[OSM-talk] Roundabouts - why is a separate segment required?

Colin Smale colin.smale at xs4all.nl
Wed Feb 14 16:50:57 UTC 2018

Based on my experiences with mkgmap it's not so much a routing problem
as a navigation problem. The router will pick the correct path through
the graph but the translation to "human instructions" get confused, like
the exit numbers and the way the roundabouts display. Turning right at a
roundabout, i.e. taking the third exit, might show as straight on and
the instructions may refer to the first exit.

On 2018-02-14 17:39, Dave F wrote:

> I think I have read it correctly.
> https://www.openstreetmap.org/node/5408566797
> It is easy to determine this shared node is part of the roundabout as well as the entrance from Wapping & can exit along Commercial, or if required, continue around the roundabout:
> How is this different from, say, two side roads joining a main road at the same node?,
> Or even cross-roads. The router has to check to find out what road it's crossing & find the appropriate exit, which, in the case of cross-roads, will be on the same node.
> DaveF
> On 14/02/2018 16:17, Maarten Deen wrote: On 2018-02-14 15:53, 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. 
> I'm not sure if you read the requirement right, but this tells mappers not to connect the entry and exit road on the same node. If you were to map it that way, the router will not see that you enter a roundabout and need to exit at the first exit. It will just tell you to go right.
> It is not (what I think you think) that there needs to be a separate way between entrance and exit, the roundabout can be mapped as one way in total.
> Maarten

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