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

Dave F davefoxfac63 at btinternet.com
Wed Feb 14 16:39:29 UTC 2018

I think I have read it correctly.


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.


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

More information about the talk mailing list