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
>> <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.
>>             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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