Why do banked curves allow Indy cars to drive faster around them?
Draw two pictures. In one, draw a car from the front (or rear) on a flat road. In the second, draw the same car, but on a road that is slanted from side to side. Now assume that both cars are accelerating around a curve. Draw a big vector pointing horizontally on each, representing the net centripetal force on the car. Now resolve that vector into the three forces acting on the car: The gravitational force, which is always straight down, the normal force from the road, which is obviously normal to the road, and the lateral force on the tires, which is parallel to the road surface. Note that the resulting lateral force on the tires is smaller on the banked turn. Note also that the normal force from the road is larger. So in the banked turn, the car has more frictional force available because the normal force is larger, and also requires less lateral force.
