Federal and state governments—and formerly high gas prices—have given consumers incentive to purchase fuel-efficient cars like the Toyota Prius, shown here at a California dealership in 2010. Those dynamics have changed over the past few months as oil and gas prices have plunged. Bloomberg News The sharp drop in fuel prices is undermining government policies that bet expensive gasoline would prod Americans to find alternatives to gas-guzzling automobiles. In recent years, the federal government required passenger cars and trucks to become much more fuel-efficient, with a goal to more than double average miles per gallon by 2025. More than a dozen states ordered auto makers to boost sales of electric vehicles, helped by billions of dollars in federal spending. And President Barack Obama ’s administration increased spending on intercity passenger rail, joining states like California in planning for Asian- and European-style bullet trains. Those policies now face a big […]