The effect of interurban movements on the spatial distribution of population in China

TL;DR

This study analyzes how interurban travel during Chinese New Year affects population distribution and epidemic spread, highlighting the impact of travel restrictions on population concentration and disease transmission dynamics.

Contribution

It provides a quantitative comparison of interurban movement patterns in 2019 and 2020, revealing how travel bans influence population distribution and epidemic risk.

Findings

Travel flows are highly fluctuating and dispersed during holidays.

Large movements create hubs that facilitate epidemic spread.

Travel restrictions slow population redistribution but increase intra-city concentration.

Abstract

Understanding how interurban movements can modify the spatial distribution of the population is important for transport planning but is also a fundamental ingredient for epidemic modeling. We focus here on vacation trips (for all transportation modes) during the Chinese Lunar New Year and compare the results for 2019 with the ones for 2020 where travel bans were applied for mitigating the spread of a novel coronavirus (COVID-19). We first show that these travel flows are broadly distributed and display both large temporal and spatial fluctuations, making their modeling very difficult. When flows are larger, they appear to be more dispersed over a larger number of origins and destinations, creating de facto hubs that can spread an epidemic at a large scale. These movements quickly induce (in about a week) a very strong population concentration in a small set of cities. We characterize quantitatively the return to the initial distribution by defining a pendular ratio which allows us to show that this dynamics is very slow and even stopped for the 2020 Lunar New Year due to travel restrictions. Travel restrictions obviously limit the spread of the diseases between different cities, but have thus the counter-effect of keeping high concentration in a small set of cities, a priori favoring intra-city spread, unless individual contacts are strongly limited. These results shed some light on how interurban movements modify the national distribution of populations, a crucial ingredient for devising effective control strategies at a national level.