In California and other US states, mitigation of the spread of COVID-19 has been implemented by cities, counties, and governors’ offices through “shelter-in-place” and “stay-at-home” orders and related actions (e.g., closure of non-essential businesses). There were several important impacts of government shelter-in-place order on traffic volumes, which in turn had impacts on fuel-use and greenhouse emissions. For all US states, I compared estimated traffic activity (vehicle miles traveled, VMT) before and after stay-at-home guidance. I compared greenhouse gas (GHG) emissions at the state and US scale before and after the guidance, using federal standards/averages for vehicle fuel-mileage and emissions of greenhouse gases (GHG). For California, I also estimated the decrease in fuel-tax revenue as a result of reduced VMT. Depending on the state, the total VMT at the county and state level declined by >50% across the US, following the various government stay-at-home orders. In the first week of March, 2020, US daily travel was equivalent to 4.6 billion gallons of fuel. Due to reduced daily travel following government guidance, the US used only 2.3 billion gallons of fuel in the second week of April. The GHG emissions that cause climate change were reduced by 13.1 MTons CO2E/week (carbon-dioxide equivalents). This is equal to a 3% reduction in total annual US emissions and 9% from transportation in the almost 8 weeks after many stay-at-home orders went into effect. This reduction put the US on track to meet its annual goals for GHG reduction under the Paris Climate Accord. California has a target of 80% reduction in GHG from 1990 levels by 2050. If traffic remained reduced for one year, the reduction in VMT would allow California to meet half of its 2050 climate change target. At an average gasoline price of $2.59 across the US (source: USDOE, Alternative Fuels Data Center, https://afdc.energy.gov/data/), this reduction in use is equivalent to a savings of $3.8 billion/week to US drivers. Every US state charges a fuel tax, which varies by state. We multiplied the state-specific tax rate by the estimated fuel use per state to calculate the total revenue per week for the first week of March and the second week of April. The revenue was reduced from $912 million/week in March to $457 million/week in April, a difference of $455 million. These findings suggest that reduced traffic during pandemic mitigation can have large negative (tax revenue) and positive (benefit to drivers and nature) consequences for a range of environmental and economic conditions. These consequences should feature in future transportation and climate planning as important variables that may stochastically appear and that are beyond the influence of transportation agencies.