What is the primary engineering purpose of applying super-elevation to a highway's horizontal curve, beyond simply aiding vehicle stability?

Super-elevation, also known as banking, is the transverse slope applied to a highway's horizontal curve, where the outer edge of the pavement is raised relative to the inner edge. Beyond simply aiding vehicle stability, its primary engineering purpose is to counteract the centrifugal force experienced by a vehicle as it navigates the curve, thereby minimizing the reliance on lateral friction between the tires and the road surface. Centrifugal force is the apparent outward force that pushes a vehicle towards the outside of a curve. By tilting the road, super-elevation utilizes a component of the vehicle's weight to provide an inward force, effectively balancing or partially balancing this outward centrifugal force. This inward force, created by the banked roadway, reduces the amount of lateral force that the tires must generate through friction to keep the vehicle from skidding outwards. The reduction in demand on lateral friction is critical because friction is a highly variable and unpredictable factor, influenced by elements such as pavement condition (dry, wet, icy), tire type and wear, and vehicle speed. By designing the curve with super-elevation, engineers create a safer and more consistent driving experience, as the road itself provides much of the necessary centripetal force – the actual inward force required for circular motion – rather than solely depending on tire grip. This engineering design allows for higher design speeds on curves while maintaining a substantial safety margin, as it prevents vehicles from skidding even if the available friction is reduced, for instance, by rain. It also contributes to reduced tire wear and improved fuel efficiency by lessening the constant lateral scrubbing action required from the tires to stay on the curve.