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

[Dave F]

On 15/02/2018 10:05, Maarten Deen wrote:
> On 2018-02-14 19:39, Dave F wrote:
>> On 14/02/2018 18:23, Johan C wrote:
>>
>>> No, they are not. Roundabouts are special types of intersections.
>>  Which is another type of intersection.
>>
>>> They have a way on which you can drive round. And round. And round.
>>> And they have other ways leading to and from this round way.
>>> Whenever you enter the roundabout you drive on this round way, even
>>> if it's just for a metre. And then you exit this round way on to a
>>> different way.
>>>
>>> The present tagging (used since 2005 or so, and all around the
>>> globe) is fine.
>>
>> To repeat myself. You can determine if you need to "drive on this
>> round way" from a single node. No need for a section between entrance
>
> You can not determine that from a single node. You need to load the 
> whole way that connects to that node



But that what happens in all roundabout situations. You enter at a node 
& read the tags of the way which contains that node. Having an exit way 
attached to that node doesn't prevent this.

https://www.openstreetmap.org/node/5408566797

> and than make a judgement call which roads connected to that node you 
> will traverse (which you don't know, because from a topology 
> standpoint you are not traversing that way).

In my example you can see you're on a roundabout & there's an exit. You 
can load both ways & calculate the correct direction to take, similar to 
a crossroads at a single node.

>
> It is like Matej's example.
> Suppose it is mapped with the entry and exit road connected to one 
> node. Yes, you can see if there also connects a roundabout to that 
> node and you can make the determination that in that case you need to 
> traverse the roundabout in the correct direction.
> But suppose there is not a roundabout connected but a (circular) way 
> with a oneway direction. Then you also need to make the decision that 
> you have to traverse that way.
> But suppose the way is not circular (making you cross or touch a 
> oneway street), than you can not do that.
> When is a road circular? Most roads are circular from a topology 
> standpoint, as in: you can reach a node on that way going in either 
> direction. So you can not determine from a topology standpoint if a 
> road is a circular road or a roundabout.

If what you say were to be true you would have problems navigating all 
types of junctions. Every junction is entered at a *node* all connected 
way's information is loaded by processing that node.

DaveF