Designing With Motion: Exploring Vestibular Cues as a Subtle Awareness Nudge Modality in Automated Vehicles

Automated driving systems, particularly at SAE Level 3, present new challenges in managing driver attention to ensure smooth transitions from automated to manual control. This paper reports on a qualitative investigation of vestibular cues—implemented via subtle deceleration events—as a form of a dynamic Human-Machine Interface (dHMI) that subtly "nudges" a user's attention away from a non-driving related task (NDRT) and towards the driving environment. Conducted as a test-track study (N=25), we explore awareness, acceptance, and design considerations related to these cues. Findings reveal that while participants showed positive attitudes toward vestibular nudges as safety features, they were unable to differentiate nudges from necessary vehicle deceleration during automated driving. The study reveals how drivers interpret the implicit interaction with the dHMI during realistic NDRT and potential limitations. The study highlights the need for multimodality HMI approaches and customisation to optimise user experience in conditional automated vehicles.