Final answer:
True, friction is necessary to prevent a car from sliding toward the inside of a banked curve when traveling below the ideal speed. Static friction supplements the horizontal component of the normal force to provide the needed centripetal force.
Step-by-step explanation:
If a car takes a banked curve at less than the ideal speed, friction is indeed needed to keep it from sliding toward the inside of the curve. When a car travels on a banked curve at the ideal speed, the centripetal force required for the car to turn is provided entirely by the horizontal component of the normal force due to the banking of the road. If the speed is less than ideal, the horizontal component of the normal force will not be sufficient to provide the necessary centripetal force, and the car would tend to slide towards the center of the curve. Therefore, static friction acts outward, along the surface of the road, to provide the additional centripetal force needed to keep the car from sliding inward.