Visual Properties of E-Mobility Devices Shape Their On-Road Vulnerability: Effects of Size and Speed

The brief visual window—often only hundreds of milliseconds—in which drivers predict the arrival of oncoming vehicles and decide whether to give way is critical for safe and efficient manoeuvring. E-mobility devices (EMDs) are a relatively new form of transportation and may challenge these decisions due to their small visual profile and relatively high operating speeds. Here, we examined whether observers show systematic biases when judging EMDs relative to other common vehicles, potentially contributing to their vulnerability in intersection crashes. Using naturalistic video stimuli spanning seven vehicle categories, we tested whether observers would overestimate the time-to-arrival (TTA) of EMDs and, consequently, accept smaller gaps for them to pass through an intersection. Consistent with this prediction, EMDs elicited greater biases in both TTA and gap judgements (N = 31 and N = 59), with these effects further amplified on a visually complex road. By comparing across vehicle categories under naturalistic viewing conditions, the results provide evidence that the amplified biases observed for EMDs arise from differences in the underlying visual signals available to observers—specifically, their smaller retinal size and potentially less reliable motion cues at higher speeds. Importantly, these biases were not explained by speed misperception: in a separate two-alternative forced-choice task (N = 49), EMDs were perceived as faster, not slower, than other vehicles at matched speeds. This dissociation indicates that TTA overestimation cannot be attributed to speed underestimation alone. Together, these findings link controlled psychophysical processes to real-world traffic behaviour, with implications for future experimental design and traffic safety policy.