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

[Maarten Deen]

On 2018-02-14 18:50, Dave F wrote:
> On 14/02/2018 17:13, Maarten Deen wrote:
>> 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.
> 
> Yes it can. It has the junction=roundabout tag on the way.
> 
>> Technically speaking you are not because you are just touching one 
>> node of the roundabout.
> 
> Yes you are. You may not be on there very long, but you approach the
> roundabout, pass the signs saying it's a roundabout, give way to those
> already on it, you enter it & then indicate that you're leaving it.

Not from a data standpoint.

>> 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.
> 
> A router has to be aware of it & know what it's attributes are, to
> decide if it needs to go along it. It does this from a *single* node.

It doesn't work like that anywhere in OSM. I can cross a road that I'm 
not allowed to drive on. The router does not need to know anything about 
the road that I'm crossing and I can always cross a road that I'm not 
allowed to enter.
It would make mapping extremely awkward if that were not so.

Maarten