Travel Emissions Across the Country
Measuring emissions by county, the smallest geography for which household, vehicle ownership, and vehicle miles traveled data are available, the results may be interpreted from two different perspectives. At the county level, measurements of VMT, and therefore CO2 emissions, tend to be higher in the places one would expect: the two coasts, the upper Midwest, and the larger American cities. At the household level, however, this relationship reverses, and precisely those regions that emit the most GHGs per unit area, emerge as the most efficient in terms of emissions per household.
The United States Environmental Protection Agency collects data on criteria pollutants generated by vehicle travel in the United States per county. Maps generated with this data do not include carbon dioxide or other greenhouse gases, because they are not regulated by current pollution control measures. The Center For Neighborhood Technology has utilized the EPA's vehicle miles traveled data to map carbon dioxide emissions from automobile use for each county in the U.S.
The EPA obtains VMT estimates that the U.S Federal Highway Administration collects from state bureaus of transportation. The states formulate the estimates by conducting traffic counts in each county and projecting those figures to arrive at an estimated miles traveled per year in each county. Motor gasoline converts to a known amount of carbon dioxide, and so the carbon dioxide emissions from vehicle miles traveled in each county can be estimated by using an average fuel consumed per miles traveled.
Emissions from travel can been approached in two different ways. Places like Los Angeles, Houston, Chicago, Atlanta and other large metropolitan areas have smog problems in the summer months because of the number of people driving each day. But how far are those urban drivers traveling each day compared to drivers in rural areas where smog is never a problem?
Analysis of county VMT figures indicate that, though total VMT is much higher in urban than in rural counties, the estimate of miles driven per household in counties with dense development is significantly less than in their rural equivalents. People who live close to jobs, shopping, and other amenities travel shorter distances than people who live where jobs, shopping, and amenities are spread out over a larger area. So, while more carbon dioxide is produced in densely populated counties, each household in dense counties is producing less CO2 than a similar rural household.
High levels of emissions can also been seen in counties that are traversed by interstate highways, most conspicuously those corridors in the Great Plains followed by interstates 70, 80, and 90. The visibility of highway corridors in maps derived from county VMT reveals a limitation in the representations drawn from the EPA data, based as it is on traffic counts.
One powerful explanation for the sharp contrast between rural and urban driving emissions is that households in urban areas tend to have multiple transportation options for a given trip. Transit is much more prevalent in urban areas: density increases transit's economic viability. When distances are closer together, people have the additional option of walking or biking to destinations instead of always getting in the car. Making regional planning decisions based on principles of sustainable development and the importance of public transportation is one way of contributing towards climate stabilization and improving the health of communities.