Today's traffic round-abouts serve a similar purpose. And, we are sure getting a lot of them around here.
Don't run over the silent cop.
Also called a "silent policeman."
It was used most recently in Australia and taken out of the Sydney streets in 2002 as "an outdated device." Motorcyclists were particularly unhappy with them. Wonder why?
According to Wikipedia, they were also referred to as "sleeping policeman" or "traffic dome." they are usually painted yellow. They are about 16 inches wide and five inches tall, usually painted yellow.
So, Now You and I Know. --RoadDog