Investigating HMIs to Foster Communications Between Conventional Vehicles and Autonomous Vehicles at Intersections

In mixed traffic environments that involve conventional vehicles (CVs) and autonomous vehicles (AVs), it is essential for CV drivers to maintain an appropriate level of situation awareness (SA) to ensure safe and efficient interactions with AVs. While previous research has established the benefits of external human–machine interfaces (HMIs) for communicating AV intent, this study extended this knowledge by focusing on the vital but underexplored interaction with CV drivers. Specifically, we investigated how AV communication through HMIs affected CV drivers by systematically comparing internal (iHMI) and external (eHMI) interfaces, and examined their impact on CV driver awareness, cognitive load, and behavior. Initially, we designed eight HMI concepts through a human-centered design process. The two highest-rated concepts were selected for implementation as eHMIs and iHMIs. Subsequently, we designed a within-subjects experiment with three conditions: a control condition without any communication HMI, and two treatment conditions using eHMIs and iHMIs as communication means. We investigated the effects of these conditions on 50 participants in a virtual environment (VR) driving simulator. Self-reported assessments and eye-tracking measures were employed to evaluate participants’ SA, trust, acceptance, and mental workload. Results indicated that the iHMI condition resulted in superior SA among participants and improved trust in the AV compared to the control and eHMI conditions. Additionally, iHMI led to a comparatively lower increase in mental workload compared to the other two conditions. Our study contributes to the development of effective AV-CV communications and has the potential to inform the design of future AV systems.