Consider the trajectories of some past energy initiatives announced with similar enthusiasm as the president’s new fuel economy plan, which calls for new cars to average 35.5 miles a gallon by 2016. In 2007, for example, Congress set quotas for ethanol made from plant matter to displace gasoline. Next year, refiners are required to use 100 million gallons. The problem is, no one has figured out yet how to produce it in commercial quantities. “The soup’s not quite cooked yet,” said Mitch Mandich, a director of Range Fuels, one of the companies trying to build such a refining operation.

In 2005, Congress passed an energy law requiring that the standard incandescent light bulb be 25 to 30 percent more efficient, beginning in 2012. But engineers are still working on a way to meet that goal.

More than half the states have mandated “renewable portfolio standards” requiring that some percentage of their electricity come from nonfossil fuel sources. But would-be developers of these energy alternatives have had trouble lining up money, equipment and production sites fast enough to meet the goals in some states.