  In recent years, researchers and manufacturers have started to investigate
ways to enable autonomous vehicles (AVs) to interact with nearby pedestrians in
compensation for the absence of human drivers. The majority of these efforts
focuses on external human-machine interfaces (eHMIs), using different
modalities, such as light patterns or on-road projections, to communicate the
AV's intent and awareness. In this paper, we investigate the potential role of
affective interfaces to convey emotions via eHMIs. To date, little is known
about the role that affective interfaces can play in supporting AV-pedestrian
interaction. However, emotions have been employed in many smaller social
robots, from domestic companions to outdoor aerial robots in the form of
drones. To develop a foundation for affective AV-pedestrian interfaces, we
reviewed the emotional expressions of non-humanoid robots in 25 articles
published between 2011 and 2021. Based on findings from the review, we present
a set of considerations for designing affective AV-pedestrian interfaces and
highlight avenues for investigating these opportunities in future studies.
