OpenStreetMap

Routing — circular junctions

Posted by daniel-j-h on 19 December 2016 in English (English)

In which I outline the need for the recent junction=circular tag.

Traffic circles are a modern feature of intersection design. They come in multiple forms and shapes.

OSRM distinguishes between roundabout intersections, named, and unnamed roundabouts. Roundabout intersections are a special case of roundabouts that are small and offer up to four clearly distinguishable exits.

Roundabout Turn

To be considered a roundabout in the strict sense the road junction has to follow certain criteria. One of the criteria for a roundabout is that traffic in the roundabout has the right of way.

There are circular junctions which may look like and be commonly identified as roundabouts, but they are not roundabouts because they do not fulfill all of the criteria for being a roundabout.

For example, traffic in the roundabout at Bersarinplatz in Berlin has to give way to incoming traffic on B 96a, disqualifying it as a roundabout. This is in conflict with the desire to provide instructions of the form "use the second exit" from a navigation engine.

Bersarinplatz Route Before

Since a junction=roundabout tag is not appropriate, mappers will often document the intentional absence of any roundabout tag on these junctions using notes. This approach means however these junctions will not be tagged in a way that lets routing engines distinguish them from non-roundabout roads.

Notes are only an unreliable pseudo-tag, however, and don't provide any clear structured data. A junction=circular tag properly documents the mappers intent and makes it possible for routing engines to handle.

Bersarinplatz Route After

Check out the tags' Wiki pages for more detailed descriptions:

Thanks to Tom Pfeifer for pushing this endeavor forward, writing up the Wiki pages and tagging circular junctions in Berlin.

Here are some more examples of junction=circular tags in use:

You can read up on the original discussion with the Routing Engine. This feature is included in the latest OSRM v5.5 Release.

Comment from TheSwavu on 19 December 2016 at 21:53

I had thought that once a "roundabout" was big enough to have a name (or has places you might want to find a route to) it stopped being a junction. So you could say:

  1. Continue Petersburger
  2. Right into Bernsarinplatz
  3. Right into Petersburger

You may also want find a route to somewhere on Bernsarinplatz.

  1. Continue Petersburger
  2. Right into Bernsarinplatz
  3. Your destination is on the right.
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Comment from trigpoint on 21 December 2016 at 12:25

Roundabouts are certainly not a new idea, they have been in existence for over 100 years.

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Comment from ElliottPlack on 5 January 2017 at 16:48

Daniel,

Thanks for the post and for looking at the one in Baltimore. Can you double check that this oval roundabout in Towson is tagged correctly? This one has multiple lanes in places.

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Comment from daniel-j-h on 6 January 2017 at 12:06

I have no local knowledge of the roundabout in Towson - from what I can see the traffic inside the roundabout always has the right of way (needs to be confirmed). Therefore a proper junction=roundabout tag is appropriate. Multiple lanes is not an issue here.

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