I saw several of these signs around central Kaunas, Lithuania. The official black text says, "Don't honk your horn", and the unofficial addition says, "But I don't have a car."
Before going to Lithuania, I heard horror stories about the traffic and crazy drivers. But to be honest, it was actually pretty calm. Downtown Kaunas was great, but maybe we spent too much time in the pedestrian areas (Laisvės alėja, Vilniaus Gatvė) to notice. :-) And yes, the signs seemed to work as I don't recall hearing any horn-honking.