The aerodynamic stability of energy-saving, lightweight, and low-drag vehicles is reduced by crosswind disturbances. In particular, crosswinds cause unsteady motion in vehicles with low-drag body shapes due to aerodynamic yaw moment. To verify fluctuations in the unsteady aerodynamic forces of a vehicle, a direct measurement method of these forces in a crosswind test was established using inertial force and tire load data. The former uses an inertia sensor comprised of a gyro, acceleration sensor, and GPS sensor, and the latter uses a wheel force sensor. Noise in the measurement data caused by the natural frequency of the tires was reduced using a spectral subtraction method. It was confirmed that aerodynamic data measured in the crosswind test corresponded to wind tunnel test data. Numerical expressions were defined to model the unsteady aerodynamic forces in a crosswind. Crosswind tests were conducted under various conditions consisting of different wind patterns and test vehicle yawing motions, to identify the model parameters in the numerical expressions by multiple linear regression analysis. The analysis results showed that dynamic aerodynamic forces can be expressed as a function of yaw rate, side slip angle, the derivative of the relative wind angle, and the relative wind angle, i.e. aerodynamic side slip angle. The transient state in a crosswind can be estimated by the proposed transient aerodynamic model more accurately than the previous static aerodynamic model. Furthermore, it was found that the dynamic aerodynamic force increases yaw rate due to crosswind disturbance, thereby deteriorating the stability of road vehicles.