  Micro-mobility transport vehicles such as e-scooters are joining current
sidewalk users and affect the safety and comfort of pedestrians as primary
sidewalk users. The lack of agreed-upon metrics to quantify people's discomfort
hinders shared public space safety research. We introduce perceived Time To
Collision (TTC) as a potential metric of user discomfort performing controlled
experiments using an e-scooter and a pedestrian moving in a hallway. The
results strongly correlate the participant's reported discomfort and the
perceived TTC. Therefore, TTC is a potential metric for public space users'
discomfort. Since the metric only uses relative velocity and position
information, it is a viable candidate for neighboring people's discomfort
estimation in advanced driver assistance systems for e-scooters and PMVs. Our
ongoing research extends the results to mobile robots.
