  Autonomous vehicles (AVs) may use external interfaces, such as LED light
bands, to communicate with pedestrians safely and intuitively. While previous
research has demonstrated the effectiveness of these interfaces in simple
traffic scenarios involving one pedestrian and one vehicle, their performance
in more complex scenarios with multiple road users remains unclear. The
scalability of AV external communication has therefore attracted increasing
attention, prompting the need for further investigation. This scoping review
synthesises information from 54 papers to identify seven key scalability issues
in multi-vehicle and multi-pedestrian environments, with Clarity of Recipients,
Information Overload, and Multi-Lane Safety emerging as the most pressing
concerns. To guide future research in scalable AV-pedestrian interactions, we
propose high-level design directions focused on three communication loci:
vehicle, infrastructure, and pedestrian. Our work contributes the groundwork
and a roadmap for designing simplified, coordinated, and targeted external AV
communication, ultimately improving safety and efficiency in complex traffic
scenarios.
