External Human-Machine Interface

A class of interfaces on the exterior of a vehicle — typically an automated or autonomous vehicle — designed to communicate the vehicle's intent, awareness, or state to pedestrians, cyclists, and other road users who would otherwise rely on cues from a human driver (eye contact, hand gestures, headlight flashes). eHMIs include visual signals such as light strips, projected symbols, and text displays, and increasingly auditory or multimodal signals like bells, speech, or vibration-equivalent cues. For accessibility, eHMIs raise significant design questions: visual-only eHMIs exclude blind pedestrians, auditory-only eHMIs exclude Deaf and Hard-of-Hearing pedestrians, and both can be masked by poor lighting, background noise, or interactions with hearing aids and cochlear implants — motivating multimodal and inclusive approaches.