A closed-loop articulated vehicle-driver model, incorporating the path errors, lateral accelerations of the two units and the rate of steering, is proposed to study the directional control behavior of the driver. The closed-loop driver-vehicle model is formulated upon integrating the yaw-plane model of a five-axle articulated vehicle and a comprehensive driver model. The driver model, incorporating the delays associated with the limb movement and muscle activities, is developed with an objective to minimize the lateral acceleration of vehicle, and the lateral position and orientation errors between the previewed and the actual path of the tractor. Various parameters required to describe the driver's contributions are identified through minimizing a weighted performance index subject to an array of limit constraints established from the reported data. The proposed model, validated using the field measured data of a seven-axle vehicle, is further analyzed to study the influence of vehicle speed and maneuver severity on the variations in the control demands posed on the driver. The results of the study may serve as an effective guide to enhance the driver's actions to improve the safety of the driver/vehicle system through enhanced directional control strategies.