The transportation sector is the second largest contributor of greenhosue gas emissions (mainly in the form of carbon dioxide) in the United States (see figure 4B [1]). This figure shows the trends in aggregate and per capita vehicle miles traveled by Americans per year.
Links:
[1] http://www.pewclimate.org/global-warming-basics/facts_and_figures/us_emissions/usghgemsector.cfm
[2] http://www.pewclimate.org/global-warming-basics/facts_and_figures/us_emissions
[3] http://www.pewclimate.org/global-warming-basics/facts_and_figures/us_emissions
[4] http://www.pewclimate.org/global-warming-basics/facts_and_figures