In the age of information, in-vehicle multitasking is inevitable. The popularity of the automobile, in combination with the present information age, create a growing demand to do more in-vehicle than simply focus on the road. Unconstrained Design, a philosophy which supports rather than constrains multitasking, is proposed as a path toward enhancing performance in-vehicle. Situation Awareness (SA), a theory allowing designers to understand how operators interact in dynamic, complex environments, is used to frame this experimental investigation. Two SA-grounded human-machine interface concepts are proposed, designed to support drivers to multitask in-vehicle when frequent task switching is required. The first focuses upon supporting preparation for a Non-Driving Related Activity (NDRA), and the second upon supporting the Driving Related Activity (DRA) when an NDRA is active. While multitasking, Contextual Cueing, using a Head-up Display, produced significant reductions in NDRA response time, while an auditory lane keeping aid increased the amount of time a driver spent in the central region of a lane. The combined evidence suggests that using SA and Unconstrained Design to create of IVIS that support drivers’ ability to multitask in-vehicle can lead to task performance improvements.