Stop A Head

Once, when young, and a passenger in an older cousin's vehicle.  He suddenly reached over and put his hand on my forehead, pushing my head back against the seat.  I was puzzled, until he said that the sign said, "Stop A...head".
I've noticed that there are signs about heads everywhere, but I don't always have a head handy.  Like, tonight.  this sign said, "walk on a..head".  I didn't have a head in the car, so I had to break the rules.