With the growth of autonomous vehicle-based sharing markets, interest in PBVs is increasing. PBVs are vehicles designed for various services and are equipped with information displays inside and outside. This study analyzed the cognitive responses and gaze distribution of the driver of a following vehicle when the leading vehicle decelerates rapidly and simultaneously displays an emergency message in a car-following situation. The main variables are display size and position (top, middle, bottom). We also conducted a preliminary experiment to understand the visual characteristics of drivers. The results showed that perception time varied depending on display position and speed. At 30 km/h, the display mounted on top had the shortest perception time, while at 60 km/h, the display mounted on bottom had the longest. Gaze distribution showed relatively little change and was not significant.