  Providing pedestrians and other vulnerable road users with a clear indication
about a fully autonomous vehicle status and intentions is crucial to make them
coexist. In the last few years, a variety of external interfaces have been
proposed, leveraging different paradigms and technologies including
vehicle-mounted devices (like LED panels), short-range on-road projections, and
road infrastructure interfaces (e.g., special asphalts with embedded displays).
These designs were experimented in different settings, using mockups, specially
prepared vehicles, or virtual environments, with heterogeneous evaluation
metrics. Promising interfaces based on Augmented Reality (AR) have been
proposed too, but their usability and effectiveness have not been tested yet.
This paper aims to complement such body of literature by presenting a
comparison of state-of-the-art interfaces and new designs under common
conditions. To this aim, an immersive Virtual Reality-based simulation was
developed, recreating a well-known scenario represented by pedestrians crossing
in urban environments under non-regulated conditions. A user study was then
performed to investigate the various dimensions of vehicle-to-pedestrian
interaction leveraging objective and subjective metrics. Even though no
interface clearly stood out over all the considered dimensions, one of the AR
designs achieved state-of-the-art results in terms of safety and trust, at the
cost of higher cognitive effort and lower intuitiveness compared to LED panels
showing anthropomorphic features. Together with rankings on the various
dimensions, indications about advantages and drawbacks of the various
alternatives that emerged from this study could provide important information
for next developments in the field.
