This study investigated levels of driver comfort and acceptance for an autonomous intelligent cruise control (AICC) system driven in an actual highway environment. Objective measures of driving performance and behavior are compared with participants' subjective assessments when operating under manual control, conventional cruise, and AICC. Included in the comparison are measures of driver velocity and braking behavior.Participants drove at slightly higher mean velocities under the manual condition as compared with AICC. Participants applied the brakes least frequently when driving manually. Participants rated the AICC system favorably for comfort, ease of use, and convenience. However, participants did express limited concerns associated with the use of AICC.