Almost everyone I see commuting is a single driver. What people should consider is lugging less car to work with them. In Canada we've had the Smart Car for a number of years, and you see them everywhere in Vancouver. I'm interested to see how they sell in the US for under $12k.
Metro/Firefly/Swift/Sprint drivers have been enjoying 55mpg economy since the 80's.
http://www.gassavers.org/garage/viewgaslog/121
As for the Volt, "Chevy Volt Designers: Shape, Not Weight, Is More Important"
http://www.autoobserver.com/2007/12/chevy-volt-desi.html
Just what we need, another bloated pig on the road, courtesy of the General.