I'm old enough to remember those signs. (Before interstate highways and freeways. Most of our highways were two lane and I and many more people looked for these signs.
I think the poster is correct in they did get your attention.
I also remember the little white crosses on the mountain curves in Arizona__especially at night! One cross for everyone that'd been killed on that curve! Well, that's what you thought when you saw them.
They worked too for we guys in our early twenties back then.
Must have. I'm still here.
Stay safe
Poli Viejo

DO NOT PASS

UPON A SLOPE

UNLESS YOU HAVE

A PERISCOPE

Burma Shave