It's like NYC's side walks. Compare pedestrian behavior at say SoHo (daylight) and say LES (nighttime). Amazingly enough, the partying and inebrieted pedestrians at night all file politely in the correct bimodal L|R formation. During the day, it's a rather wild and somewhat uncivilized dynamic slalom formation. My theory: Fangs. The night creatures know someone potentially dangerous maybe in the midst.
https://journals.plos.org/plosone/article?id=10.1371/journal...