This says 40% of America's GHG is from Transportation:
Reducing Greenhouse Gases Through Traffic Management and Smart Growth
Environmental Defense Fund, Michael Replogle, Transportation Director May 21, 2008
But this table says that overall in the largest 100 US municipalities 59% of per capita contribution to GHG is from transportation, and 3/4 of that is from cars (you'll have to do your own math).
AND the companion brief says metro area residents have smaller carbon footprints than the average American:
Shrinking the Carbon Footprint of Metropolitan America
The Brookings Institution, Andrea Sarzynski, Marilyn A. Brown, Frank Southworth May 2008
(page two of their brief says 33% of GHG comes from transportation across the nation)
How come the discrepancies?
If you look closer at the table, taking ratios city-by-city (for example San Francisco's ratio is a bit over 75%), we are seeing the effect of the predominance of the private vehicle as the producer of GHG. Metro dwellers may have a slightly smaller footprint, but that budget is largely spent on their cars. That will be exacerbated by sprawl, congestion, lousy transit, low transit patronage, outsized vehicles, congestion, longer average commutes, diminished walking, poor bicycle paths and congestion.
The point? In America, metro dwellers may have a slightly smaller footprint, but they could have a very much smaller footprint. Furthermore, since these 100 largest metro areas occupy a tiny fraction of the United States, wide-area cordon pricing could make a huge difference.