We investigated drivers' braking behavior when the preceding vehicle unexpectedly brakes, and evaluated the effectiveness of forward vehicle collision warning signals during the use of adaptive cruise control systems. Then, we analyzed the toral error frequency of the man-machine system with consideration of driver's over dependence on the warning signals. Finally, we calculated and evaluated the effectiveness of forward vehicle collision warning signals using this model. In the study, the increase of driver's braking latency or collosion frequency to the preceding vehicle generated by the over dependence on the warning signals was observed. Even though the increase of driver's error frequency was observed, it was revealed that the total error frequency decreased to 1/500 when the warning signals were presented, compared to when they were not given, at a slpeepiness level is high.