Cool images from the Center for Neighborhood Technology show that people who live in dense urban areas—downtown San Francisco, the denser parts of LA, or the Chicago city core—emit less climate-warming emissions from cars and trucks. See for yourself:
I’m showing San Francisco above because it’s closest to the Northwest, but I think the Chicago map is coolest..
The simple explanation for the pattern is that people who live in denser neighborhoods—places where stores, services, and jobs tend to be a short drive away, or even within walking distance—simply drive less. City dwellers also tend to drive smaller cars, which helps, too—see, e.g., this pdf.