I think both would be appropriate?

Looking at https://wiki.openstreetmap.org/wiki/Key:traffic_calming , I see there are already some values in use for purely painted-on measures (for example "painted_island"). There's not a value that would indicate an artwork though.

This seems to be a not uncommon thing: https://duckduckgo.com/?t=ffab&q=street+mural+to+slow+traffic&ia=web . I think it makes sense to come up with a good tag combo.

Maybe 

traffic_calming=artwork
tourism=artwork
artwork_type=ground_mural

