This paper analyzes the behavior of drivers using Adaptive Cruise Control (ACC) within the theoretical framework of Human-Machine Cooperation. The study was carried out on a driving simulator. Driving task performance data and responses to a trust questionnaire were analyzed in order to examine the relationship between driver reliance on ACC and such intervening variables as trust, perceived workload and perceived risk. The participants were divided a posteriori into two groups according to their use of the ACC device during the experimental run. The results show that high-use drivers seemed to cooperate more with ACC than low-use drivers, who tended to perceive more risk and a higher workload. These findings are discussed in the light of Riley's theory of operator reliance on automation.