For the development of a traffic-simulation model to estimate the effect of adaptive cruise control (ACC) systems on traffic safety, throughput, and environment, data of a field operational test (FOT) were analyzed, in which vehicles were equipped with ACC and lane-departure warning (LDW) systems. The objective of this paper is to use this FOT to investigate the deactivation or (re)activation of the ACC on driver behavior in a real traffic environment. By taking these results into account in the traffic simulation environment, a more realistic evaluation of the impact of ACC on safety, throughput, and environment can be achieved. Some of the conclusions that were found are that after the participants deactivated the ACC by pressing the brake pedal, the gap with the lead vehicle was decreased. Resuming the ACC by activating the system or by releasing the throttle after overruling the system resulted in a larger gap between participant and lead vehicle than an overruled ACC or the ACC turned off. The participants overruled the ACC by pressing the throttle mainly to overtake the lead vehicle. © 2010 IEEE.