This paper is concerned with a new control method which can share the control authority between a human driver and a lane keeping assist system. Based on the concept of multiple controller switching, this shared control method is composed of two levels: operational and tactical. At the operational level, two local optimal-based controllers are designed to satisfy their own predefined control goals. At the tactical level, a supervisor is designed to orchestrate a smooth control authority transition between two local controllers. The closed-loop properties of the human-in-the-loop vehicle system are guaranteed via Lyapunov stability arguments. In particular, the design of both local controllers is recast as a convex optimization problem, easily solved with numerical solvers. The effectiveness of the proposed shared control method is experimentally validated with a human driver and a dynamic driving simulator.