  Unlike the lockdown measures taken in some countries or cities, the Japanese
government declared a "State of Emergency" (SOE) under which people were only
requested to reduce their contact with other people by at least 70 %, while
some local governments also implemented their own mobility-reduction measures
that had no legal basis. The effects of these measures are still unclear. Thus,
in this study, we investigate changes in travel patterns in response to the
COVID-19 outbreak and related policy measures in Japan using longitudinal
aggregated mobile phone data. Specifically, we consider daily travel patterns
as networks and analyze their structural changes by applying a framework for
analyzing temporal networks used in network science. The cluster analysis with
the network similarity measures across different dates showed that there are
six main types of mobility patterns in the three major metropolitan areas of
Japan: (I) weekends and holidays prior to the COVID-19 outbreak, (II) weekdays
prior to the COVID-19 outbreak, (III) weekends and holidays before and after
the SOE, (IV) weekdays before and after the SOE, (V) weekends and holidays
during the SOE, and (VI) weekdays during the SOE. It was also found that travel
patterns might have started to change from March 2020, when most schools were
closed, and that the mobility patterns after the SOE returned to those prior to
the SOE. Interestingly, we found that after the lifting of the SOE, travel
patterns remained similar to those during the SOE for a few days, suggesting
the possibility that self-restraint continued after the lifting of the SOE.
Moreover, in the case of the Nagoya metropolitan area, we found that people
voluntarily changed their travel patterns when the number of cases increased.
