[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
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