On 14/02/2018 16:46, Marcus Wolschon wrote:


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.

I think I understand what you're saying but for clarity could you provide an example.

Only if you calculate the angle in an euclidian XY-plane for each one and then sort them in clockwise
or counterclockwise fassion.

The geometry is irrelevant. Entrances/exits can be determined because they don't contain a junction=roundabout tag.

Assuming you can find out what side of the road people drive on in this part of your route.

Any person writing a routing/navigation shouldn't be doing it if they can't determine that. And anyway it's irrelevant to my point - it's the same in either direction.

Something that can be avoided altogether with oneway segments making up the roundabout.

All ways with junction=roundabout are one way.



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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