A street design, originated in Detroit,MI and can be found in several other parts of the state.
Used on a divided street, where traffic in either direction is separated by some sort of median, it provides a fairly simple way of changing directions. It is essentially a sanctioned U-turn: there is a paved curve across the median, usually some sort of stop sign or yield. They alternate these every few hundred feet, so that either direction gets a fair shot at turning around.
This is not quite the same as certain sections where there is a large open space, and either direction can turn around. Those are usually found where there is another street, which needs to go either direction.