This paper describes a method for estimating the crosswind disturbance and the transverse slope affecting a passenger vehicle that is travelling in a straight line. It also discusses a compensation control system that uses electric power steering to provide steering assistance to the driver. Under ordinary driving conditions, the aerodynamic sideslip angle remains small, and therefore, the center of pressure remains reasonably stable. In this study, a constant center of pressure is assumed, and a lower-order disturbance observer is designed so as to reduce the computational cost of the implementation. The transverse slope is estimated independently of the crosswind, and the compensation torque of the electric power steering system is determined by the crosswind component. Vehicle dynamics simulations confirmed that the yaw rate could be reduced by using the proposed quartic disturbance observer.