The purpose of this paper is to clarify the suitable warning timing of “Lane Change Decision Aid Systems (LCDAS)” for a driver’s lane change maneuver. The relationship between lane-change tasks and closing vehicles in the passing lane was investigated by field experiments on the Chuo expressway in Japan. The driver’s steering during the lane change was simulated using a linear prediction model. Based on these results, the system requirements of warning timing and sensing area for LCDAS are proposed.