An eHMI Presenting Request-to-Intervene and Takeover Status of Level 3 Automated Vehicles to Support Surrounding Traffic Safety

Liu, Hailong; Kuge, Masaki; Hiraoka, Toshihiro; Wada, Takahiro · 2026 · arXiv

archive: archived pipeline: cataloged verified

Get this paper ↗ (full text — opens at the source; we link to it, we don't host it)

Summary

This study addresses the safety risks posed to surrounding manually driven vehicles (MVs) when Level 3 automated vehicles (AVs) issue a Request to Intervene (RtI). While RtI indicates that the AV’s automated driving system is approaching its operational limits and requires human takeover, this critical transition is typically invisible to other road users. This lack of visibility creates uncertainty for surrounding drivers, potentially leading to delayed reactions and increased collision risks. The authors propose an external human-machine interface (eHMI) called “eHMI C+O” to externalize the AV’s takeover status, hypothesizing that it will improve surrounding drivers’ situation awareness and promote safer defensive driving behaviors. The researchers conducted a driving simulator experiment with 40 participants acting as MV drivers following a Level 3 AV. The study employed a within-subject design comparing three eHMI conditions: no eHMI, an eHMI displaying only ADS status via a cyan light bar (similar to existing Automated Driving Marker Lights), and the proposed eHMI C+O, which added a flashing orange light bar to indicate the RtI state and subsequent takeover status. Participants encountered scenarios involving both successful and unsuccessful AV takeovers on curve and diverging roads. Data collection included post-trial questionnaires assessing cognition and confidence, measurements of time headway (THW) changes, and binary outcomes for accident involvement in crash scenarios. Statistical analyses included mixed-effects ANOVAs and exploratory piecewise structural equation modeling. The results demonstrated that the proposed eHMI C+O significantly outperformed the other conditions. Participants reported higher understanding of the AV’s driving intention, better prediction of its behavior, and greater perceived sufficiency of information. Behaviorally, the eHMI C+O reduced hesitation and increased driver confidence, leading to earlier and larger increases in time headway after the RtI was issued. Crucially, in scenarios where the AV failed to take over and crashed, the eHMI C+O significantly reduced the odds of the following MV being involved in the accident by 76.8% compared to the no-eHMI condition. Exploratory path analysis suggested that these safety benefits were mediated by improved situation awareness and earlier defensive responses. The findings indicate that explicitly communicating the RtI and takeover status of Level 3 AVs to surrounding traffic can substantially enhance safety in mixed-traffic environments. By reducing uncertainty and enabling earlier defensive actions, such eHMIs can mitigate the risk of secondary collisions caused by AV takeover failures. This study highlights the importance of externalizing internal vehicle states, particularly during safety-critical transitions, to support the safe integration of automated vehicles into existing traffic systems.

Key finding

Externalizing Level-3 AV takeover status to surrounding drivers via a cyan/orange eHMI yielded a 76.8% reduction in following-MV accident odds in an AV-accident scenario and produced earlier, larger increases in time headway after the RtI, mediated by improved situation awareness.

Methodology

simulator

Sample size: N=40 MV-driver participants (ages 22–24)

Provenance

The full processing record for this entry. Every stage of this paper's journey through the pipeline is logged — what ran, with which tool and model, how many attempts it took, and when it last completed. Discovered via discover_arxiv_cs.HC on 2026-05-04 (3 acquisition events logged).

Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

Ranked by relevance to this paper. Hover a topic for its definition.

• ehmi external hmi
• automation
• vru facing ehmi
• situational awareness
• takeover transitions
• automation surprise

Information type

What kind of knowledge this paper contributes, grouped by family — independent of topic (what it is about) and method (how it was studied).

• Applied Guidance: design guidelines
• Empirical Findings: behavioral performance data
• Theoretical Contribution: conceptual framework