They found that 72 percent of county workers drove to work alone,” says Shoup, “but 60 percent of federal employees carpooled, took public transportation, or even walked. These were workers in the same professions, driving to the same location.” When forced to pay a practical value for their parking, drivers were twice as likely to carpool—traffic congestion was halved, carbon emissions were halved. “The more I thought of that,” says Shoup, “the more I thought there was a perfect storm here. No one can tell you why parking prices are set as they are. But when people pay comparatively little for something that’s expensive to produce, the result is collective irrational behavior.