Pavement serviceability or ride quality indices have been widely applied to monitor pavement performance and deterioration for pavement rehabilitation, design, and other purposes. It is known that the ride quality or serviceability index of roads can be explained mainly by the vertical jerk experienced by raters sitting in a moving vehicle. However, it is not known to what extent that the indices have to do with some external traffic loading variables (e.g., the rate of dynamic forces or the dynamic forces exerted on a pavement surface by an axle load). Employing some actual road profiles and a half-axle load model, both the dynamic forces and the rate of dynamic forces exerted on a pavement surface are estimated using a spectral density method and a finite difference method, respectively. A very good correlation exists between these physical variables and the ride quality index or the serviceability index, indicating that the deterioration of pavement structures is closely related to the rate of dynamic forces.