The World Resources Institute has a geekalicious image (see page 15 of this pdf) that shows how human activities contribute to global warming. Click on the image below to get a legible, full-sized version.
The most interesting bit, to me, is the massive contribution of agriculture and land use -- the green and purple areas on the bottom left -- to global climate change. Worldwide, the net impacts of land use are far more significant than global transportation emissions.
But in the Northwest, the situation is reversed. At the moment, forests in this part of the world appear to be carbon sinks, and transportation is far and away the largest single source of climate warming emissions. So just because transportation isn't the biggest deal globally, it's still where state and regional policymakers should focus their attention.