                 FOR PUBLICATION

  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT


NATURAL RESOURCES DEFENSE               No. 12-56467
COUNCIL, INC., a non-profit
corporation; EAST YARD                     D.C. No.
COMMUNITIES FOR ENVIRONMENTAL           2:09-cv-08055-
JUSTICE, a non-profit corporation;        JAK-MAN
COALITION FOR A SAFE
ENVIRONMENT, a non-profit
corporation,                              OPINION
               Plaintiffs-Appellants,

                 v.

U.S. DEPARTMENT OF
TRANSPORTATION; RAY LAHOOD, in
his official capacity as Secretary of
Transportation; STATE OF
CALIFORNIA, DEPARTMENT OF
TRANSPORTATION; VICTOR MENDEZ,
Administrator, Federal Highway
Administration; JEFFREY PANIATI, in
his official capacity as Acting
Deputy Director of the Federal
Highway Administration,
                Defendants-Appellees,

ALAMEDA CORRIDOR
TRANSPORTATION AUTHORITY,
    Real Party in Interest-Appellee.
2                        NRDC V. USDOT

         Appeal from the United States District Court
            for the Central District of California
         John A. Kronstadt, District Judge, Presiding

                   Argued and Submitted
             May 14, 2014—Pasadena, California

                      Filed October 30, 2014

      Before: John T. Noonan, Kim McLane Wardlaw,
          and Raymond C. Fisher, Circuit Judges.

                   Opinion by Judge Wardlaw


                           SUMMARY*


                       Environmental Law

    The panel affirmed the district court’s summary judgment
in favor of federal and state defendants in an action brought
by environmental groups alleging that the defendants violated
the Clean Air Act and the National Environmental Policy Act
by failing to properly evaluate and disclose the potential
environmental impact of a planned expressway connecting
the Ports of Los Angeles and Long Beach to the I-405
freeway.

   Pursuant to the Clean Air Act (CAA), the states must
adopt a State Implementation Plan that provides for the

  *
    This summary constitutes no part of the opinion of the court. It has
been prepared by court staff for the convenience of the reader.
                      NRDC V. USDOT                            3

implementation and maintenance of national air quality
standards. The CAA contains a “conformity” provision that
prohibits federal participation in any project that fails to
conform to an approved State Implementation Plan. The CAA
delegated to the Environmental Protection Agency (EPA) and
the U.S. Department of Transportation the duty to establish
procedures to assure conformity for transportation projects.
Pursuant to that authority, the EPA promulgated regulations
mandating a “hot-spot analysis” for certain pollutants,
including PM2.5 - the pollutant at issue. In the course of the
expressway project’s approval process, the defendants
conducted an air quality Conformity Determination, which
involved a qualitative hot-spot analysis of existing
concentration of PM2.5, and an Environmental Impact
Statement (EIS) as required by the National Environmental
Policy Act.

    The panel held that the defendants were not required to
estimate PM2.5 increases within the area immediately adjacent
to the proposed expressway, and concluded that the
defendants’ Conformity Determination complied with the
CAA. Specifically, the panel held that the CAA’s statutory
phrase “any area” was ambiguous, and the governing
regulations did not decisively answer whether the CAA
required qualitative hot-spot analysis within the immediate
vicinity of the project area during the time period at issue, but
the EPA’s and Department of Transportation’s interpretation
of the regulations¯permitting the type of analysis performed
here by the defendants¯were entitled to considerable
deference. The panel further held that the defendants’
Conformity Determination was neither arbitrary nor
capricious.
4                   NRDC V. USDOT

    Finally, the panel held that the EIS prepared by the
defendants took the requisite “hard look” at the freeway
project’s likely consequences and probable alternatives, and
therefore the EIS comported with the National Environmental
Policy Act.


                       COUNSEL

Adriano Martinez (argued), David Pettit, Natural Resources
Defense Council, Santa Monica, California; Robert E.
Yuhnke, Robert Yuhnke & Associates, Boulder, Colorado,
for Plaintiffs-Appellants.

J. David Gunter II (argued), Ignacia S. Moreno, David
Glazer, Norman Rave, United States Department of Justice,
Washington, D.C., for Defendants-Appellees.

Jocelyn Denise Thompson (argued), Sharon Rubalcava,
Shiraz D. Tangri, Marisa Blackshire, Alston & Bird LLP, Los
Angeles, California, for Real Party in Interest-Appellee.


                        OPINION

WARDLAW, Circuit Judge:

    Natural Resources Defense Council, East Yard
Communities for Environmental Justice, and Coalition for a
Safe Environment (collectively “NRDC”) appeal the district
court’s grant of summary judgment in favor of the U.S.
Department of Transportation and other federal and state
defendants (collectively “Defendants”). NRDC argues that
Defendants violated the federal Clean Air Act (“CAA”) and
                         NRDC V. USDOT                                5

the National Environmental Policy Act (“NEPA”) by failing
to properly evaluate and disclose the potential environmental
impact of a planned expressway connecting the Ports of Los
Angeles and Long Beach to the I-405 freeway. We have
jurisdiction under 28 U.S.C. § 1291, and we affirm the district
court’s grant of summary judgment to Defendants.

                                   I.

    The Port of Los Angeles is our nation’s busiest container
port.1 Considered together with the adjacent Port of Long
Beach, this port complex is among the ten busiest in the
world, and it accounts for roughly forty percent of all
waterborne cargo that enters the United States. BILL
SHARPSTEEN, THE DOCKS 53-54 (2011). The port’s cargo
volume is projected to continue rising for decades.2

    Although the ports are an economic boon for the Los
Angeles region, they also affect air quality in the surrounding
area, especially in the adjacent communities of San Pedro and
Wilmington. These impacts are projected to worsen with the
rise in container volume at the ports. The State Route 47
Expressway Project (“Project”) is one of several port-related
infrastructure projects designed to ease traffic congestion and
mitigate air pollution. If built, the Project will connect the
ports to the I-405 freeway via an elevated, 1.7 mile-long


  1
    Strategic Plan 2012-2017, THE PORT OF LOS ANGELES, at 1 (2012),
http://www.portoflosangeles.org/pdf/strategic_plan_2012_lowres.pdf (last
visited Sept. 24, 2014).
 2
    See Port Master Plan, THE PORT OF LOS ANGELES, at 9 (Feb. 2014),
http://www.portoflosangeles.org/planning/pmp/Amendment%2028.pdf
(last visited Sept. 24, 2014).
6                     NRDC V. USDOT

expressway. The Project’s sponsors, which are now the
Defendants in this litigation, assert that the Project will better
integrate the ports with the freeway system, thereby reducing
the need for surface-street travel by trucks carrying shipping
containers, and the pollution generated while they run idle at
traffic signals and railroad crossings.

    In the course of the Project’s approval process,
Defendants conducted an air quality Conformity
Determination and an Environmental Impact Statement
(“EIS”). As one component of the Conformity Determination
study, Defendants performed a qualitative “hot-spot” analysis
that measured existing concentrations of PM2.5, a type of fine
particulate matter, and estimated the Project’s likely impact
on PM2.5 levels. Because there was no PM2.5 receptor located
within the immediate vicinity of the Project, Defendants
based their qualitative hot-spot analysis on data from a
receptor five miles away from the project area. Defendants
released a draft Conformity Determination in November
2008. Following a round of comments and revisions, the
final Conformity Determination was issued in May 2009.

    Meanwhile, Defendants prepared an EIS as required by
NEPA. The EIS process began in 2004, and Defendants
released a draft EIS in August 2007. The draft EIS prompted
numerous comments, including comments from NRDC, to
which Defendants responded at length. The comments also
spurred Defendants to conduct additional studies, such as a
Traffic Sensitivity Analysis and a Health Risk Assessment
that detailed the Project’s likely health impacts, including an
increased risk of cancer in the areas immediately adjacent to
the Project. Defendants released the final version of the EIS
in May 2009, and signed the Record of Decision in August
2009.
                      NRDC V. USDOT                           7

    In November 2009, NRDC filed a complaint in the
Central District of California alleging that Defendants’
approval of the Project violated CAA, NEPA, and the
Administrative Procedure Act. At the district court’s request,
the parties briefed and argued cross-motions for summary
judgment. On June 29, 2012, the district court issued an
order granting summary judgment in Defendants’ favor. This
appeal follows.

                              II.

   A district court’s decision on cross-motions for summary
judgment is reviewed de novo. Am. Civil Liberties Union of
Nev. v. City of Las Vegas, 466 F.3d 784, 790 (9th Cir. 2006).
“We view the evidence in a light most favorable to the non-
moving party and decide whether there are any genuine issues
of material fact and whether the district court correctly
applied the substantive law.” FTC v. Stefanchik, 559 F.3d
924, 927 (9th Cir. 2009).

    Under the Administrative Procedure Act, we must “hold
unlawful and set aside agency action, findings, and
conclusions found to be . . . arbitrary, capricious, an abuse of
discretion, or otherwise not in accordance with law.”
5 U.S.C. § 706(2)(A). “Review under the arbitrary and
capricious standard is narrow, and we do not substitute our
judgment for that of the agency.” Barnes v. U.S. Dep’t of
Transp., 655 F.3d 1124, 1132 (9th Cir. 2011).

    “When Congress has ‘explicitly left a gap for an agency
to fill, there is an express delegation of authority to the
agency to elucidate a specific provision of the statute by
regulation,’ and any ensuing regulation is binding in the
courts unless procedurally defective, arbitrary or capricious
8                    NRDC V. USDOT

in substance, or manifestly contrary to the statute.” United
States v. Mead Corp., 533 U.S. 218, 227 (2000) (quoting
Chevron, U.S.A., Inc. v. Natural Res. Def. Council, Inc.,
467 U.S. 837, 843-44 (1984)). Even if the agency has not
formally interpreted the statute, Chevron deference applies
when it has “promulgated a rule based on an implicit
interpretation of the statute.” Schleining v. Thomas, 642 F.3d
1242, 1246 (9th Cir. 2011). An agency’s interpretation of its
own regulation is “controlling unless plainly erroneous or
inconsistent with the regulation.” Auer v. Robbins, 519 U.S.
452, 461 (2000) (internal quotation marks omitted).

                             III.

                             A.

    The CAA mandates joint efforts between the federal
government and the states to combat air pollution. Under the
CAA’s statutory framework, the federal Environmental
Protection Agency (“EPA”) begins by establishing National
Ambient Air Quality Standards (“NAAQS”) for certain types
of pollutants. 42 U.S.C. § 7409. Then, the EPA designates
areas throughout the United States as “attainment,”
“nonattainment,” or “maintenance” for each type of pollutant
depending on whether these national standards have been
met. See 40 C.F.R. § 93.101 (defining these terms). For
PM2.5, the pollutant at issue here, the EPA has designated the
South Coast Air Basin—which encompasses the ports as well
as most of the greater Los Angeles area—as “nonattainment.”
The details of NAAQS enforcement are left to the states, each
of which must adopt a State Implementation Plan that
provides for the implementation and maintenance of national
air quality standards. 42 U.S.C. § 7410(a)(1).
                     NRDC V. USDOT                          9

   The CAA also contains a “conformity” provision that
prohibits federal participation in any project that fails to
conform to an approved State Implementation Plan.
42 U.S.C. § 7506(c)(1). The statute defines “conformity” to
mean, in relevant part, that:

       such activities will not—

       (i) cause or contribute to any new violation of
       any standard in any area;

       (ii) increase the frequency or severity of any
       existing violation of any standard in any area;
       or

       (iii) delay timely attainment of any standard
       or any required interim emission reductions or
       other milestones in any area.

Id. § 7506(c)(1)(B). The CAA does not define the term “any
area.” Instead, it delegates to the EPA and the U.S.
Department of Transportation (“DOT”) the duty to
“promulgate, and periodically update, criteria and procedures
for demonstrating and assuring conformity in the case of
transportation plans, programs, and projects.”            Id.
§ 7506(c)(4)(B).

    Acting pursuant to that authority, the EPA has
promulgated regulations that mandate a “hot-spot analysis”
for several air pollutants, including PM2.5. Hot-spot analysis
is described as

       an estimation of likely future localized . . .
       PM2.5 pollutant concentrations and a
10                    NRDC V. USDOT

        comparison of those concentrations to the
        national ambient air quality standards. Hot-
        spot analysis assesses impacts on a scale
        smaller than the entire nonattainment or
        maintenance area . . . and uses an air quality
        dispersion model to determine the effects of
        emissions on air quality.

40 C.F.R. § 93.101. Project sponsors must carry out this hot-
spot analysis to ensure that the project does not “cause or
contribute to any new localized . . . PM2.5 violations, increase
the frequency or severity” of such violations, or “delay timely
attainment” of air quality goals. Id. § 93.116(a). When a
project’s sponsors conduct a hot-spot analysis, concentrations
“must be estimated and analyzed at appropriate receptor
locations in the area substantially affected by the project.” Id.
§ 93.123(c)(1).

    Hot-spot analyses may be either qualitative or
quantitative. “Where quantitative analysis methods are not
available,” as was true of PM2.5 during the period at issue
here, the demonstration required by § 93.116(a) “must be
based on a qualitative consideration of local factors.” Id.
§ 93.123(b)(2). The parties agree that Defendants were
required to conduct a qualitative hot-spot analysis rather than
a quantitative analysis.

    In March 2006, the EPA and DOT jointly published the
Transportation Conformity Guidance for Qualitative Hot-spot
Analyses in PM2.5 and PM10 Nonattainment and Maintenance
Areas (“Conformity Guidance”). According to the EPA, all
qualitative PM2.5 analyses “should be completed” according
to the Conformity Guidance. 71 Fed. Reg. 12468, 12471
(Mar. 10, 2006). The Conformity Guidance does not define
                     NRDC V. USDOT                          11

“any area,” nor does it refer to “appropriate receptor
locations.” It does, however, provide a detailed methodology
for how project sponsors should conduct qualitative PM2.5
hot-spot analyses, and it features several examples of
permissible analyses.

    NRDC’s CAA claim turns on whether the statute’s use of
the phrase “any area” means that Defendants were required
to estimate PM2.5 increases within the area immediately
adjacent to the proposed expressway. If Defendants were
required to do so, then it follows that their approval of the
Project was contrary to law and must be set aside. If not, then
their Conformity Determination complied with the CAA.

                              B.

                              1.

    NRDC argues that the plain meaning of § 7506(c)(1)(B)
compels the conclusion that “any area” means “‘all’ or
‘every’ part of the ‘area’ affected by project emissions,” and
thus that Defendants were required to measure PM2.5
concentrations within the immediate vicinity of the Project.
We agree that “read naturally, the word ‘any’ has an
expansive meaning.” Ali v. Fed. Bureau of Prisons, 552 U.S.
214, 219 (2008) (alterations and internal quotation marks
omitted). However, NRDC’s plain meaning argument
ignores a critical, obvious ambiguity in the phrase “any area”:
the word “area.”

   On its own, the word “area” does not tell us whether
Defendants were required to measure PM2.5 concentrations
within the area immediately adjacent to the proposed
expressway or in any other “area.” The plain meaning of
12                   NRDC V. USDOT

“area” encompasses a wide variety of sizes. “Area” is
defined as “a region or part of a town, a country, or the
world,” “a space allocated for a specific purpose,” or “the
extent or measurement of a surface or piece of land.” NEW
OXFORD AMERICAN DICTIONARY 83 (3rd ed. 2010).
Additionally, the statutory context does not allow us to
pinpoint the meaning of “area.” A different provision of the
CAA requires that states send the EPA “a list of all areas (or
portions thereof) in the State,” 42 U.S.C. § 7407(d)(1)(A), but
this usage of “area” appears to refer to entire air quality
regions, which often include multiple counties. Likewise, the
statutory section concerning State Implementation Plans
repeatedly uses the term “areas” to describe air quality
regions. E.g., id. § 7410(a)(2)(I). Thus, we are convinced
that the statutory phrase “any area” is ambiguous.

                              2.

    Because Congress has not “directly spoken to the precise
question at issue,” Chevron, 467 U.S. at 842, we look to the
relevant agencies’ interpretations of the ambiguous phrase
“any area” during the period at issue in this appeal. NRDC
asserts that the EPA has adopted a regulatory interpretation
that supports its understanding of “any area,” and it argues
that EPA and DOT interpretations to the contrary are not
entitled to judicial deference.

    NRDC’s argument begins with an EPA rule interpreting
the key statutory phrase “increase the frequency or severity
of any existing violation of any standard in any area.” 42
U.S.C. § 7506(c)(1)(B)(ii). According to the EPA’s
regulatory definition,
                          NRDC V. USDOT                                  13

         [i]ncrease the frequency or severity means to
         cause a location or region to exceed a
         standard more often or to cause a violation at
         a greater concentration than previously
         existed . . . .

40 C.F.R. § 93.101 (emphasis added). NRDC argues the term
“a location” refers to an area smaller than the project area,
such as the area immediately adjacent to the expressway. In
March 2010, nearly a year after the Defendants issued their
Conformity Determination, the EPA issued an amended rule
which “specifically clarifies that the term ‘any area’ in CAA
section 176(c)(1)(B) applies to any portion of a nonattainment
or maintenance area, including the local area affected by a
transportation project.”3 75 Fed. Reg. 14,260, 14,276 (Mar.
24, 2010). NRDC argues that the EPA’s 2010 amendment to
40 C.F.R. § 93.116 confirms that the term “a location”
referred to an area smaller than the project area.

    NRDC also points to the regulatory requirement that hot-
spot analyses estimate pollutant concentrations at
“appropriate receptor locations in the area substantially
affected by the project.” 40 C.F.R. § 93.123(c)(1). In
NRDC’s view, this provision’s use of the plural “locations,”
and its reference to “the area significantly affected by the
project,” impliedly interprets the CAA’s “any area”
requirement to mandate collection and analysis of data drawn


  3
     In explaining its amended rule, the EPA noted that “[i]n practice,
EPA’s regulations will ensure that any project that creates a new violation
or worsens an existing violation of the NAAQS in the local area affected
by the project (either by increasing the number of violations or the severity
of an existing violation) will not be found to conform.” 75 Fed. Reg.
14,260, 14,278 (Mar. 24, 2010).
14                   NRDC V. USDOT

from the project’s immediate vicinity. Finally, NRDC cites
the EPA’s 1997 response to a public comment on proposed
hot-spot rules. In this response, the EPA noted that some
commenters had proposed that “a project should be allowed
to make a violation worse in a place not frequented by the
public if it improves air quality and eliminates violations
where public exposure is more likely.” 62 Fed. Reg. 43,780,
43,798 (Aug. 15, 1997). The EPA, however, rejected this
view, explaining that

       Clean Air Act section 176(c)(1)(B) states that
       transportation projects must not cause or
       contribute to any new violation of any
       standard in any area, or increase the frequency
       or severity of any existing violation of any
       standard in any area. It is not public exposure
       to a violation of a standard that the Clean Air
       Act language prohibits; it prohibits any
       violation of any standard in any area. The
       conformity rule cannot override the Clean Air
       Act to make exceptions that create new or
       worsen existing violations.

Id.

    NRDC’s arguments, while not without merit, fail to
persuade us that during the period at issue in this appeal the
EPA interpreted “any area” in the manner NRDC proposes.
First, that 40 C.F.R. § 93.101 refers to “a location or region”
does not persuade us that the “area” referenced is the area
immediately adjacent to the expressway as opposed to the
broader project area. Although the EPA clarified the term
“any area” in its 2010 amendments to mean “any portion of
a nonattainment or maintenance area, including the local area
                     NRDC V. USDOT                         15

affected by the transportation project,” the EPA did not
clearly indicate that this interpretation should apply
retroactively. 75 Fed. Reg. at 14,276; cf. Kankamalage v.
INS, 335 F.3d 858, 862 (9th Cir. 2003) (observing that a
regulation may not apply retroactively unless it
“unambiguously directs retroactive application”).

    Nor do we find the language of 40 C.F.R. § 93.123(c)(1)
particularly relevant. To begin with, it is not clear from the
language or structure of this provision whether its
“appropriate receptor locations” requirement applies to all
hot-spot analyses or only the more rigorous quantitative
analyses. Section 93.123(c)(1) is listed as one of several
“General requirements,” but § 93.123(b)(2) explicitly carves
out an exception for qualitative analyses, which “must be
based on a qualitative consideration of local factors.” The
EPA’s own statements and publications also indicate that the
“appropriate receptor locations” provision is inapplicable to
qualitative hot-spot analyses. The qualitative Conformity
Guidance says nothing about “receptors” or “appropriate
receptor locations.” In a 2010 response to public comments,
the EPA noted that it “intends to describe appropriate
receptor locations in its forthcoming quantitative PM hot-spot
guidance.” 75 Fed. Reg. 14,260, 14,282 (Mar. 24, 2010)
(emphasis added). The EPA’s recent quantitative Conformity
Guidance made good on this promise.                 See EPA,
Transportation Conformity Guidance for Quantitative Hot-
spot Analyses 113-15 (Nov. 2013) (defining and explaining
appropriate receptor locations).

   Finally, the EPA’s 1997 response does not settle the
matter. The response does not discuss hot-spot analysis,
much less the proper application of qualitative PM2.5 hot-spot
analysis. In addition, the response simply reiterates “that
16                    NRDC V. USDOT

transportation projects must not cause or contribute to any
new violation of any standard in any area, or increase the
frequency or severity of any existing violation of any
standard in any area.” 62 Fed. Reg. at 43,798. It does not
interpret the term “any area” or state that the term refers to an
area immediately adjacent to a transportation project. Thus,
the 1997 response does not establish the meaning of the term
“any area” at the time the Defendants issued their Conformity
Determination.

    Therefore, the governing regulations do not decisively
answer whether the CAA required qualitative hot-spot
analysis within the immediate vicinity of the project area
during the time period at issue.

                               3.

    The EPA and DOT’s Conformity Guidance implicitly, but
authoritatively, fills this void by interpreting these ambiguous
regulations to permit the type of analysis Defendants
performed here. Although the Conformity Guidance does not
explicitly interpret terms such as “any area,” “a location,” or
“appropriate receptor locations,” the methodological
examples it sets forth make clear that a qualitative PM2.5 hot-
spot analysis may be performed by analyzing data from an
existing air quality monitor in a location similar to the project
area, even if that monitor is not located within the immediate
vicinity of the new project. Because this interpretation is not
“plainly erroneous or inconsistent with the regulation,” Auer,
519 U.S. at 461, we must afford it considerable deference.

   The Conformity Guidance, which the two agencies jointly
published in March 2006, was “developed . . . to help state
and local agencies meet the [regulatory] hot-spot analysis
                      NRDC V. USDOT                          17

requirements.” Conformity Guidance at 2. Indeed, it states
that “future qualitative PM2.5 . . . hot-spot analyses should be
based on today’s new guidance.” Id.; see also 71 Fed. Reg.
at 12471 (referring to the Conformity Guidance and stating
that “[q]ualitative PM2.5 hot-spot analyses should be
completed according to joint EPA and DOT guidance”). The
Conformity Guidance “highlights two methods for
completing qualitative PM2.5 . . . analyses,” including one in
which the project sponsor compares the project location to
“another location with similar characteristics.” Conformity
Guidance at 17. The comparison method “involves
reviewing existing highway or transit facilities that were
constructed in the past and built in locations similar to the
proposed project and, whenever possible, near an air quality
monitor (a ‘surrogate’) to allow a comparison of PM2.5 . . . air
quality concentrations.” Id.

    The Conformity Guidance then provides several examples
of permissible comparisons. Two of these examples suggest
that a project can conform even if it increases PM2.5
concentrations in the area immediately surrounding the
project, and even if the surrogate is in violation of NAAQS.
In Example A, a project sponsor plans to build a bus terminal
that will “significantly increase diesel bus traffic at the
project’s location.” Id. at 27. The project sponsor measures
PM2.5 emissions in “the vicinity” of a similar, already-existing
bus terminal. Id. The project sponsor also uses a “nearby”
air monitor to determine existing air-quality conditions. Id.
The “similar” bus terminal’s PM2.5 emissions violate NAAQS
standards. Id. However, because the new bus terminal will
include “mitigation measures,” the project sponsor concludes
that the new terminal will conform. Id. In Example B, the
project sponsor plans to modify a highway interchange
“connecting a primary route to an interstate” which will be
18                   NRDC V. USDOT

used by a “significant number of diesel vehicles.” Id. at 28.
The project sponsor collects air quality information from the
project’s “location.” Id. The project is found to meet the
conformity hot-spot requirements because “any increase in
the emissions due to traffic changes associated with the
project[] would be offset by decreases in the emissions from
the transportation facility.” Id.

    Only one published decision has addressed the
Conformity Guidance, but it is well-reasoned and highly
instructive. In Audubon Naturalist Society of the Central
Atlantic States, Inc. v. U.S. Department of Transportation,
524 F. Supp. 2d 642 (D. Md. 2007), environmental advocacy
groups challenged the PM2.5 hot-spot analysis for a highway
project, in part because the project sponsor used an existing
air monitor located outside the immediate vicinity of the
proposed highway. Id. at 701. The district court held that the
regulations governing qualitative PM2.5 hot-spot analyses are
ambiguous and that the Conformity Guidance is entitled to
Auer deference as a reasonable interpretation of those
regulations. Id. at 697–99. The court then noted that the
Conformity Guidance explicitly recommends the “monitor
comparison method,” in which data from “another location
with similar characteristics” is used to project the likely
impact of the new project. Id. at 700. Ultimately, the court
upheld the project sponsors’ analysis, reasoning that the
Conformity Guidance neither mentions a distance
requirement nor requires installation of new air monitors;
rather, it only requires project sponsors to use nearby air
monitors at “locations similar to the proposed project.” Id. at
701 (quoting Conformity Guidance at 17).

   We also note that the Federal Highway Administration
(“FHWA”), an agency within the DOT, has published several
                        NRDC V. USDOT                              19

examples of permissible qualitative PM2.5 hot-spot analyses.
According to FHWA’s website, these are analyses that “could
be replicated in other areas of the country.”4 Two of these
examples—the Woodrow Wilson Bridge and Prairie Parkway
projects—are of particular interest. In the Woodrow Wilson
Bridge hot-spot analysis, the project sponsor based its
estimated PM2.5 concentrations on a surrogate air monitor
twenty miles away from the project site.5 Because the
estimated traffic levels for the Woodrow Wilson Bridge
project were similar to the traffic levels at the surrogate
monitor, and because the surrogate did not violate PM2.5
standards, the sponsor concluded that the project would not
violate PM2.5 standards. Id. at 21. Similarly, in the Prairie
Parkway hot-spot analysis, the project sponsor based its
estimated PM2.5 concentrations on a surrogate air monitor
roughly ten miles away from the project site.6 Traffic levels
near the surrogate were similar to estimated traffic levels for
the project, and the surrogate did not violate PM2.5 standards.
Id. at 23. Thus, the sponsor concluded that Prairie Parkway
project would not violate PM2.5 standards. Id. While we
acknowledge that these examples are less authoritative than
the Conformity Guidance—in part because the studies


         4
         FHWA, Examples of Transportation Conformity Practices,
http://www.fhwa.dot.gov/environment/air_quality/conformity/practices/
(last visited Sept. 24, 2014).
 5
   Woodrow Wilson Bridge PM2.5 Conformity Analysis, at 15 (Oct. 2006),
http://www.fhwa.dot.gov/environment/air_quality/conformity/practices/
woodrow_wilson.pdf (last visited Sept. 24, 2014).
     6
     Fine Particulate Matter (PM2.5) Project Level Hot-Spot Analysis:
Prairie Parkway Study, ILL. DEP’T OF TRANSP. 9 (Jan. 4, 2008),
http://www.fhwa.dot.gov/environment/air_quality/conformity/practices/
prairie_parkway.pdf (last visited Sept. 24, 2014).
20                   NRDC V. USDOT

themselves were conducted by state agencies—FHWA’s
endorsement provides further indication that Defendants were
permitted to rely on a surrogate air monitor outside the
immediate vicinity of the project. See United States v. Mead
Corp., 533 U.S. 218, 234 (2001) (“[A]n agency’s
interpretation may merit some deference whatever its form.”);
J.G. v. Douglas Cnty. Sch. Dist., 552 F.3d 786, 797 n.8 (9th
Cir. 2008) (“Although a state agency’s interpretation of
federal law is not entitled to deference, the Secretary’s
approval of that agency’s interpretation is due some deference
because it shows a federal agency’s interpretation of the
federal statute that it is charged to administer.” (citation
omitted)).

    NRDC offers no persuasive reason why we should not
rely upon these agency interpretations. Having already
concluded that the language of the CAA and the hot-spot
regulations are ambiguous, we address NRDC’s three
remaining arguments: that the CAA delegates interpretive
authority to the EPA, not FHWA; that the agencies’
interpretations were not adopted through notice-and-comment
rulemaking; and that FHWA’s application of the Conformity
Guidance represents an inconsistent and unexplained change
in policy. NRDC’s first remaining objection makes little
sense. The EPA and DOT, to whom Congress expressly
delegated interpretive authority, jointly published the
Conformity Guidance on which Defendants relied. 42 U.S.C.
§§ 7506(c)(4)(B); EPA, Transportation Conformity Guidance
for Quantitative Hot-spot Analyses 1 (Nov. 2013). Moreover,
FHWA—which performed the hot-spot analysis here and
published examples of permissible qualitative analyses on its
website—is an agency within the DOT that reports to the
Secretary of Transportation.
                       NRDC V. USDOT                              21

    NRDC’s second objection fares no better. We afford
Auer deference to an agency’s interpretation of its own
regulations regardless of whether that interpretation was
adopted through notice-and-comment rulemaking. See, e.g.,
Chase Bank USA, N.A. v. McCoy, 131 S. Ct. 871, 881 (2011)
(deferring to agency’s interpretation of its own regulation that
was advanced in an amicus brief).7

    NRDC’s third remaining argument, its objection to the
agency’s purported change in policy, is more properly
analyzed as a challenge to this particular Conformity
Determination, not as a challenge to the agency’s general
interpretation of the permissible methodology for conducting
qualitative hot-spot analyses. We therefore turn to the review
of the Conformity Determination before us.

                                 C.

    Having concluded that the agencies’ interpretation of the
appropriate hot-spot analysis governs, it is clear that
Defendants’ Conformity Determination was neither arbitrary
nor capricious.

   Defendants performed a qualitative PM2.5 hot-spot
analysis using the comparison method described in the
Conformity Guidance. Defendants chose the North Long
Beach air monitoring station, located roughly five miles away
from the far end of the project, as a surrogate because its
PM2.5 concentrations were “representative of the project


 7
   NRDC’s reliance on High Sierra Hikers Ass’n v. Blackwell, 390 F.3d
630 (9th Cir. 2004), is misplaced. High Sierra Hikers stands for the
uncontroversial—but inapposite—proposition that an informal agency
interpretation is not entitled to Chevron deference. Id. at 648.
22                       NRDC V. USDOT

area.” To reach this determination, Defendants compared the
North Long Beach station to a second monitoring station
located closer to the project area.8 They found that annual
average daily traffic, trucks as a percentage of all traffic, and
PM2.5 concentrations at the closer station were all similar to
those at the North Long Beach station, and therefore
concluded that “the North Long Beach station reflects the
same traffic conditions as at the project location, and the
monitoring data are shown to be representative of ambient air
quality for the project area.”

    Defendants then used the North Long Beach PM2.5 data as
a baseline to estimate the Project’s likely impacts on PM2.5
concentrations. Defendants projected that although the
Project might increase total vehicle miles traveled in
comparison to the no-build alternative, this impact would be
offset by faster vehicle speeds and reduced traffic congestion.
As a result, Defendants concluded, “PM2.5 emissions of the
build alternatives would be the same or less than the No Build
alternative,” and “[b]ased on the current ambient PM2.5
concentrations in the project area, the project is not expected
to have [a] significant localized PM2.5 concentration increase
when compared to the No Build alternative.” Thus, because
the Project would not cause a new PM2.5 violation, increase
the severity of an existing violation, or delay the
implementation of national air quality standards for PM2.5,
Defendants concluded that the Project conforms to statutory
and regulatory requirements.

   Defendants’ Conformity Determination using this
comparison method was a reasonable application of the EPA

 8
   This closer station was not chosen for the hot-spot analysis because it
was relatively new and thus lacked extensive historical data.
                     NRDC V. USDOT                          23

and DOT’s Conformity Guidance.             The Conformity
Determination compares favorably to the hot-spot analysis
approved in Audubon, as well as to the Woodrow Wilson
Bridge and Prairie Parkway analyses endorsed by the FWHA.
The Conformity Guidance makes clear that Defendants were
permitted to use a surrogate air monitor, and this monitor’s
distance from the Project—about one mile from the near end
of the Project, and five miles from the far end—was well
within the ranges approved in Audubon and the Woodrow
Wilson and Prairie Parkway examples. In addition,
Defendants used a second air monitor to verify that the North
Long Beach station was representative of air quality in the
project area, further bolstering the accuracy of their
qualitative analysis.

                             IV.

    Under NEPA, federal agencies must prepare an EIS when
considering “major Federal actions significantly affecting the
quality of the human environment.” 42 U.S.C. § 4332(C).
Federal regulations require that the EIS “provide full and fair
discussion of significant environmental impacts,” 40 C.F.R.
§ 1502.1, as well as “state how alternatives considered in it
and decisions based on it will or will not achieve the
requirements of [NEPA] and other environmental laws and
policies,” id. § 1502.2(d). The EIS’s discussion of
alternatives “should present the environmental impacts of the
proposal and the alternatives in comparative form, thus
sharply defining the issues and providing a clear basis for
choice among options by the decisionmaker and the public.”
Id. § 1502.14.

   Generally, our review is limited to whether the EIS
contains “a reasonably thorough discussion of the significant
24                   NRDC V. USDOT

aspects of the probable environmental consequences.” City
of Carmel-by-the-Sea v. U.S. Dep’t of Transp., 123 F.3d
1142, 1150 (9th Cir. 1997) (internal quotation marks
omitted). “Once satisfied that a proposing agency has taken
a ‘hard look’ at a decision’s environmental consequences, our
review is at an end.” Id. at 1151 (alterations and internal
quotation marks omitted). NRDC contends that Defendants
violated NEPA because their EIS failed to address whether
the potential increase in PM2.5 concentrations would violate
2006 NAAQS standards and failed to fully disclose the
Project’s likely effects on public health.

    Contrary to NRDC’s argument, the EIS did not
impermissibly rely on outdated air quality standards. In
2006, the EPA updated the NAAQS standard for PM2.5,
cutting the maximum permissible level to 35 micrograms per
cubic meter. 71 Fed. Reg. at 61,144 (Oct. 17, 2006). These
new standards did not go into effect for transportation
conformity purposes until December 2010, over a year after
the Conformity Determination was completed. See 75 Fed.
Reg. 14260, 14262 (Mar. 24, 2010) (“Transportation
conformity for the 2006 PM2.5 NAAQS does not apply until
December 14, 2010.”).

    Further, Defendants correctly contend that the EIS was
forthright in discussing the new standard. For example, the
EIS acknowledged that even though PM2.5 levels were below
the old NAAQS standard in the two preceding years, “the
current federal 24-hour PM2.5 standard of 35 [micrograms per
cubic meter] was exceeded each year in the past 3 years.”
The EIS also discussed at length the results of Defendants’ air
quality study, and concluded that any localized increase in
PM2.5 would be offset by reduced vehicle congestion and
idling in the project area as a whole. The EIS also
                      NRDC V. USDOT                           25

incorporated Defendants’ response to NRDC’s comment on
this point. Defendants noted that according to the results of
the air quality study, “the project will not adversely affect the
human environment by contributing to increased PM2.5
concentrations in the study area or delay attainment of the
NAAQS regardless of whether the study area is subject to the
existing or revised 24-hour NAAQS.”

    In addition, Defendants’ EIS adequately disclosed the
Project’s likely health effects. The EIS included a Health
Risk Assessment that was subject to the public comment and
review process. In the Health Risk Assessment, Defendants
disclosed that the Project would lead to increased PM10 and
PM2.5 concentrations in the immediate vicinity of the Project,
and how those increased concentrations could have adverse
health effects for local residents. The Health Risk
Assessment also acknowledged that this type of
transportation project usually leads to increased PM2.5
concentrations in the area immediately adjacent to the project.

    Defendants also conducted detailed studies based on
2006-2007 meteorological data, where they estimated cancer-
and other health-risk increases at thousands of residences,
schools, parks, and other areas in the immediate vicinity of
the Project. Defendants explained the study results with
color-coded diagrams illustrating the precise locations where
adverse health effects would be the greatest. They also
included statistical discussions and tables illustrating that
roughly 97% of the adverse health affects would be due to
diesel particulate matter concentrations. Additionally,
Defendants determined that a heating, ventilation, and air
conditioning retrofit program for residences within the
vicinity of the significant impact zone would be a feasible
mitigation measure.
26                  NRDC V. USDOT

    Because we are satisfied that Defendants took a “hard
look” at the Project’s likely consequences and probable
alternatives, see Carmel-by-the-Sea, 123 F.3d at 1151, we
agree with the district court that the EIS comported with
NEPA requirements.

                           V.

  Defendants’ Conformity Determination did not violate the
CAA, nor did their EIS violate NEPA. Accordingly, we
AFFIRM the district court’s grant of summary judgment.
