This paper proposes an active steering assistance system that could act as lane departure avoidance and lane keeping system. The assistance has been designed initially for a lane departure avoidance purpose. In this paper the driver-assistance interaction is studied with the objective to allow a permanent system activation and hence a lane keeping functionality. The lane keeping performance of the assistance system is analyzed while the driver is intervening. More specifically, bounds for the lane keeping performance are theoretically computed and experimentally validated.