[Routing] Roundabouts - why is a separate segment required?
Dave F
davefoxfac63 at btinternet.com
Wed Feb 14 17:21:13 UTC 2018
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
>>> <davefoxfac63 at 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.
>>> 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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>>>
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