eHMI for All - Investigating the Effect of External Communication of Automated Vehicles on Pedestrians, Manual Drivers, and Cyclists

Paper Title

eHMI for All - Investigating the Effect of External Communication of Automated Vehicles on Pedestrians, Manual Drivers, and Cyclists

Publication Info

• Topic area: External Human-Machine Interfaces (eHMIs) for automated vehicles and their impact on diverse road users.
• Keywords: Automated vehicles, eHMI, pedestrians, cyclists, manual drivers, virtual reality, trust, perceived safety, usability, distraction.

Background and Problem

• Problem / challenge: Existing research on eHMIs focuses predominantly on pedestrian-AV interactions, neglecting other road users like cyclists and manual drivers. No prior studies have compared eHMI effects across these roles.
• Significance: A unified eHMI design could streamline communication between AVs and diverse road users, reducing ambiguity and improving safety in mixed traffic environments.
• Motivation and related work: Prior studies have demonstrated positive effects of eHMIs on trust and clarity but have not addressed scalability or role-specific challenges. This paper addresses the gap by evaluating a unified eHMI design across pedestrians, cyclists, and drivers under varying distraction levels.

Solution

• Proposed approach: A unified eHMI design using a Slow-Pulsing Light Band (SPLB) to communicate AV intentions (e.g., yielding) to pedestrians, cyclists, and manual drivers.
• Novelty:
  1. First comparative study of eHMI effects across three road user roles (pedestrians, cyclists, drivers).
  2. Evaluation of eHMI performance under different distraction conditions (none, visual noise, interference).
  3. Use of a virtual reality (VR) setup to simulate realistic traffic scenarios for all roles.
  4. Insights into the feasibility of a standardized eHMI design for diverse road users.
• Procedure and key techniques:
  • Conducted a within-subject VR experiment with 40 participants.
  • Tested scenarios with and without eHMI across three roles and three distraction levels.
  • Measured subjective metrics (e.g., trust, perceived safety, usability) and objective metrics (e.g., time to intersection, gaze data).

Results

• Concrete findings:
  • eHMIs significantly improved perceived safety (F(1, 39) = 47.88, p < 0.001), trust (F(1, 39) = 40.96, p < 0.001), and usability (F(1, 39) = 37.19, p < 0.001) across all roles.
  • Mental demand was lower with eHMIs (M = 7.81) compared to no eHMI (M = 10.24, p < 0.001).
  • Cyclists reported the lowest trust and perceived safety compared to pedestrians and drivers.
• Advantage over baselines:
  • Scenarios with eHMIs consistently outperformed those without in terms of trust, perceived safety, and usability.
  • eHMIs reduced time to first fixation (TTFF) on AVs, improving attentional capture.
• Experiments / evaluation:
  • Participants experienced 18 conditions (3 roles × 3 distraction levels × 2 eHMI states).
  • Subjective metrics included NASA-TLX, Trust in Automation, System Usability Scale (SUS), and perceived safety.
  • Objective metrics included gaze tracking and interaction times.
• Limitations and future work:
  • Limited demographic diversity (participants aged 21–30, mostly German).
  • Realism of VR scenarios and bicycle simulator braking system could be improved.
  • Only one eHMI design (SPLB) was tested; future work should explore other designs and non-yielding scenarios.
  • Cultural differences in eHMI interpretation were not addressed.

Summary

This study evaluated the effectiveness of a unified eHMI design (Slow-Pulsing Light Band) for communicating AV intentions to pedestrians, cyclists, and manual drivers. Results from a VR-based experiment with 40 participants showed that eHMIs significantly enhanced perceived safety, trust, and usability across all roles, even under distraction. Cyclists reported lower trust and safety compared to other roles, highlighting role-specific challenges. The findings support the feasibility of a standardized eHMI design, with implications for AV design, regulation, and adoption. Future work should address cultural differences, alternative eHMI designs, and real-world validation.