I'm an essential worker, so I kept commuting pretty much like normal throughout the pandemic.

During the initial lockdowns I was averaging a whole MPG better just from not having any traffic.

And the real kicker is that my schedule is kind of weird, so I already commute at times when traffic isn't too bad, I normally start at 2:30 in the afternoon and work 12 hours until the 2:30 in the morning (before anyone asks, my job isn't very physically demanding, and I have more and more frequent days off, so 12 hour shifts aren't too bad) so I'm going in after people have been running out to do stuff on their lunch breaks and before schools let out so traffic is minimal then, and I usually don't even see a half dozen other cars on the road when I'm heading home, and some of my shifts are weekends so traffic is usually even lighter during the day. And my commute is only about 10 miles/20 minutes, no highways or anything, just normal semi-rural to suburban main roads.

And so a slight reduction in traffic during my commute into work (and no real difference to my commute home) got me a small but noticeable difference in my average fuel economy. Now all told that means I probably only saved a few gallons, maybe a tank of gas myself, but think of all of the millions of people who commute in much heavier traffic both ways, possibly even further, and how much extra gas they're burning releasing CO2 and other pollutants into the atmosphere.

Imagine what more people being able to work from home, better public transit and carpooling to reduce number of cars on the road, companies staggering the start/end times of their business days so that everyone isn't commuting at the same time, etc. could do.