  Understanding of the mechanisms driving our daily face-to-face encounters is
still limited; the field lacks large-scale datasets describing both individual
behaviors and their collective interactions. However, here, with the help of
travel smart card data, we uncover such encounter mechanisms and structures by
constructing a time-resolved in-vehicle social encounter network on public
buses in a city (about 5 million residents). This is the first time that such a
large network of encounters has been identified and analyzed. Using a
population scale dataset, we find physical encounters display reproducible
temporal patterns, indicating that repeated encounters are regular and
identical. On an individual scale, we find that collective regularities
dominate distinct encounters' bounded nature. An individual's encounter
capability is rooted in his/her daily behavioral regularity, explaining the
emergence of "familiar strangers" in daily life. Strikingly, we find
individuals with repeated encounters are not grouped into small communities,
but become strongly connected over time, resulting in a large, but
imperceptible, small-world contact network or "structure of co-presence" across
the whole metropolitan area. Revealing the encounter pattern and identifying
this large-scale contact network are crucial to understanding the dynamics in
patterns of social acquaintances, collective human behaviors, and --
particularly -- disclosing the impact of human behavior on various
diffusion/spreading processes.
