The data for the past four weeks show a significant decline in gasoline usage compared to a year earlier -- down 2.2 million barrels per day over four weeks, or 25%. There are 42 gallons in a barrel, so this represents (take the two...times two, times seven...four plus eight...carry the one...) a savings of 2.6 Bgal in about a month (four weeks), or 23 Mt CO2. That's equivalent (EPA says) to 4.9 M car-years.
For comparison, the US emitted about 5,140 Mt CO2 last year, about 25% from transportation. Of course there will also be savings from the industrial sector and from aviation, but an increase from the residential sector, electricity maybe a wash. The total monthly CO2 emissions for February won't be out until late May, but I won't be surprised to see a drop in US emissions for the year just because of the coronavirus.