Allowed CD is set below the value that would be allowed based on safety in order to reduce wheel and rail wear and to reduce the rate of degradation of geometry of ballasted track. Choice of design CD will be less constrained by passenger comfort in the case of vehicles that have tilting capability. One historical approach to determining safe cant deficiency was the requirement that the projection to the plane of the track of the resultant of the inertial and gravitational forces acting on a vehicle fall within the middle third of the track gauge. Contemporary engineering studies would likely use vehicle motion simulation including cross wind conditions to determine margins relative to derailment and rollover.