"My guess is that old polluting vehicles are slowly dying out and being replaced with the clean burning cars we have today that just so happen to get much better gas mileage than before. Also, low sulfur diesel fuel has started to hit the market."

Cars aren't nor were they ever a major source of CO2 emission. And though short term graph- it shows growth in gasoline use:

Thought this article might explain some of it:
"Using electronics to telecommute saves the equivalent of 9 to 14 billion kilowatt-hours of electricity per year - the same amount of energy used by roughly 1 million U.S. households every year, according to a study commissioned by the Consumer Electronics Association.

The findings also indicate the estimated 3.9 million telecommuters in the U.S. reduced gasoline consumption by about 840 million gallons, while curbing carbon dioxide emissions by nearly 14 million tons. This level of CO2 reduction is equal to removing 2 million vehicles from the road every year. "
U.S. Telecommuters

I would say the "9 to 14 billion kilowatt-hours" saving is more significant than "the 840 million gallons" gasoline in terms of CO2. But I haven't done the math.