I found two signs that were made in the "Baby' On Board" style that was a fad in the mid 1980's.
(Although I saw a Baby's on Board sign on a car this past week!)
These hung on your rear or side windows by a little suction cup.
Here are the two I found.
These are well worn because they hung on the rear window of my Nisan pickup truck until it died. (Over 250,000 miles)
After the truck had given up they ghost they went in a box of things that were in the truck and then moved around until this past week.
I am sure I got these around the release of Batman in 1989.
Look pretty good hanging in Gotham.
So now they get placed where they belong. With the rest of my Bat-collection.