A vehicle moving at a fast speed on a straight rail/road has a large inertia, i.e. tendency to keep moving in the same direction at the same speed. When it takes a circular turn on a curved rail/road, it experiences an additional force, termed as centripetal force, acting towards the centre of the circle. While rounding the curve as the vehicle has tendency to leave curved path and regain straight line path, force of friction between wheels and ground provides necessary centripetal force. In order that the vehicle can go round the curved track at a reasonable speed without skidding, the sufficient centripetal force is managed for it by banking the rail/road.