  While large scale mobility data has become a popular tool to monitor the
mobility patterns during the COVID-19 pandemic, the impacts of non-compulsory
measures in Tokyo, Japan on human mobility patterns has been under-studied.
Here, we analyze the temporal changes in human mobility behavior, social
contact rates, and their correlations with the transmissibility of COVID-19,
using mobility data collected from more than 200K anonymized mobile phone users
in Tokyo. The analysis concludes that by April 15th (1 week into state of
emergency), human mobility behavior decreased by around 50%, resulting in a 70%
reduction of social contacts in Tokyo, showing the effectiveness of
non-compulsory measures. Furthermore, the reduction in data-driven human
mobility metrics showed correlation with the decrease in estimated effective
reproduction number of COVID-19 in Tokyo. Such empirical insights could inform
policy makers on deciding sufficient levels of mobility reduction to contain
the disease.
