This paper presents the design and experimental implementation of a longitudinal control system for the operation of automated vehicles in platoons. The control system on each vehicle is designed to have a hierarchical structure consisting of an upper level controller and a lower level controller. The upper controller determines the desired acceleration for each vehicle in the platoon so as to maintain constant accurate spacing between vehicles and ensure string stability in the platoon. The lower controller determines the throttle and/or brake commands required to track the desired acceleration. The design of the lower level controller for low speed operation is made difficult by the fact that the torque converter remains unlocked and the engine speed is not directly proportional to vehicle speed. In addition, the drive torque drops to zero for a brief time interval during each gear shift. Solutions to overcome these low speed problems are described. The design of safe intra-platoon maneuvers so as to allow any car in the platoon to exit by making a lane change are presented. Design of the co-ordination between vehicles for these maneuvers is described. The paper presents extensive experimental results from the public NAHSC demonstration of automated highways conducted in August 1997 at San Diego, California. The demonstration included an eight-car platoon operating continuously over several weeks with passenger rides being given to visitors. The maneuvers demonstrated included starting the automated vehicles from complete rest, accelerating to cruising speed, allowing any vehicle to exit from the platoon, allowing new vehicles to join the platoon and bringing the platoon to a complete stop at the end of the highway.