a vehicle speed control system is provided to maintain the vehicle speed at a target value . the control system includes a target vehicle speed determining section , a section for calculating a driving torque necessary to drive the vehicle at the target vehicle speed , a section for calculating a target engine rotational speed and an engine load torque and a section for calculating a control amount for controlling an opening of the throttle valve of the engine . the driving torque calculating section and the control amount calculating section are designed based on linear control theory on physical models describing the behavior of the vehicle and the engine , respectively , thereby improving the responsiveness of the control system compared to conventional pid control . the design of the control system is characterized by separate sections which reduces designing labor and which facilitate standardization of every section .