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

```
Am 14.02.2018 um 18:21 schrieb Dave F:
> On 14/02/2018 16:46, Marcus Wolschon wrote:
>>
>> 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.
>
> I think I understand what you're saying but for clarity could you
> provide an example.```
```
The wiki page talks about them as "one way lateral shortcuts" just above
the seldom case of roads crossing the middle of the
roundabout. (I often see that as special ways for oversized transports
near industrial areas.)

The lane on the very right is such one.

>
>> Only if you calculate the angle in an euclidian XY-plane for each one
>> and then sort them in clockwise
>> or counterclockwise fassion.
>
> The geometry is irrelevant. Entrances/exits can be determined because
> they don't contain a junction=roundabout tag.

It is not irrelevant as you need to tell the user "enter the roundabout
and use the THIRD exit".
How do you plan to determine how many exists to pass?

>
>> Assuming you can find out what side of the road people drive on in
>> this part of your route.
>
> 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.

It's not the same. Using the second exit from the right or the fourth
from the left is different.
Also this is one of the very few places where the direction of driving
actually matters for routing and only in your node-based aproach.
Simple because the tag oneway=yes is explicitely implied in the
segment-based aproach as stated on the Wiki page.

>
>> Something that can be avoided altogether with oneway segments making
>
> All ways with junction=roundabout are one way.

Yes and your proposal is a single node instead of a circle of segments.
How can a node be a oneway and thus determine a direction for counting
the exits?

>
>>
>>>
>>> DaveF.
>>>
>>>>
>>>> Phil (trigpoint)
>>>>
>>>> On 14 February 2018 15:38:01 GMT+00:00, Dave F
>>>> <davefoxfa...@btinternet.com> wrote:
>>>>
>>>>     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
>>>>
>>
>> A nice example of 2 shared nodes making up 1 exit.
>>
>>
>>>>     Dave F.
>>>>
>>>>     On 14/02/2018 15:21, Dave F wrote:
>>>>
>>>>         On 14/02/2018 15:02, Marcus Wolschon wrote:
>>>>
>>>>             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.
>>>>
>>
>> Irrelevant to your point but not to mine.
>>
>> 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.
>> 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?
>>
>> 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.
>>
>> 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.
>>
>>
>>
>>>>         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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>>>>
>>>>
>>>> ------------------------------------------------------------------------
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>>>>
>>>>
>>>>
>>>> ------------------------------------------------------------------------
>>>>
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>>>>
>>>> --
>>>> Sent from my Android device with K-9 Mail. Please excuse my brevity.
>>>>
>>>>
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