On March 16th, 2020 the public health officers of Alameda, Contra Costa, Marin, San Francisco, San Mateo and Santa Clara counties, along with the City of Berkeley, announced a legal order directing residents to shelter at home for 3 weeks beginning March 17th, 2020. This order was a unified step taken in order to mitigate the spread of the novel coronavirus and it's resulting illness (COVID-19) and protect health care capacity across the Bay Area region. Even before the county orders went into place, individual counties began taking steps to limit nonessential travel in their jurisdictions.

This progression of travel limitations resulted in the Bay Area experienced an unprecedented drop in vehicle traffic, as residents stayed home from work and limited their car travel to essential trips. The effect of this immediate, significant decrease in traffic can be observed in the air quality data captured by the BEACO₂N nodes during the time leading up to and following the shelter in place order. In the top figure below, we can see the significant decrease in CO₂ concentrations during the shelter-in-place time compared to the previous weeks. This difference is distributed spatially in the second figure, where it is evident that the largest decreases in CO₂ are from decreased traffic flow on the major highways.

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