Augmented reality (AR) in automobiles has the potential to significantly alter the driver's user experience. Prototypes developed in academia and industry demonstrate a range of applications from advanced driver assist systems to location-based information services. A user-centered process for creating and evaluating designs for AR displays in automobiles helps to explore what collaborative role AR should serve between the technologies of the automobile and the driver. In particular, we consider the nature of this role along three important perspectives: understanding human perception, understanding distraction and understanding human behavior. We argue that AR applications should focus solely on tasks that involve the immediate local driving environment and not secondary task spaces to minimize driver distraction. Consistent depth cues should be supported by the technology to aid proper distance judgement. Driving aids supporting situation awareness should be designed with knowledge of current and future states of road users, while focusing on specific problems. Designs must also take into account behavioral phenomena such as risk compensation, inattentional blindness and an over-reliance on augmented technology in driving decisions.