But, as far as I can imagine, the other way around, all pedestrian refuges are traffic islands, right? Or can anyone think of a case where this is indeed not the case?
Otherwise I would add a hint in the wiki for data consumers that they should also look at crossing:island=yes (and the deprecated crossing=island since its usage is still in the thousands).
And finally, if any pedestrian refuge is anyway also a traffic_calming=island, would you add that tag on that crossing?
Currently, it looks like about only 3% of pedestrians crossings with an island also have traffic_calming=island tagged.
However, the combination of a kerb extension and pedestrian crossing is found often found in the real world1, yet, still just about 6% of all traffic_calming=choker are also tagged as a pedestrian crossing, so this might as well just be a case of that it hasn’t been mapped yet.
1 at least where I live this is done almost at any intersection that leads into a residential street when a road is being renovated
My understanding so far has been that these two tags are mutually exclusive. While a crossing:island=yes also calms traffic, that is not the purpose of it. The wiki of crossing:island says:
traffic_calming=island - A traffic island that is not built as a refuge at pedestrian crossings, but to calm traffic.
I would not add traffic_calming=island to these crossings.
For vast majority: yes, though I have not considered tagging them also as traffic_calming=island
I have seen some build (deliberately or due to incompetence, hard to say) that provide refugee for pedestrians and allow them to cross road in parts but result in no traffic calming whatsoever, cars can drive like they have not existed anyway.
There are also cases where there is traffic island that protects cyclists, especially on roads with contraflow, but does impede cars at all. Is it qualifying as traffic calming?
By the way: I’ve seen a lot of traffic islands that were tagged with traffic_calming=island, but whose purpose is not to calm traffic, but to “guide” the traffic, especially on crossings.
1 is a separator that also acts as a pedestrian crossing and 2 is just a separator, yet both were tagged with traffic_calming=island. Even the triangle bit is/was tagged this way. I feel like we have an over-tagging of these, maybe because they are all called the same in certain Regions? At least in Germany, they are all called „Verkehrsinsel“ (traffic island), no matter the purpose. Most actual traffic calming islands I’ve seen in Germany, are very much like the picture in our Wiki: at the entrance of a town/village, before a curve.
So contrary to what I’ve said before, I’m sure a pedestrian crossing island can also act as a traffic calming, but it’s definitely not always like this. So maybe it really does make sense to tag this for pedestrian crossings that act as traffic calmers as well. But we should definitely not tag every island as traffic calming as seen in the picture. I guess the use is disputed, but area:highway=traffic_island just makes much more sense.
Yeah, that is an open discussion on the discussion page for traffic_calming - many traffic islands do not really calm traffic, but guide traffic.
In my opinion it is too much to ask of OSM mappers to draw a distinction between the function of a traffic island (and it may be a fluent transition / both at the same time anyway). Also, whether or not a traffic island actually makes the road narrower is something that can be derived from other tags, namely width.
So anyway, what I want to say is: there is no traffic_guide=island, so I expect that everyone just uses the traffic_calming=island for traffic islands of any sort.
I fully agree. And for this reason, either all crossing:island=yes could be considered to be traffic_calming=island, or can be tagged as such, because the current tagging practice seems to be “traffic island, no matter which” But I might be seeing this too pessimistic.
From a data consumers point of view, it doesn’t matter if crossings with crossing:island=yes are also tagged with traffic_calming=island because right now, any data consumer that is interested in finding all traffic islands would be very well advised to also look at crossing:island=yes anyway because only 3% of such crossings also have the traffic_calming=island tag set.
It might be worth creating a distinction between devices designed to slow traffic (traffic_calming) vs those meant to keep higher speed lanes seperated without impeding them (traffic_guiding). That way road islands can be mapped for more than just calming. For example those use to enforce turning lanes.
as seen in the picture. I guess the use is disputed, but 
But I might be seeing this too pessimistic.