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

Johan C osmned at gmail.com
Wed Feb 14 18:23:39 UTC 2018


2018-02-14 18:50 GMT+01:00 Dave F <davefoxfac63 at btinternet.com>:

> 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.
>
>
No, they are not. Roundabouts are special types of intersections. 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.


> DaveF
>
>
>
>
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