In this paper the effect of aerodynamic modifications that influence the unsteady aerodynamic properties of a vehicle on the response of the closed loop system driver-vehicle under side wind conditions is investigated.In today's aerodynamic optimization the side wind sensitivity of a vehicle is determined from steady state values measured in the wind tunnel. There, the vehicle is rotated with respect to the wind tunnel flow to create an angle of attack. In this approach however, the gustiness that is inherent in natural wind is not reproduced. Further, unsteady forces and moments acting on the vehicle are not measured due to the limited dynamic response of the commonly used wind tunnel balances.Therefore, a new method is introduced, overcoming the shortcomings of the current steady state approach. The method consists of the reproduction of the properties of natural stochastic crosswind that are essential for the determination of the side wind sensitivity of a vehicle. Further, a fast-response wind tunnel balance is used to measure unsteady forces on the model resulting from the crosswind excitation. The results for a notchback model are presented in terms of aerodynamic transfer functions for side force and yawing moment and the effect of aerodynamic modifications is discussed.With the method described, the optimization of the aerodynamic properties of a vehicle is no longer restricted to steady state values, but extended to the frequency range that is relevant from a driving dynamics point of view. To estimate the influence of an aerodynamic modification affecting the unsteady response of a vehicle, aerodynamic transfer functions are included in a vehicle dynamics model. Further, the effect on the driver's subjective assessment is evaluated by simulation of the closed-loop system composed of driver and vehicle.