For the development of a micro-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) was analyzed in which vehicles were equipped with ACC and Lane Departure Warning (LDW) systems. The objective of the research described in this paper was to use this FOT to investigate the effects of deactivation or (re)activation of the ACC on driver behaviour in a real traffic environment. It was found 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. By taking these results into account in the micro-simulation environment, a more realistic evaluation of the impact of ACC on safety, throughput and environment could be done.