Among the various approaches available for the study of traffic flow, the optimal velocity (OV) model, which consists of simple equations, has been recognized as being effective for simulating traffic congestion. This model determines the reference velocity of a vehicle as a function of the distance to the preceding vehicle. Further, adaptive cruise control (ACC) has already been applied in practical situations, and the effects of the mixture of the ACC activated ones and ordinary vehicles would become very important in the near future. Considering such a scenario, this study modifies the OV model to simulate traffic congestion more accurately, and examines the effects of using the ACC system for alleviating traffic congestion using the improved model.