Flow trace: A novel representation of intra-urban movement dynamics

The dynamics of human activities can illustrate how a city operates at different times. In today's highly connected urban systems, the travel flow rhythms can help uncover the functionalities of urban spaces. For efficient land use and traffic planning, it is necessary to understand how outflows and inflows of intra-urban regions change under the impact of different land uses. Many efforts have been made to characterize flow rhythms and correlate mixed land uses with intra-urban mobility, but they are limited because they separately analyze outflows and inflows and treat land use impacts as static throughout the day. We thus introduce the concept of ‘flow trace’ to synthesize and compare rhythms of outflows and inflows. The geometric features of flow traces help extract spatiotemporal variations of diurnal outflows/inflows and uncover the impact of land uses on local flows at different times. Using Beijing as the study area, our results demonstrate the effectiveness of flow traces in revealing monocentric flow patterns and changes of functionalities in intra-urban regions. We also investigated how the connections between land uses and flow characteristics change within a day. This useful analytical framework can be generalized to new cities and different datasets for sustainable urban development.