Rethinking External Communication of Autonomous Vehicles: Is the Field Converging, Diverging, or Stalling?

TL;DR

The paper investigates whether external human–machine interfaces for autonomous vehicles are converging, diverging, or stalling. It analyzes 620 eHMI studies (2014–2025) alongside industry deployments and regulatory documents to map field evolution, consensus, and alignment with practice. The findings reveal a safety-first convergence on simple, allocentric cues, but persistent fragmentation across modalities and debates on necessity, with a funnel-like progression from exploration to deployment and regulation. The authors argue for shifting from new prototypes to consolidating evidence, developing flexible standardisation, and integrating across industry and policy to support ecologically valid, context-sensitive eHMI design. This work provides a field-level blueprint for researchers, regulators, and industry to coordinate toward safe, adaptable external communication in diverse mobility contexts.

Abstract

As autonomous vehicles enter public spaces, external human-machine interfaces are proposed to support communication with external road users. A decade of research has produced hundreds of studies and reviews, yet it remains unclear whether the field is converging on shared principles or diverging across approaches. We present a multi-dimensional analysis of 620 publications, complemented by industry deployments and regulatory documents, to track research evolution and identify convergence. The analysis reveals several field-level patterns. First, convergence on a safety-first core: simple visual cues that clarify intent. Second, sustained divergence in necessity and implementation. Third, a progressive filtering funnel: broad exploration in research and concepts narrows in deployment and is codified by regulation into a minimal set of permitted signals. These insights point to a shift in emphasis for future work, from producing new prototypes toward consolidating evidence, clarifying points of contention, and developing frameworks that can adapt across contexts.

Figures (10)

• Figure 1: PRISMA flow diagram showing the identification, screening, and inclusion process for eHMI studies. The final dataset comprised 512 articles from database searches and 108 additional articles identified through backward snowballing (total = 620).
• Figure 2: Annual distribution of 620 eHMI publications from 2014–2025.
• Figure 3: Top 10 countries by first-author affiliation in the eHMI dataset.
• Figure 4: Distribution of road-user and vehicle-type categories in the reviewed studies. Counts exceed the number of studies because a single paper may include multiple mentions.
• Figure 5: Heatmap overview of the high-level dimensions used to characterise AV external communication research. The framework distinguishes three orientations: Design Inquiry, Situated Investigation, and Field Consolidation. Values range from 11 (smallest) to 506 (largest).