The federal government has played a major role in influencing transportation options. Land grants subsidized railroad construction in the nineteenth century. In the twentieth century, billions have been spent on highway construction. Federal funds have also aided the construction of airports.

Assistance from all levels of government to the air, rail, bus, and auto industries is a complicated subject that cannot be tackled in this book. Special attention needs to be given to how government decisions and government spending influence the transportation decisions of ordinary people. The outcome of present policies has been to encourage or at least not significantly discourage the American penchant for flying in airplanes and driving cars—the two most energy intensive ways to travel.

Energy intensiveness in travel (energy use per passenger mile) is the outcome of three factors: the amount of fuel needed to propel the vehicle, the vehicle's speed, and the number of passengers carried. Estimates of energy use per passenger mile differ, and a change in the estimate for one of these factors can alter the estimate of energy use per passenger mile considerably (see Table 4-2). For example, a city bus that is one-fifth full uses an estimated 3,800 Btu's per passenger mile. That same bus travelling with a full passenger load only uses 760 Btu's per passenger mile (Table 4-5). If all vehicles were driven only when every passenger seat was full, the energy intensiveness of each transportation