Those first appeared in the mid 80s. They irritated me because of the apparent implication that their baby's life was more valuable than anyone else's. They were rapidly joined with every copycat sign imaginable. Some I remember:
Alien on Board
Mother-in Law in Trunk
Stupid Yellow Sign on Board
Alien on Board
Mother-in Law in Trunk
Stupid Yellow Sign on Board
Only two things are infinite, the universe and human stupidity, and I'm not sure about the former.
Albert Einstein
Albert Einstein