Thanks largely to the hydraulic fracturing boom in the United States, it would be far cheaper today to run those heavy trucks on natural gas than diesel—and potentially cleaner, too. (See related: "Natural Gas Nation: EIA Sees Future Shaped By Fracking.") Those rumbling rigs are the fastest-growing contributors to the nation's greenhouse gas emissions from transportation, according to the U.S. Environmental Protection Agency. Widespread use of cleaner-burning natural gas is a step that could address climate change, although the environmental impact is complex and still under study. (See related quiz: "What You Don't Know About Natural Gas.")

There's no question that the trucking industry would present an enormous new market for natural gas: Heavy-duty trucks account for more