In total, 8.5 percent of fuel used by the transportation sector came from non-petroleum sources in 2014. Biomass from corn-based ethanol - still supported by generous government subsidies - represented the largest non-petroleum energy source and was used primarily to fuel cars and other light vehicles. Use of natural gas to operate pipelines followed close behind. The report also shows smaller but still significant increases in the use of electricity, biodiesel and natural gas in vehicles.

Climate change and fluctuating oil prices has made moving away from petroleum when possible a priority for governments and corporations alike. But it’s still uncertain which fuel will be the best and greenest replacement, according to Christopher R. Knittel, an MIT professor of energy economics. Ethanol, natural gas, hydrogen and electricity are all possibilities.

"We don’t know where we’ll be 50 years from now," said Knittel. "There are four potential replacement for petroleum, and, ultimately, we don’t what's going to win out."

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