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

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

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

Marcus Wolschon
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
> _______________________________________________
> Routing mailing list
> [hidden email]
> https://lists.openstreetmap.org/listinfo/routing

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

Dave F
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.

DaveF

> 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
>> _______________________________________________
>> Routing mailing list
>> [hidden email]
>> https://lists.openstreetmap.org/listinfo/routing


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

Dave F
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:
>> 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.
>> 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
>>> _______________________________________________
>>> Routing mailing list
>>> [hidden email]
>>> https://lists.openstreetmap.org/listinfo/routing
>
>
> _______________________________________________
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> [hidden email]
> https://lists.openstreetmap.org/listinfo/routing


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

Philip Barnes
If an entering way shares a node with an exiting way there is no need to pass through a roundabout way to navigate between them hence the roundabout will not be seen. It also messes up the exit count in navigation instructions.

Phil (trigpoint)

On 14 February 2018 15:38:01 GMT+00:00, Dave F <[hidden email]> 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

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.

DaveF

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

Dave F


On 14/02/2018 15:46, Philip Barnes wrote:
If an entering way shares a node with an exiting way there is no need to pass through a roundabout way

If that shared node is also part of junction=roundabout, then it does "need to pass through".



It also messes up the exit count in navigation instructions.

How? It has the same number of exits/entrances, no matter if they share nodes. They're all still countable.

DaveF.


Phil (trigpoint)

On 14 February 2018 15:38:01 GMT+00:00, Dave F [hidden email] 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

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

Marcus Wolschon



Am 14.02.2018 um 16:55 schrieb Dave F:


On 14/02/2018 15:46, Philip Barnes wrote:
If an entering way shares a node with an exiting way there is no need to pass through a roundabout way

If that shared node is also part of junction=roundabout, then it does "need to pass through".


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.





It also messes up the exit count in navigation instructions.

How? It has the same number of exits/entrances, no matter if they share nodes. They're all still countable.

Only if you calculate the angle in an euclidian XY-plane for each one and then sort them in clockwise
or counterclockwise fassion.
Assuming you can find out what side of the road people drive on in this part of your route.
Something that can be avoided altogether with oneway segments making up the roundabout.


DaveF.


Phil (trigpoint)

On 14 February 2018 15:38:01 GMT+00:00, Dave F [hidden email] 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. 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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Re: [Routing] Roundabouts - why is a separate segment required?

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.

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.

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.

Something that can be avoided altogether with oneway segments making up the roundabout.

All ways with junction=roundabout are one way.



DaveF.


Phil (trigpoint)

On 14 February 2018 15:38:01 GMT+00:00, Dave F [hidden email] 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. 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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Re: [Routing] Roundabouts - why is a separate segment required?

Marcus Wolschon



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.
https://upload.wikimedia.org/wikipedia/commons/thumb/a/a3/Blackwood_roundabout.jpg/1482px-Blackwood_roundabout.jpg


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 up the roundabout.

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 [hidden email] 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. 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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Re: [Routing] Roundabouts - why is a separate segment required?

Dave F
In reply to this post by Marcus Wolschon
On 14/02/2018 16:46, Marcus Wolschon wrote:
>
> A nice example of 2 shared nodes making up 1 exit.

Err. it's clearly one node, with one entrance, one exit, & one continue.

>
>
>
> Irrelevant to your point but not to mine.

Err.. but it was my point being discussed. I started the thread!

> Or do you proclaim that e.g. accurate graphical rendering of a map is
> not important for anyone?

Are you serious? The reason I mapped it as such was to make it accurate.
Adding a separate section would make it *inaccurate*

> 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.

Again, I think you've gone way off piste.

DaveF

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