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.

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.

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.

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

• junction=roundabout and possible misinterpretations
• junction=circular

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:

• Bersarinplatz
• Kottbusser Tor
• Strausberger Platz
• Grosser Stern

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