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

Maarten Deen mdeen at xs4all.nl
Wed Feb 14 17:13:38 UTC 2018

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?,

Because a machine can not determine if you are actually entering the 
roundabout or not. Technically speaking you are not because you are just 
touching one node of the roundabout.

Suppose your example with the two side roads, if you were to go from 
Side Road A to Side Road B via the node that is connected to Main Road 
A, would you want the router to give you instructions to "enter Main 
Road A and turn into Side Road B"? No, it gives you directions to "turn 
into Side Road B". The same at this roundabout, going from Wapping Road 
to Commercial Road it will tell you to "turn left into Commercial Road" 
and not to "enter the roundabout and exit at the first exit into 
Commercial Road".
At may be that the road layout is such that you don't enter the 
roundabout, I don't know but when you do have to enter the roundabout, 
you have to leave a segment between the entry and exit or the routing 
instructions will be wrong.

Just connecting to a road on a node does not mean you enter that road. 
The same at intersections, if you cross a road (connected by a node) you 
do not enter that road so you do not need instructions for it.


> 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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