The US also imports quite a bit of food (I'm not finding a definitive figure right now, but seeing 15% in several places), so that complicates the calculation a bit. Also, you'd want to see a breakdown of how much transportation comes from agricultural activities as well, to get a better idea of the full picture.

Still, agriculture is a big player no matter how you cut it, and your own eating habits are a lot easier to control than overall policy or law changes (and probably even your transportation, given the strong need for cars).