University of Pennsylvania law professor Herbert Hovenkamp said regulators need to be creative to determine a gas station's market. They consider who is driving in and out of a community, for example, tapping U.S. Census Bureau survey data about commuter patterns as well as any traffic data that local transportation departments may provide.
Regulators also may seek data from the gas station owners, determining who their customers are. In some cases, the owners may volunteer the data, hoping to prove they aren't exerting monopoly power.
That happed in a famous antitrust case involving Casey's, Hovenkamp said. In the mid-1980s, when annual sales dropped, the company's executives decided to sell gas at a loss in some small towns, according to a U.S. Court of Appeals ruling.