The Federal Highway Administration (FHWA) (defendant) was required by statute, 23 U.S.C. § 151, to set maximum intervals for state-highway-bridge inspections. The FHWA set the maximum interval at two years. In 1984, the FHWA affirmed the importance of the two-year interval. In 1988, a new FHWA rule provided criteria by which states could inspect some bridges less frequently than two years, or not at all. The FHWA said it changed its 1984 position based on unspecified later studies. Also, for the first time, the 1988 rule required inspection of underwater bridge supports. Lacking better data about the appropriate frequency of underwater inspections, the FHWA relied on professional engineer recommendations to set the underwater-bridge inspections at five-year intervals. The FHWA denied a petition from the Center for Auto Safety, Inc. (Center) (plaintiff) to review the amended 1988 regulations. The Center sued the FHWA, arguing that the FHWA's amended regulations were adopted arbitrarily and capriciously in violation of the Administrative Procedure Act, 5 U.S.C. § 551 et seq. Specifically, the Center argued the amendment providing exemptions from the two-year interval violated 23 U.S.C. § 151 because the exemption provided no maximum inspection interval at all. The Center also argued the underwater-bridge inspection intervals should be two years, like the other bridges. The district court upheld the FHWA's amended rules. The Center appealed to the United States Court of Appeals for the District of Columbia Circuit.