Berkeley researchers have pointed out that urban density is good and that suburban sprawl is bad for the environment -
"A key finding of the UC Berkeley study is that suburbs account for half of all household greenhouse gas emissions, even though they account for less than half the U.S. population. The average carbon footprint of households living in the center of large, population-dense urban cities is about 50 percent below average, while households in distant suburbs are up to twice the average.
“Metropolitan areas look like carbon footprint hurricanes, with dark green, low-carbon urban cores surrounded by red, high-carbon suburbs,” said Christopher Jones, a doctoral student working with Kammen in the Energy and Resources Group. “Unfortunately, while the most populous metropolitan areas tend to have the lowest carbon footprint centers, they also tend to have the most extensive high-carbon footprint suburbs.”"Spatial Distribution of U.S. Household Carbon Footprints Reveals Suburbanization Undermines Greenhouse Gas Benefits of Urban Population Density