I Evaluation of Natural Gas as an Alternative Fuel in

The energy issue in transportation for the next 20 years is not one of adequate supply but rather it is a combination of overdependence on imported petroleum and of managing a transition to economically, strategically and environmentally desirable energy sources. More than a quarter of all energy consumption in the United States is attributable to transportation, as is nearly two-thirds of all petroleum use.1 In 1991 nearly 12 million of the 18 million barrels of U.S. daily oil was consumed by 185 million cars, trucks and buses as well as aircraft and locomotives. During the first seven months of 1990 more than half the oil used in the United States was imported, the highest level ever. Oil imports are projected to rise in the future as old U.S. oil fields run dry,z The interdependence between transportation and the environment has been recognized, and greater care is being exercised to keep the two appropriately balanced. A host of environmental laws establish the legal framework that guides transportation decisions towards that goal. The Clean Air Act requires that, beginning in 1998, large-city operators of centrally fueled fleets of 10 or more vehicles begin to use “clean” fuels. In Texas, one-third of