making up 1 exit.

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```    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.

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Irrelevant to your point but not to mine.

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The purpose of this map is much more then just routing for motorized vehicles.
```Representing the real road as accurate as possible is a major point here.
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Or do you proclaim that e.g. accurate graphical rendering of a map is not important for anyone? That pedestian crossings on the legs of a roundabout are not important for anyone?
```That the roundabout-segment a postbox is at is not important for anyone?

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Also for vehicle routing, calculating the metrics as preicsely as possible is a major quality factor in good routing. So if using a roundabout is much slower itself and slows you down in front of (decellerating) and behind the roundabout (accelerating) compared to a simple right-turn, then this is an imporant thing to model correctly.
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If a construction site or traffic jam blocks one exit, your model would block the entire roundabout instead of just that exist. Causing the driver to be routed way around that intersection while for
```his/her particular route it poses not much of an issue.

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```        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 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
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
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
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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