[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
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>
>
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