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 <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
>
>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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