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On March 16, 2020, the United States government issued new guidelines promoting public health social social distancing interventions to reduce the spread of the COVID-19 epidemic in the country [1]. In addition, many state and local governments in the United States have enacted stay-at-home policies banning mass gatherings, enforcing school closures, and promoting smart working. So far, however, the extent to which these policies have resulted in reduced people’s mobility has not been quantified. By analyzing data from millions of (anonymized, aggregated, privacy-enhanced) devices, we estimate that by March 23 the the policies have generally reduced by half the overall mobility in several major US cities. In order to gauge the observed results we know events, we note that the commuting volume on Monday, March 16, approached those of a typical snow day or analogous day when public schools are partially closed (ie January 2). By Friday, March 20, we observe commuting numbers that resemble those measured on federal holidays (ie Martin Luther King Jr. Day in January or Presidents’ Day in February). Currently, we are unable to quantify the extent

∗ a. vespignani@ northeastern. edu