  The COVID-19 pandemic offers an unprecedented natural experiment providing
insights into the emergence of collective behavioral changes of both exogenous
(government mandated) and endogenous (spontaneous reaction to infection risks)
origin. Here, we characterize collective physical distancing -- mobility
reductions, minimization of contacts, shortening of contact duration -- in
response to the COVID-19 pandemic in the pre-vaccine era by analyzing
de-identified, privacy-preserving location data for a panel of over 5.5 million
anonymized, opted-in U.S. devices. We define five indicators of users' mobility
and proximity to investigate how the emerging collective behavior deviates from
the typical pre-pandemic patterns during the first nine months of the COVID-19
pandemic. We analyze both the dramatic changes due to the government mandated
mitigation policies and the more spontaneous societal adaptation into a new
(physically distanced) normal in the fall 2020. The indicators defined here
allow the quantification of behavior changes across the rural/urban divide and
highlight the statistical association of mobility and proximity indicators with
metrics characterizing the pandemic's social and public health impact such as
unemployment and deaths. This study provides a framework to study massive
social distancing phenomena with potential uses in analyzing and monitoring the
effects of pandemic mitigation plans at the national and international level.
