The City of Petaluma (Petaluma) (defendant) in California responded to a nearly 25 percent increase in population between 1970 and 1972 by adopting several resolutions collectively known as the Plan. The Plan was intended to limit population increase in order to protect the small-town character and open spaces of Petaluma. The Plan limited the amount of new housing units in projects involving five or more units to 500 within a five-year period. The Plan also positioned a greenbelt around Petaluma and required building permits to be evenly allocated between the eastern and western sections of Petaluma, as well as between single-family dwellings and buildings with multiple residential units. The Construction Industry Association of Sonoma County and two landowners (plaintiffs) filed suit in federal district court, challenging the Plan as: (1) arbitrary; (2) in violation of substantive due-process rights under the Fourteenth Amendment, including the right to travel; and (3) an unreasonable burden on interstate commerce. The district court found that the express purpose and actual effect of the Plan was to exclude substantial numbers of people who would otherwise move to Petaluma. The district court further ruled that certain aspects of the Plan unconstitutionally denied the right to travel by limiting the natural population growth of the area. Accordingly, the district court enjoined Petaluma from implementing the unconstitutional aspects of the Plan. Petaluma appealed.