Are everywhere - and most years they stay right wherethey are until the wind takes them - someone snags some of the signs for the politician they're not voting for and they toss 'em somewhere along their way.
I say that every sign left after the Friday after the elections (in public places like along the highways etc.) equal some sort of penalty/fine for the person/bill etc. they represent and that those parties' representativies etc. could also be fined.
Bottom line - clean 'em up.