[Routing] Roundabouts - why is a separate segment required?

[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".



> 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.

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
>
>     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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