Direction-dependent bias in the perceived approach speed and its implications for robot motion design

Humans frequently interact with robots that approach from different directions; however, how the approach direction systematically biases the perceived speed remains unclear. Here, we demonstrate a robust direction-dependent bias in the perceived approach speed, whereby objects approaching from behind are consistently perceived as moving faster than those approaching from the front are. Across five experiments using both physical robots and immersive virtual reality, this bias manifests as a systematic shift in the point of subjective equality without corresponding changes in discrimination sensitivity, indicating a perceptual bias rather than altered precision. We further show that this effect can be explained by body-induced differences in interocular velocity that arise when observers orient themselves toward stimuli approaching from behind. This bias persists in virtual environments and for nonrobotic objects, suggesting that it is driven primarily by visual factors rather than multisensory cues or robot-specific appearance. Together, these findings provide a perceptual foundation for incorporating approach direction into the design and evaluation of the motion behavior of robots in human-centered environments.