The roughly 200 million automobiles and light trucks currently in use in the United States travel approximately 2.4 trillion miles every year, and consume almost two-thirds of the U.S. oil supply. They also produce about two-thirds of the carbon monoxide, one-third of the lead and nitrogen oxides, and a quarter of all volatile organic compounds (VOCs). More efficient transportation energy use could have dramatic effects on environmental quality as well as saving billions of dollars every year in our payments to foreign governments. In response to high gasoline prices in the 1970s and early 1980s, automobile gas-mileage averages in the United States more than doubled from 13 mpg in 1975 to 28.8 mpg in 1988.

Unfortunately, cheap fuel prices and the popularity of sport utility vehicles (SUVs) and light trucks in the 1990s caused fuel efficiency to slide back below where it was 25 years earlier. By 2002, the average mileage for U...