Traffic Island: landuse=grass vs surface=grass

I prefer surface=grass and the traffic island is part of the land reserved (i.e. used for) a highway.

Actually, I much prefer landcover=grass but the landcover tagging has not gained much traction and I am not aware of a major map rendering site or app that recognized landcover tags.

Technically they are all different tags.

landuse=grass mostly exists because of the easily unpleasing semantics of natural=, and lacking support of landcover=. There were no other top-level feature in the past. Problem is it is inside a landuse=highway in this case, and the rest are inside other landuse= (including landuse=railway).

surface= alone is not valid syntax. It is an attribute describing a feature, usually highway= and leisure=pitch, also natural=beach. landcover= would be more orthodox on its own.

There has been area:highway=traffic_island + surface=grass for a long time (a decade). That's what should be used.

(A deeper issue is a "traffic island" is not the same as a median / central reservation, if that's what you are looking for. The definition is less distinctive on short sections, or at junctions. In any case, don't use traffic_calming=island if it's not traffic-calming.)