Abstract
The communication of an automated vehicle (AV) with human road users can be realized by means of an external human–machine interface (eHMI), such as displays mounted on the AV’s surface. For this purpose, the amount of time needed for a human interaction partner to perceive the AV’s message and to act accordingly has to be taken into account. Any message displayed by an AV must satisfy minimum size requirements based on the dynamics of the road traffic and the time required by the human. This paper examines the size requirements of displayed text or symbols for ensuring the legibility of a message. Based on the limitations of available package space in current vehicle models and the ergonomic requirements of the interface design, an eHMI prototype was developed. A study involving 30 participants varied the content type (text and symbols) and content color (white, red, green) in a repeated measures design. We investigated the influence of content type on content size to ensure legibility from a constant distance. We also analyzed the influence of content type and content color on the human detection range. The results show that, at a fixed distance, text has to be larger than symbols in order to maintain legibility. Moreover, symbols can be discerned from a greater distance than text. Color had no content overlapping effect on the human detection range. In order to ensure the maximum possible detection range among human road users, an AV should display symbols rather than text. Additionally, the symbols could be color-coded for better message comprehension without affecting the human detection range.
Highlights
The process of introducing automated vehicles (AVs) into road traffic is progressing
Symbols can be identified at a greater distance than text, which means that in the same scenario the human interaction partner has more time to perceive and process an AV’s message in the form of a symbol
We state that the height of the display content should be 170 mm (6.64 min of arc (MOA))
Summary
The process of introducing automated vehicles (AVs) into road traffic is progressing. One approach for enabling communication of AVs with their environments is to use external human–machine interfaces (eHMIs). These are displays mounted on the surface of the vehicle [2,3], light strips [4,5,6], and projections on the road [7,8]. These devices enable AVs to indicate, for instance, their status, perception, or intention [9] in relevant scenarios, such as at intersections, in parking lots, in narrow spaces, or in merging traffic [10,11]
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