They make what they call "D16" roadtrack. The diameter of the outer rail is about 16 inches - the outside edge of the rad track itself makes a circle about 17" in diameter.
They also make D21, which is 5 inches more in diameter.
When K-line first announced Superstreets their catalog listed D12 track - but they never made it. D16 is the tightest diameter "track" I know of for O gauge.
I have run clown cars on them. A note: the power supplies provided with Superstreets RTR sets are DC (the SS cars will run on either DC or AC). The Lionel/K-Line clown cars I have run on DC or AC also, so they will run with the RTR set's power supply - I don't know if all of clown cars will, though.