Why do banked curves allow Indy cars to drive faster around them?

There's a whole lot of physics that goes along with an answer to a question like this, but the short answer is that if the track were not banked in the corners, the car would slide outward, and if the track is banked, the same force that is acting on the car to move it outward on the curve is now being used to push it down tighter to the track. The car can move faster through the corner because a lot of that lateral force is sort of converted to downforce. Unfortunately, Yahoo! Answers doesn't allow us to draw pictures.