On March 24, 1989, the Exxon Valdez supertanker struck Bligh Reef in Prince William Sound, dumping eleven million gallons of oil into Alaska's once-pristine coastal ecosystem. Congress passed the Oil Production Act (OPA). In part, the act established substantive tanker design and evaluation requirements to prevent spills. Section 4110--requires D, by August 18, 1991, to promulgate regulations establishing minimum compliance standards and use requirements for tank level and pressure monitoring (TLPM) devices. Section 4116(c)--requires D, by February 18, 1991, to initiate the issuance of regulations to define waters, including Prince William Sound and two other named areas, over which single-hulled tankers must be escorted by at least two towing vessels. Ps filed the present mandamus petition in December 1999, seeking to compel D to comply with its obligations under both § 4110 and § 4116(c) of the OPA. D has episodically engaged in some rulemaking and promulgated some regulations, pursuant to each of the provisions at issue. Approximately three months before the statutorily-imposed deadline on § 4110, D issued an advanced notice of proposed rulemaking seeking comments and suggestions regarding possible proposed rules for complying with §§ 4110(a) and (b). D also commissioned a technical feasibility study of existing TLPM devices, released in early 1993, which confirmed that, as of 1993, 'existing level detectors [were] not sufficiently sensitive to detect leakage before a large discharge occurred.' The study found that 'attainable accuracy is expected to be within 1.0-2.0% of the actual level.' D deemed this would provide 'insufficient warning to allow prompt action by the crew.' D called for a public hearing to augment comments to the original advanced notice. In 1995 D limited its proposed rule to the establishment of standards for TLPM devices pursuant to § 4110(a), leaving questions of installation and use of compliant devices, pursuant to § 4110(b), for another day. D acknowledged that 'currently available devices may not meet the proposed standards for meaningful leak detection. In March 1997, D adopted the proposed standards in the form of a temporary rule, effective for two years beginning April 28, 1997. D let the temp rule expire because the technology wasn’t available. There are currently no regulations in place under either of § 4110's two provisions. Moreover, the Coast Guard never even attempted rulemaking pursuant § 4110(b). For section 4116(c) D issued a final rule in August of 1994. The final rule did not expand coverage beyond the statutorily-mentioned areas. To date, the Coast Guard has not promulgated final 'other waters' escort requirements. D has stated that 'extending escort requirements beyond the OPA 90 mandated areas is discretionary.' The lower court refused to grant mandamus. Ps appealed.