The main loop of Prospect Park is exactly 3.35 miles, and it's shared by walkers, joggers, bikers, and cars. However, cars can only enter the park at certain hours in the morning and in the evening, which makes the park more pedestrian-friendly and less like an Indiana 500 speedway.
Hence, the "Do Not Enter" sign on this particular entrance, which is ocassionally ignored by some errant motorists who drive around it (and quickly pulled aside by alert police who are just waiting to pounce on such scofflaws).
The statue on the pedestal is called The Horse Tamers, by Frederick MacMonnies. I'll likely take more detailed photos of this statue, and all the others that adorn Prospect Park.