Longitudinal controllers are derived for an automated car driving. Two types of information sources are considered: the first-order information includes locations and speeds of following and leading cars, while the second-order information contains accelerations/decelerations in addition to the first-order information. A proper headway distance is defined for a variable speed of a leading car, sliding mode and adaptive sliding mode control theories then become applicable, and stable and robust longitudinal controllers are derived. The acceleration/deceleration information available through the modern inter-vehicle communication and sensing technologies significantly improve the control performance. The adaptive sliding mode controller yields better results because it includes a stable and swift estimation mechanism for the maximum disturbance value.