How about the sun hits, the black lettering gets hotter than the white, melts that portion more... to produce that effect.if it is an embossed it would be real
I doubt they were embossed. All the clients I have who produce roadsigns use layers of vinyl. There's the reflective base signs and then the lettering itself. I have no problem believing the ice adhered to the sign, conformed itself to the vinyl layers, then melted from the sign heating underneath it and slipping off.