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

[Dave F]

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.

>
> 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".

Noting my comment above, if a router doesn't tell you the latter, then 
it's a poor program. Commercial Rd is the first exit

> 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. If 
it can do it at intersections it can do it on roundabouts.

Roundabouts are just another type of intersection.

DaveF