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

2026-04-30

AI Summary

This study addresses a critical safety concern in Level 3 automated driving: the invisibility of the vehicle’s request-to-intervene (RtI) state to external road users, which can lead to hazardous misunderstandings. To enhance surrounding drivers’ situational awareness, the authors propose an innovative external human-machine interface (eHMI) termed C+O, which, for the first time, explicitly communicates RtI-related states through color-coded cyan and orange light bars. Using a driving simulator experiment combined with behavioral and trajectory analyses, the results demonstrate that, compared to baseline conditions with no eHMI or only generic automated-driving indicators, the C+O eHMI significantly improves other drivers’ comprehension and prediction of the automated vehicle’s intentions. This enhancement reduces rear-end collision risk by 76.8% and elicits earlier, more decisive defensive driving responses.

Abstract

Level 3 automated vehicles (AVs) issue a request to intervene (RtI) when the automated driving system approaches its system limitations. Although this takeover transition is safety-critical, it is usually invisible to surrounding manually driven vehicle (MV) drivers. This study proposes an external human-machine interface (eHMI) called eHMI C+O that externalizes the RtI-related takeover status of a Level~3 AV using cyan and orange light bars. A driving-simulator experiment with 40 participants examined whether the proposed eHMI supports surrounding MV drivers during AV takeover scenarios. The results showed that, compared with the ADS-status-only eHMI condition, which is similar to ``Automated Driving Marker Lights,'' and the no-eHMI condition, the proposed eHMI C+O significantly improved participants' understanding of the AV's driving intention, their prediction of its behavior, and their perceived sufficiency of the information presented by the AV. It also reduced hesitation, increased confidence, and promoted earlier and larger increases in time headway after the RtI was issued. In the AV accident scenario, eHMI C+O significantly reduced the odds of accident involvement for the following MV compared with the no-eHMI condition, corresponding to a 76.8% reduction in accident odds. Exploratory path analysis suggested that the safety benefit of the proposed eHMI C+O may be associated with improved situation awareness and earlier defensive driving responses. These findings indicate that externalizing RtI-related takeover status can help surrounding drivers better understand Level 3 AVs and respond more safely during safety-critical takeover transitions.

Problem

• Level 3 automated vehicles
• request-to-intervene
• takeover transition
• external human-machine interface
• traffic safety

Innovation

• eHMI
• Level 3 automated vehicles
• request-to-intervene
• takeover transition
• traffic safety