  Recently, with the availability of various traffic datasets, human mobility
has been studied in different contexts. Researchers attempt to understand the
collective behaviors of human movement with respect to the spatio-temporal
distribution in traffic dynamics, from which a gravitational scaling law
characterizing the relation between the traffic flow, population and distance
has been found. However, most studies focus on the integrated properties of
gravitational scaling, neglecting its dynamical evolution during different
hours of a day. Investigating the hourly traffic flow data of Beijing subway
network, based on the hop-count distance of passengers, we find that the
scaling exponent of the gravitational law is smaller in Beijing subway system
compared to that reported in Seoul subway system. This means that traffic
demand in Beijing is much stronger and less sensitive to the travel distance.
Furthermore, we analyzed the temporal evolution of the scaling exponents in
weekdays and weekends. Our findings may help to understand and improve the
traffic congestion control in different subway systems.
