fkv
There is no clear distinction between roads and plazas. You can see Graben as a plaza, with its width of 29 m where you can freely walk around, and you can see Stephansplatz as the road around the cathedral. Stephansplatz and Stock-im-Eisen-Platz are currently mapped as roads (ways 311792006 etc.) and as areas (relation 152813, way 27497960). Having a feature mapped twice with the same tags is generally a bad idea. When the highway=* exists as a line, consensus is that the area should be tagged with area:highway=*, although this is not yet approved.
With my previous comment I didn't intend to trigger such a fundamental tagging discussion. I certainly just meant the 10m gap between Stock-im-Eisen-Platz (way 27497960) and the Haas-Haus.