Dirt track racing is the single most common form of auto racing in the United States. According to the National Speedway Directory, there are over 700 dirt oval tracks in operation in the US. The composition of the dirt on tracks has an effect on the amount of grip available. Many tracks use clay with a specific mixture of dirt. Tracks are sometimes banked in the turns and on the straights. This banking is utilized primarily to allow vehicles to carry more speed through the corners. However, some tracks prefer less banked turns.