Hi Simon,
That's a Swiss question indeed, as most other countries only know sign 2.13 (car and motorcycle). I would tag 2.13 as motorcar=no and motorcycle=no and 2.14 as motor_vehicle=no.
Maybe a better way would be to tag 2.13 as motor_vehicle=no, moped/mofa=yes, as it is not yet clear if moped/mofa is a child of motorcycle or not.
http://www.admin.ch/ch/d/sr/741_21/a19.html
http://www.admin.ch/ch/d/sr/741_21/app2.html
Motorcycle definitely does not include mofa (Motorfahrrad).
Sorry, I probably wasn't clear enough in that point:
I meant it's not clear in the OSM-hierarchy, whether motorcycle includes mofas or not.
We either need a clear international hierarchy (and there mofa will be a child of motorcycle) or we ned national hierarchies. But as there are (in Switzerland) relatively few motorcar&motorcycle sings, I would vote for motor_vehicle=no & mofa=yes.
Regards, Thomas