Yes, heard that one before, but now I think about it there have been lots of amusing comments written on vehicles but I can't think of them now. Also on the side of yachts - like 'This way Up' (with arrow).
The one that made me laugh last year on a white van on the M1 also had a familiar ring to it,
A DOG IS NOT JUST FOR CHRISTMAS,
HAVE IT COLD ON BOXING DAY AS WELL
Saw this poster on the back of a bus "please leave room for this bus to pull out" some wag had scribbled the words "to reduce mini buses" infront of the sign . Made me laugh anyway!!