Understanding metropolitan patterns of daily encounters

TL;DR

This study leverages large-scale travel card data to uncover the structure and regularity of face-to-face encounters in a city, revealing a large, interconnected contact network that influences social dynamics and diffusion processes.

Contribution

It constructs the first large-scale, time-resolved social encounter network from public bus data, revealing regular encounter patterns and the emergence of a city-wide small-world contact structure.

Findings

Physical encounters show reproducible temporal patterns.

Repeated encounters are regular and identical.

Individuals with repeated encounters form a large, interconnected network.

Abstract

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.