From the Macquarie Dictionary.
This traffic device was used in the U.S. during the early days of autoists, but dropped. It was most recently used in Australia.
A circular steel plate about 60 cm in diameter with a pattern of steel projections on top, set in the center of the road at an intersection where there are no traffic lights. This encourages traffic to keep to the left. (Remember, Australia where they drive on the other side of the road.)
I've Never seen One. --RoadDog