and local environmental requirements. Urban areas with particularly dirty air are required by federal law to sell "reformulated gas" for all or part of the year, and because the cleaner-burning fuel is refined through a special process, it tends to be a little more expensive than regular gas. Within Missouri, for example, St. Louis and Kansas City have higher prices compared with the rest of the state because of mandates for cleaner gas. California uses a unique blend of gas due to environmental regulations and is generally cut off from supplies east of the Rockies; there, prices can spike to more than 40 cents above the national average.

Yet one piece of legislation designed to help the environment may have had the unusual effect of reducing gas prices inthat additional capacity.

Explainer thanks Severin Borenstein of the University of California Energy Institute, Hayley Chouinard of Washington State University, Michael Davis of the Missouri University of Science and Technology, Tom Kloza of the Oil Price Information Service, Matthew Lewis of Ohio State University, Erich Muehlegger of Harvard University, and the Missouri Department of Natural Resources Energy Center.

Correction, June 10, 2008: Due to an error introduced during editing, diesel fuel was incorrectly described as a kind of gasoline. (Return to the corrected sentence.)