Traversable roundabouts

in Germany we want to tag roundabouts which are traversable and which are too large to be tagged as “mini_roundabout”. In a voting the favourite tag was:

Is this term correct in the english language?


As a native speaker of US English (where we don’t have a lot of roundabouts, but we do have them, especially in the state of Indiana), the term “central island” does resonate as “the part in the middle of the roundabout,” usually cars don’t drive here, and it may be offset with a kerb/curb or a paint line, or nothing.

My dictionary (US English, includes other dialects with notations like “Chiefly British”) does contain the word “traversable” as a derivative form (spelled exactly like that) of (the verb, noun, adjective) “traverse.”

So, putting these together as a tag (key=value), the semantic created in my mind (as a native US English speaker who also knows that British English is a preferred dialect for OSM tags) is “the part of the middle of the roundabout where people usually do not ‘drive’ (traverse) is actually able to be traversed (say, in an emergency, or if other rules / law allows it), without ‘hopping a kerb/curb,’” for example. More than that would require additional tagging, but I do get at least that much from central_island=traversable and it doesn’t have any spelling or grammar errors (to my sense of English).

I hope this helps.


Before that, I have to say I disagree with =mini_roundabout not being applicable when “too large”. There are even two-lane (entry) mini-roundabouts. And surely the ones at the Magic Roundabout are very big too. That being said, whether mountable kerbs and slightly raised surface fits the definition is a good question.
Yes, “Central island” is the term used in US’s MUTCD and UK’s DMRB. It is adopted by a Netherlands roundabout manual as well.
But first of all, you need to ask: where does central_island= comes from? Is it meaningful on the roads themselves? Are you going to tag splitter islands as well? If not, eg roundabout:island= (similar to crossing:island=), or even roundabout= (has been suggested) can already do the job first. The idea of central_island:diameter= raises the question of how to show the traversable (apron) and non-traversable (raised island) diameter on compact roundabouts. Indeed, how do you show the different surfacing and kerbing in that case?
On the other hand, central_island= might arguably be applicable to intersections. In Japan, it’s common to have hatched area markings at center to channelize right-turn vehicles. I forgot whether there are examples for physically raised islands.
I see questions being asked about traffic_calming=island for the area, which is not accurate. For central island alone, too small will result in insufficient deflection from the straight path; too large is a large diameter design promoting higher speeds. Wide splitter islands is a tangential connection geometry, that again promotes high entry or exit speed. area:highway=traffic_island can be used, without needing to determine whether it is “traffic calming”.

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The idea is to be able to map large “mini roundabouts” (that sounds so wrong) as ways, rather than nodes (for various reasons, but trust me there are valid reasons). We cannot put highway=mini_roundabout on a way, because we’d lose the road-classification. So there was a discussion, how to express that a roundabout has a traversable center island, because, ultimately, that’s the difference between a “mini roundabout” and a “regular roundabout”.
One of the suggested taggings was to add center_island=traversable to the junction=roundabout. I hope that clarify things a little bit.

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Struggling with the subject and classification of ‘mini’ roundabout and I’m giving 3 samples where physical separation exists between all or most of the on/off ways, with a pre-ample that all 3 have a traversable central island to facilitate long articulated or long non-articulated vehicles like city buses and heavy lorries, particular the first which is just 50 meters away from a railway loading dock entrance.

  1. This one was since long mapped as a full roundabout and remapped recently as a mini. 2 parallel one-way streets with physical separation and 2 regular streets meet at a (mini) roundabout. Flat painted central island.

This looks awful on Carto standard rendered map and in edit mode. Truckers might think twice when route planning with the sharp angle to the right when approaching from the west.

  1. Mini roundabout with raised traffic islands on all but one on/off way and painted central island changed to full roundabout mapping in recent days.

  1. Same as 2 except, all on/off are split by raised traffic islands, the central island is a bulge. Roundabout mapped in recent days.

To see the physical state switch to edit mode in ID editor, Bing or ESRI imagery.

Followed a German language discussion which centred on the ‘large’ and the pragmatic view was in summary, “it’s been mapped that way… just leave it alone” which was what was done.

Does the mini-roundabout wiki need a revision or amplification (?) as when there area physically separating traffic islands for the on/off it seems to me the full junction=roundabout mapping method is the one appropriate for the offered conditions, the absence of a raised central island one of facilitation for large/long vehicle traffic.

It could be conceived as an area, covering the lines render. The problem is then the entry & exit arms distortion in routing. Their geometry should somehow be ignored at a =mini_roundabout by applications?
It’s quite standard to have physically raised splitter islands for small ones. Including together in a mix with ghost islands (hatch-marked). (bottom 2)
In fact, it might be argued larges ones in question by this post are less severe, because the arms would be longer. The angle distortion is thus reduced. Only the stretching of distance for RHT right-turns is maintained, but that’s the same in non-raised splitter island =mini_roundabout otherwise.

You’re still describing a mini roundabout that should be mapped as a mini roundabout.

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That is the objective. But as a way instead of a node.

It’s best to think of a mini roundabout as less “roundabout” and more “weird intersection”, since the most major practical difference is whether left turns are supposed to quarter around the far side of the intersection and behind oncoming traffic turning left, or in front of it.

Routers that are vehicle-dimension-aware are usually checking to see if some reasonable estimate of steering geometry is going to offtrack too far to clear a turn. Which, in a mini roundabout, it’s not the island that’s a factor at all, it’s still the outsides of the intersection like any other intersection.

I’m not sure what problem y’all are trying to solve, but, for the reasons outlined in the previous paragraph, I strongly suspect solving that one’s only going to create a new set of related, harder to solve problems.

In Germany, a mini-roundabout is a normal roundabout, with exactly the same rules. But the central island is traversable. Only very large vehicles are allowed to drive over it.

Some mini-roundabouts are exactly the same size as normal roundabouts but have a traversable central island. For most users, there is no difference at all to a roundabout with a central island that cannot be driven over. And they are so big that we want to map the geometry.