Motivated by the question "Why do people behave strangely while driving cars with GPS navigation systems?", we reviewed previous work in the field and paid attention to the psychological (short- and long-term) effects of "mindlessness" in human minds while interacting with these machines. This psychological phenomenon is analyzed in the framework of mindlessness/mindfulness theory (Ellen Langer, 1989), and we suggest a new approach to the user-interface (UI) design based on the analysis. In this project, we explore problems found in GPS-based car-navigation systems, and suggest new design ideas—such as illusion of control or perceived control—in UI, via the preliminary experiment of "Mindful Walking." In addition, we present some other design possibilities based on Langer's work. Lastly, we suggest a new role for an intelligent agent.