What is the purpose of superelevation on curves?

• A. To improve tire wear.
• B. To increase drainage by lowering the outer edge.
• C. To raise the inner edge for better stability.
• D. To counterbalance lateral acceleration by banking the outer edge higher.

Answer: (D)

Banking the roadway on a curve, with the outer edge raised higher than the inner edge, counterbalances the lateral acceleration a vehicle feels when negotiating a bend. As a car goes around a curve, inertia pushes it outward; by tilting the surface, gravity provides a stabilizing component that helps press the tires into the road and keeps the vehicle from sliding outward or relying solely on friction. This makes turning safer and smoother at the design speed and curve radius. The amount of banking is chosen so that the weight component along the slope reduces the required lateral friction, often conceptually connected to tan(theta) ~ v^2/(gR). This purpose is distinct from drainage or tire wear considerations and is not about raising the inner edge or lowering the outer edge for drainage.