(EIA 2006b), and diesel (primarily in medium- and heavy-duty vehicles) accounted for approximately 17% of energy used.
The past four decades have seen a substantial national effort to regulate the emissions from transportation, starting with light-duty vehicles in the 1970s, and moving to heavy-duty on-road vehicle, and most recently to a range of other transportation sources, including construction and agricultural equipment, locomotives, boats, and ships (NRC 2004c). These efforts have been driven in part by even stricter standards adopted by California, which have in turn been adopted by a number of states. The result has been substantial reductions in emissions and ambient levels of a number of pollutants, even as vehicle miles have increased. For example, there have been substantial reductions of ambient levels of carbon monoxide (CO), in most cases to levels below2 the current National Ambient Air Quality Standards (NRC 2003b).