An improved lane-changing model based on the Nagatani's method is proposed. The threshold of the safety headway in lane-changing process is calibrated. Both the lane-changing process and the impact of the lane-changing vehicle on the adjacent vehicles are incorporated into the proposed model. Therefore, simulated results using the proposed model agree with the surveyed results. The marginal decrease of lane capacity with the number of lanes is analysed and validated upon the proposed lane-changing model. We find that the vehicle velocity decreases and the lane changing probability increases with the increasing number of lanes under the congested condition of highways in cities.