Hey, Kendall....why was your friend driving so close that his rig got sprayed?

I can (almost) see the utility of one of those signs - except for the fact that they block vision out of the rear window when one is trying to check the next lane for a lane change. Of course, the ones who (1) have sign(s) and (2) are talking on their telephones probably don't look when they change lanes anyway - without benefit of turn signals, of course.