Before 1988, states were required to inspect their respective highway bridges at least every two years. Title 23 U.S.C. § 151(a) requires D to establish 'national bridge inspection standards' to provide for 'the proper safety inspection and evaluation of all highway bridges.' Section 151(b) imposes various minimum requirements that the inspection standards must satisfy. Section 151(b)(2), the focus of much of this controversy, provides that the standards must 'establish the maximum time period between inspections.' The Secretary has delegated his section 151 responsibilities, among others, to the Federal Highway Administration. See 23 C.F.R. § 1.37 (1991). In 1988, D amended the bridge inspection standards. States were allowed to apply for, and D to approve, bridge-specific exemptions from the two-year inspection rule. D justified the new section 650.305(c) as a means of providing the states 'greater flexibility with which to utilize available inspection resources in a cost-effective manner.' D reasoned that these savings could be redirected into bridge replacement programs. D asserted that the two-year inspection interval 'can be increased for some categories of bridges with only a minimal or negligible increase in risk to the public.' In 1984, D had concluded that the safety benefits of a strict two-year rule 'far outweigh' the potential economic benefits of less frequent inspections. In justifying the change, D cited with little elaboration its 'further review and analysis since April of 1984.' Section 650.303(e) also required for the first time that bridges with 'underwater members,' be inspected at least every five years. This new requirement was based on the collective best judgment of professional bridge, hydraulic and geotechnical engineers as expressed by the current AASHTO Guide for Bridge Maintenance Inspection along with comments from rulemaking. P petitioned for review and D denied the petition. Ps sought declaratory and injunctive relief in the district court in that they were arbitrary and capricious. The court granted D summary judgment, and P appealed.