Cool images from the Center for Neighborhood Technology show that people who live in dense urban areas — downtown San Francisco, the denser parts of L.A., or the Chicago city core — emit less CO2 for transportation. See for yourself:
I’m showing San Francisco above because it’s closest to my heart, 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 — just drive less. City dwellers also tend to drive smaller cars, which helps, too — see, e.g., this PDF.