C. Noise Measurement Methodology

Section 4(f) of the DOT Act, 49 U.S.C. s 303 (1983), makes it "the policy of the United States Government that special effort should be made to preserve the natural beauty of the countryside and public park and recreation lands, wildlife and waterfowl refuges, and historic sites." Accordingly, s 303(c) provides:

The Secretary [of Transportation] may approve a transportation program or project requiring the use ... of publicly owned land of a public park, recreation area, or wildlife and waterfowl refuge of national, State, or local significance, or land of an historic site of national, State, or local significance ... only if --

(1) there is no prudent and feasible alternative to using that land; and

(2) the program or project includes all possible planning to minimize harm to the park, recreation area, wildlife and waterfowl refuge, or historic site resulting from the use.

The FAA, applying its standard "Part 150" noise measurement technique, as set forth at Appendix A to 14 C.F.R. s 150, found that the protected areas -- viz., parks and historic sites--potentially affected by the DFW Airport expansion would not be "used" within the meaning of s 4(f). The Part 150 guidelines rely upon weighted average day and night sound level measurements (so-called Ldn) "for the analysis and characterization of multiple aircraft noise events and for determining the cumulative exposure of individuals to noise around airports." 14 C.F.R. Pt. 150, App. A. s A150.3(b). The guidelines provide that a recreational area is "used" by noise at a level of 70 to 75 Ldn. There is no guideline specifically for historic sites in Part 150. As it turns out, however, all the historic sites at issue in this case are in fact residential properties, and the FAA therefore used the guideline for residences, which is 65 Ldn. Id. at App. A Table 1.

The petitioners argue that use of the Part 150 guidelines is inappropriate because they were not designed for the purpose of s 4(f) but in connection with other types of land use and funding decisions. They also contend that it was arbitrary and capricious for the FAA not to rely upon indicators measuring the intensity of single noise events, or upon "a computer simulation technique" that "replicate[s] the aircraft noise that people actually will hear."

As set forth in its Decision, the FAA takes the position that the Ldn system is the "best measure of noise exposure to identify significant impact on the quality of the human environment and is the only noise metric with a substantial body of scientific data on the reactions of people to noise." This court has previously deferred to the agency's expertise in choosing the appropriate way to measure noise; indeed we have specifically approved the FAA's use of the Ldn metric. See Sierra Club v. DOT, 753 F.2d 120, 128 (D.C.Cir.1985); Citizens Against Burlington, 938 F.2d at 201. The petitioners offer no distinction upon which we could withhold our approval in this case.

Furthermore, we note that in 1990 the EPA and the FAA undertook a joint study of noise measurement methodology, and that pending the completion of their study the FAA agreed to include a single event noise analysis in its assessment of impacts at a number of airport projects, including the DFW Airport expansion project. Accordingly, although the FAA's noise analysis in this case "was primarily based upon the development of Ldn contours," in considering the environmental effect of noise caused by the expansion project the agency also "estimat[ed] levels of annoyance, conduct[ed] a time-above threshold analysis, determin[ed] single-event noise levels and evaluat[ed] the potential noise impacts of airspace actions occurring at altitudes of up to 18,000 feet above ground level." In view of our prior decisions, and of the FAA's ongoing cooperation with the EPA to study different methods of evaluating noise pollution, the agency's "primary reliance" upon the Ldn metric in this case was surely neither arbitrary nor capricious.

The petitioners also challenge the FAA's application to historic sites of the Part 150 standard for "residential properties." As they point out, we have previously noted that "while it is reasonable for the agency to rely on guidelines in determining whether a 'use' of section 4(f) lands has occurred, [the standard used] must bear some relevance to the value, significance, and enjoyment of the lands at issue." Allison v. DOT, 908 F.2d at 1029. In that case we held that the DOT could not apply the standard for "recreational parks," which include amusement parks and recreational waters, to determine whether airport noise would constructively use a wildlife preserve, the purpose of which is to provide "tranquility and the opportunity to observe nature undisturbed by human activity." Id.

In this case the historic sites at issue are in daily use as "residential properties." There might well be instances in which the 65 Ldn standard for residential properties would be inadequate to protect the particular values that led to the designation of a site as historic. Consider, for example, a village preserved specifically in order to convey the atmosphere of rural life in an earlier (and presumably a quieter) century. Cf. Communities, Inc. v. Busey, 956 F.2d 619, 624 (6th Cir.1992) (suggesting no s 4(f) use because "noise would not affect the relevant characteristics of [an historic neighborhood]--its architecture and its place in history"). There is no reason, however, to believe that the use standard applicable to a private home is inapposite merely because the home is historic. Therefore, we uphold in all respects the noise measurement methodology that the FAA used in this case.

D. Conditional Approval of the West Runway

The petitioners and the National Trust for Historic Preservation, amicus, argue that the FAA unlawfully approved the proposed West Runway before completion of the review process required by the NHPA, 16 U.S.C. ss 470-470w-6. That Act requires each federal agency to take responsibility for the impact that its activities may have upon historic resources, and establishes the Advisory Council on Historic Preservation (ACHP) to administer the Act. Section 470f (Title I, Section 106) requires that:

The head of any Federal agency ... shall, prior to the approval of the expenditure of any Federal funds on the undertaking ... take into account the effect of the undertaking on any district, site, building, structure, or object that is included in or eligible for inclusion in the National Register [of Historic Places]. The head of any such Federal agency shall afford the [ACHP] ... a reasonable opportunity to comment with regard to such undertaking.

In its Decision the FAA conditioned final approval of the West Runway upon its subsequent reevaluation, pursuant to Order 5050.4A, of the runway's impact; the agency specifically noted that as part of that reevaluation it would "take into account the conclusions and recommendations arising out of the consultation process required by Section 106" of the NHPA. Seven months later (in November 1992) the FAA transmitted to the Texas Historical Commission its final assessment of the effects of the West Runway. After a further six-month period of deliberation the FAA, the Texas Historical Commission, and the DFW Airport Board entered into an agreement finding that there would be no adverse effect within the meaning of the NHPA, and submitted that finding to the ACHP. The ACHP then informed the FAA that it disagreed with the finding and that it wished to consult with the FAA about alternatives and mitigation measures.

Throughout these events the FAA followed the procedure prescribed by the ACHP's regulations. Much of the relevant activity, however, took place after the FAA had issued its Decision. Although it is of course desirable for the s 106 process to occur as early as possible in a project's planning stage, we do not agree with the petitioners that in this case the FAA's conditional approval of the West Runway violated any requirement of the NHPA. Merely by issuing its Decision the FAA did not "approv[e] the expenditure of any Federal funds" for the runway. Recall that the FAA is not the sponsor of the airport project; if the DFW Airport Board commits its own resources to the West Runway -- for further planning, engineering, or what have you short of construction -- although the runway was only conditionally approved, then it does so at the risk of losing its investment should the s 106 process later turn up a significant adverse effect and the FAA withdraw its approval. In sum, because the FAA's approval of the West Runway was expressly conditioned upon completion of the s 106 process, we find here no violation of the NHPA.

The scope of the FEIS and of the alternatives that the FAA considers therein demonstrate that the agency took a "hard look" at the DFW Airport expansion project and fully complied with its own and the CEQ's regulations implementing the NEPA. The FAA's primary reliance upon its established Ldn noise measurement methodology for the purpose of determining whether the project would "use" any historic property within the meaning of s 4(f) of the DOT Act was not arbitrary or capricious. Nor do we find a violation of the NHPA in the FAA's conditional approval of the West Runway pending completion of the consultations required by s 106 of that Act. For these reasons, the petitions for review are

Denied.