DEPARTMENT OF TRANSPORTATION et al. v. PUBLIC CITIZEN et al.

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752 OCTOBER TERM, 2003
Syllabus
DEPARTMENT OF TRANSPORTATION et al. v.
PUBLIC CITIZEN et al.
certiorari to the united states court of appeals for
the ninth circuit
No. 03–358. Argued April 21, 2004—Decided June 7, 2004
The National Environmental Policy Act of 1969 (NEPA) requires federal
agencies to analyze the environmental impact of their proposals and
actions in an Environmental Impact Statement (EIS), but Council of
Environmental Quality (CEQ) regulations allow an agency to prepare a
more limited Environmental Assessment (EA) if the agency’s proposed
action neither is categorically excluded from the EIS production re-
quirement nor would clearly require production of an EIS. An agency
that decides, pursuant to an EA, that no EIS is required must issue a
“finding of no significant impact” (FONSI). The Clean Air Act (CAA
or Act) leaves States to develop “implementation plan[s]” to comply with
national air quality standards mandated by the Act, and requires fed-
eral agencies’ actions to “conform” to those state plans, 42 U. S. C.
§ 7506(c)(1). In 1982, Congress enacted a moratorium, prohibiting, inter
alia, Mexican motor carriers from obtaining operating authority within
the United States and authorizing the President to lift the moratorium.
In 2001, the President announced his intention to lift the moratorium
once new regulations were prepared to grant operating authority to
Mexican motor carriers. The Federal Motor Carrier Safety Adminis-
tration (FMCSA) published one proposed rule addressing the applica-
tion form for such carriers and another addressing the establishment of
a safety-inspection regime for carriers receiving operating authority.
Congress subsequently provided, in § 350 of the Department of Trans-
portation and Related Agencies Appropriations Act, 2002, that no funds
appropriated could be obligated or expended to review or process any
Mexican motor carrier’s applications until FMCSA implemented specific
application and safety-monitoring requirements. Acting pursuant to
NEPA, FMCSA issued an EA for its proposed rules. The EA did not
consider the environmental impact that might be caused by the in-
creased presence of Mexican trucks in the United States, concluding
that any such impact would be an effect of the moratorium’s modifica-
tion, not the regulations’ implementation. Concluding that the regula-
tions’ issuance would have no significant environmental impact, FMCSA
issued a FONSI. In subsequent interim rules, FMCSA relied on the
EA and FONSI to demonstrate compliance with NEPA, and determined

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753 Cite as: 541 U. S. 752 (2004)
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that any emissions increase from the regulations would fall below the
Environmental Protection Agency’s (EPA) threshold levels needed to
trigger a conformity review under the CAA. Before the moratorium
was lifted, respondents sought judicial review of the proposed rules,
arguing that their promulgation violated NEPA and the CAA. The
Court of Appeals agreed, finding the EA deficient because it did not
consider the environmental impact of lifting the moratorium, when that
action was reasonably foreseeable at the time FMCSA prepared the EA
and directing FMCSA to prepare an EIS and a full CAA conformity
determination for the regulations.
Held: Because FMCSA lacks discretion to prevent cross-border opera-
tions of Mexican motor carriers, neither NEPA nor the CAA requires
FMCSA to evaluate the environmental effects of such operations.
Pp. 763–773.
(a) FMCSA did not violate NEPA or the relevant CEQ regulations.
Pp. 763–770.
(1) An agency’s decision not to prepare an EIS can be set aside
only if it is arbitrary and capricious, see 5 U. S. C. § 706(2)(A). Re-
spondents argue that the issuance of a FONSI was arbitrary and ca-
pricious because the EA did not take into account the environmental
effects of an increase in cross-border operations of Mexican motor
carriers. The relevant question, under NEPA, is whether that increase,
and the correlative release of emissions, is an “effect,” 40 CFR § 1508.8,
of FMCSA’s rules; if not, FMCSA’s failure to address these effects in
the EA did not violate NEPA, and the FONSI’s issuance cannot be
arbitrary and capricious. Pp. 763–764.
(2) Respondents have forfeited any objection to the EA on the
ground that it did not adequately discuss potential alternatives to the
proposed action because respondents never identified in their com-
ments to the rules any alternatives beyond those the EA evaluated.
Pp. 764–765.
(3) Respondents argue that the EA must take the increased cross-
border operations’ environmental effects into account because § 350’s ex-
penditure bar makes it impossible for any Mexican truck to operate in
the United States until the regulations are issued, and hence the trucks’
entry is a “reasonably foreseeable” indirect effect of the issuance of the
regulations. 40 CFR § 1508.8. Critically, that argument overlooks
FMCSA’s inability to countermand the President’s lifting of the morato-
rium or otherwise categorically to exclude Mexican trucks from operat-
ing in the United States. While § 350 restricted FMCSA’s ability to
authorize such operations, FMCSA remains subject to 49 U. S. C.
§ 13902(a)(1)’s mandate that it register any motor carrier willing and

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754 DEPARTMENT OF TRANSPORTATION v. PUBLIC
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able to comply with various safety and financial responsibility rules.
Only the moratorium prevented it from doing so for Mexican trucks
before 2001. Respondents must rest on “but for” causation, where an
agency’s action is considered a cause of an environmental effect even
when the agency has no authority to prevent the effect. However, “but
for” causation is insufficient to make an agency responsible for a particu-
lar effect under NEPA and the relevant regulations. NEPA requires a
“reasonably close causal relationship” akin to proximate cause in tort
law. Metropolitan Edison Co. v. People Against Nuclear Energy, 460
U. S. 766, 774. Also, inherent in NEPA and its implementing regula-
tions is a “rule of reason,” which ensures that agencies determine
whether and to what extent to prepare an EIS based on the usefulness
of any new potential information to the decisionmaking process. The
underlying policies behind NEPA and Congress’ intent, as informed by
the “rule of reason,” make clear that the causal connection between the
proposed regulations and the entry of Mexican trucks is insufficient to
make FMCSA responsible under NEPA to consider the environmental
effects of entry. Neither of the purposes of NEPA’s EIS requirement—
to ensure both that an agency has information to make its decision and
that the public receives information so it might also play a role in the
decisionmaking process—will be fulfilled by requiring FMCSA to con-
sider the environmental impact at issue. Since FMCSA has no ability
to prevent such cross-border operations, it lacks the power to act on
whatever information might be contained in an EIS and could not act
on whatever input the public could provide. This analysis is not
changed by the CEQ regulation requiring an agency to evaluate the
“cumulative impact” of its action, 40 CFR § 1508.7, since that rule does
not require FMCSA to treat the lifting of the moratorium itself or the
consequences from that lifting as an effect of its rules promulgation.
Pp. 765–770.
(b) FMCSA did not act improperly by not performing a full conform-
ity analysis pursuant to the CAA and relevant regulations. To ensure
that its actions are consistent with 42 U. S. C. § 7506, a federal agency
must undertake “a conformity determination . . . where the total of
direct and indirect emissions in a nonattainment or maintenance area
caused by [the] action would equal or exceed” certain threshold levels
established by the EPA. 40 CFR § 93.153(b). “Direct emissions” “are
caused or initiated by the Federal action and occur at the same time
and place as the action,” § 93.152; and “indirect emissions” are “caused
by the Federal action” but may occur later in time, and may be practica-
bly controlled or maintained by the federal agency, ibid. Some sort of
“but for” causation is sufficient for evaluating causation in the conform-
ity review process. See ibid. Because it excluded emissions attribut-

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able to the increased presence of Mexican trucks within the United
States, FMCSA concluded that its regulations would not exceed EPA
thresholds. Although arguably FMCSA’s proposed regulations would
be “but for” causes of the entry of Mexican trucks into the United
States, such trucks’ emissions are not “direct” because they will not
occur at the same time or place as the promulgation of the regulations.
And they are not “indirect” because FMCSA cannot practicably control
or maintain control over the emissions: FMCSA has no ability to coun-
termand the President’s decision to lift the moratorium or to act cate-
gorically to prevent Mexican carriers from registering and Mexican
trucks from entering the country; and once the regulations are promul-
gated, FMCSA will not be able to regulate any aspect of vehicle exhaust
from those trucks. Pp. 771–773.
316 F. 3d 1002, reversed and remanded.
Thomas, J., delivered the opinion for a unanimous Court.
Deputy Solicitor General Kneedler argued the cause for
petitioners. With him on the briefs were Solicitor General
Olson, Assistant Attorney General Sansonetti, Deputy So-
licitor General Hungar, Deputy Assistant Attorney Gen-
eral Clark, Austin C. Schlick, John L. Smeltzer, David
C. Shilton, Jeffrey A. Rosen, and Peter J. Plocki.
Jonathan Weissglass argued the cause for respondents.
With him on the brief were Stephen P. Berzon, Gail Ruder-
man Feuer, Julie Masters, Adrianna Quintero Somaini,
Melissa Lin Perrella, David C. Vladeck, Patrick J. Szyman-
ski, David Rosenfeld, William S. Lerach, Patrick J. Cough-
lin, Albert H. Meyerhoff, and Thomas O. McGarity.*
*Briefs of amici curiae urging affirmance were filed for the State of
California et al. by Bill Lockyer, Attorney General of California, Susan
L. Durbin and Gordon B. Burns, Deputy Attorneys General, Manuel M.
Medeiros, Solicitor General, Tom Greene, Chief Assistant Attorney Gen-
eral, Theodora Berger, Senior Assistant Attorney General, and Craig C.
Thompson, Supervising Deputy Attorney General, and by the Attorneys
General for their respective States as follows: Terry Goddard of Arizona,
Lisa Madigan of Illinois, Thomas F. Reilly of Massachusetts, Patricia
A. Madrid of New Mexico, W. A. Drew Edmondson of Oklahoma, Hardy
Myers of Oregon, Christine O. Gregoire of Washington, and Peggy
A. Lautenschlager of Wisconsin; for the American Public Health Associa-

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756 DEPARTMENT OF TRANSPORTATION v. PUBLIC
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Opinion of the Court
Justice Thomas delivered the opinion of the Court.
In this case, we confront the question whether the Na-
tional Environmental Policy Act of 1969 (NEPA), 83 Stat.
852 (codified, as amended, at 42 U. S. C. §§ 4321–4370f), and
the Clean Air Act (CAA), 42 U. S. C. §§ 7401–7671q, require
the Federal Motor Carrier Safety Administration (FMCSA)
to evaluate the environmental effects of cross-border opera-
tions of Mexican-domiciled motor carriers, where FMCSA’s
promulgation of certain regulations would allow such cross-
border operations to occur. Because FMCSA lacks discre-
tion to prevent these cross-border operations, we conclude
that these statutes impose no such requirement on FMCSA.
I
Due to the complex statutory and regulatory provisions
implicated in this case, we begin with a brief overview of the
relevant statutes. We then turn to the factual and proce-
dural background.
A
1
Signed into law on January 1, 1970, NEPA establishes a
“national policy [to] encourage productive and enjoyable har-
mony between man and his environment,” and was intended
to reduce or eliminate environmental damage and to promote
“the understanding of the ecological systems and natural re-
sources important to” the United States. 42 U. S. C. § 4321.
“NEPA itself does not mandate particular results” in order
to accomplish these ends. Robertson v. Methow Valley Citi-
zens Council, 490 U. S. 332, 350 (1989). Rather, NEPA im-
poses only procedural requirements on federal agencies with
a particular focus on requiring agencies to undertake anal-
tion et al. by Hope M. Babcock; for Defenders of Wildlife et al. by Pamela
S. Karlan and Sanjay Narayan; for the Eagle Forum Education & Legal
Defense Fund by Karen B. Tripp; and for South Coast Air Quality Man-
agement District et al. by Barbara Baird and Patricia V. Tubert.

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yses of the environmental impact of their proposals and
actions. See id., at 349–350. At the heart of NEPA is a
requirement that federal agencies
“include in every recommendation or report on propos-
als for legislation and other major Federal actions sig-
nificantly affecting the quality of the human environ-
ment, a detailed statement by the responsible official
on—
“(i) the environmental impact of the proposed action,
“(ii) any adverse environmental effects which cannot
be avoided should the proposal be implemented,
“(iii) alternatives to the proposed action,
“(iv) the relationship between local short-term uses of
man’s environment and the maintenance and enhance-
ment of long-term productivity, and
“(v) any irreversible and irretrievable commitments of
resources which would be involved in the proposed ac-
tion should it be implemented.” 42 U. S. C. § 4332(2)(C).
This detailed statement is called an Environmental Impact
Statement (EIS). The Council of Environmental Quality
(CEQ), established by NEPA with authority to issue regula-
tions interpreting it, has promulgated regulations to guide
federal agencies in determining what actions are subject to
that statutory requirement. See 40 CFR § 1500.3 (2003).
The CEQ regulations allow an agency to prepare a more lim-
ited document, an Environmental Assessment (EA), if the
agency’s proposed action neither is categorically excluded
from the requirement to produce an EIS nor would clearly
require the production of an EIS. See §§ 1501.4(a)–(b).
The EA is to be a “concise public document” that “[b]riefly
provide[s] sufficient evidence and analysis for determining
whether to prepare an [EIS].” § 1508.9(a). If, pursuant to
the EA, an agency determines that an EIS is not required
under applicable CEQ regulations, it must issue a “finding
of no significant impact” (FONSI), which briefly presents

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the reasons why the proposed agency action will not
have a significant impact on the human environment. See
§§ 1501.4(e), 1508.13.
2
What is known as the CAA became law in 1963, 77 Stat.
392. In 1970, Congress substantially amended the CAA into
roughly its current form. 84 Stat. 1713. The 1970 amend-
ments mandated national air quality standards and deadlines
for their attainment, while leaving to the States the develop-
ment of “implementation plan[s]” to comply with the federal
standards. Ibid.
In 1977, Congress again amended the CAA, 91 Stat. 749,
to prohibit the Federal Government and its agencies from
“engag[ing] in, support[ing] in any way or provid[ing] finan-
cial assistance for, licens[ing] or permit[ting], or approv[ing],
any activity which does not conform to [a state] implementa-
tion plan.” 42 U. S. C. § 7506(c)(1). The definition of “con-
formity” includes restrictions on, for instance, “increas[ing]
the frequency or severity of any existing violation of any
standard in any area,” or “delay[ing] timely attainment
of any standard . . . in any area.” § 7506(c)(1)(B). These
safeguards prevent the Federal Government from interfer-
ing with the States’ abilities to comply with the CAA’s
requirements.
3
FMCSA, an agency within the Department of Transpor-
tation (DOT), is responsible for motor carrier safety and
registration. See 49 U. S. C. § 113(f). FMCSA has a vari-
ety of statutory mandates, including “ensur[ing]” safety,
§ 31136, establishing minimum levels of financial responsi-
bility for motor carriers, § 31139, and prescribing federal
standards for safety inspections of commercial motor vehi-
cles, § 31142. Importantly, FMCSA has only limited discre-
tion regarding motor vehicle carrier registration: It must
grant registration to all domestic or foreign motor carriers

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that are “willing and able to comply with” the applicable
safety, fitness, and financial-responsibility requirements.
§ 13902(a)(1). FMCSA has no statutory authority to impose
or enforce emissions controls or to establish environmental
requirements unrelated to motor carrier safety.
B
We now turn to the factual and procedural background of
this case. Before 1982, motor carriers domiciled in Canada
and Mexico could obtain certification to operate within the
United States from the Interstate Commerce Commission
(ICC).1 In 1982, Congress, concerned about discriminatory
treatment of United States motor carriers in Mexico and
Canada, enacted a 2-year moratorium on new grants of oper-
ating authority. Congress authorized the President to ex-
tend the moratorium beyond the 2-year period if Canada or
Mexico continued to interfere with United States motor car-
riers, and also authorized the President to lift or modify the
moratorium if he determined that doing so was in the na-
tional interest. 49 U. S. C. § 10922(l) (1982 ed.). Although
the moratorium on Canadian motor carriers was quickly
lifted, the moratorium on Mexican motor carriers remained,
and was extended by the President.
In December 1992, the leaders of Mexico, Canada, and the
United States signed the North American Free Trade
Agreement (NAFTA), 32 I. L. M. 605 (1993). As part of
NAFTA, the United States agreed to phase out the morato-
rium and permit Mexican motor carriers to obtain operating
authority within the United States’ interior by January 2000.
On NAFTA’s effective date (January 1, 1994), the President
began to lift the trade moratorium by allowing the licensing
1 In 1995, Congress abolished the ICC and transferred most of its re-
sponsibilities to the Secretary of Transportation. See ICC Termination
Act of 1995, § 101, 109 Stat. 803. In 1999, Congress transferred responsi-
bility for motor carrier safety within DOT to the newly created FMCSA.
See Motor Carrier Safety Improvement Act of 1999, 113 Stat. 1748.

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of Mexican carriers to provide some bus services in the
United States. The President, however, did not continue to
ease the moratorium on the timetable specified by NAFTA,
as concerns about the adequacy of Mexico’s regulation of
motor carrier safety remained.
The Government of Mexico challenged the United States’
implementation of NAFTA’s motor carrier provisions under
NAFTA’s dispute-resolution process, and in February 2001,
an international arbitration panel determined that the
United States’ “blanket refusal” of Mexican motor carrier
applications breached the United States’ obligations under
NAFTA. App. 279, ¶ 295. Shortly thereafter, the Presi-
dent made clear his intention to lift the moratorium on Mexi-
can motor carrier certification following the preparation of
new regulations governing grants of operating authority to
Mexican motor carriers.
In May 2001, FMCSA published for comment proposed
rules concerning safety regulation of Mexican motor carri-
ers. One rule (the Application Rule) addressed the estab-
lishment of a new application form for Mexican motor carri-
ers that seek authorization to operate within the United
States. Another rule (the Safety Monitoring Rule) ad-
dressed the establishment of a safety-inspection regime for
all Mexican motor carriers that would receive operating au-
thority under the Application Rule.
In December 2001, Congress enacted the Department of
Transportation and Related Agencies Appropriations Act,
2002, 115 Stat. 833. Section 350 of this Act, id., at 864, pro-
vided that no funds appropriated under the Act could be obli-
gated or expended to review or to process any application
by a Mexican motor carrier for authority to operate in the
interior of the United States until FMCSA implemented spe-
cific application and safety-monitoring requirements for
Mexican carriers. Some of these requirements went beyond
those proposed by FMCSA in the Application and Safety

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Monitoring Rules. Congress extended the § 350 conditions
to appropriations for Fiscal Years 2003 and 2004.
In January 2002, acting pursuant to NEPA’s mandates,
FMCSA issued a programmatic EA for the proposed Appli-
cation and Safety Monitoring Rules. FMCSA’s EA eval-
uated the environmental impact associated with three
separate scenarios: where the President did not lift the
moratorium; where the President did but where (contrary to
what was legally possible) FMCSA did not issue any new
regulations; and the Proposed Action Alternative, where the
President would modify the moratorium and where FMCSA
would adopt the proposed regulations. The EA considered
the environmental impact in the categories of traffic and con-
gestion, public safety and health, air quality, noise, socioeco-
nomic factors, and environmental justice. Vital to the EA’s
analysis, however, was the assumption that there would be
no change in trade volume between the United States and
Mexico due to the issuance of the regulations. FMCSA did
note that § 350’s restrictions made it impossible for Mexican
motor carriers to operate in the interior of the United States
before FMCSA’s issuance of the regulations. But, FMCSA
determined that “this and any other associated effects in
trade characteristics would be the result of the modifica-
tion of the moratorium” by the President, not a result of
FMCSA’s implementation of the proposed safety regulations.
App. 60. Because FMCSA concluded that the entry of the
Mexican trucks was not an “effect” of its regulations, it did
not consider any environmental impact that might be caused
by the increased presence of Mexican trucks within the
United States.
The particular environmental effects on which the EA fo-
cused, then, were those likely to arise from the increase in
the number of roadside inspections of Mexican trucks and
buses due to the proposed regulations. The EA concluded
that these effects (such as a slight increase in emissions,
noise from the trucks, and possible danger to passing motor-

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ists) were minor and could be addressed and avoided in the
inspections process itself. The EA also noted that the in-
crease of inspection-related emissions would be at least par-
tially offset by the fact that the safety requirements would
reduce the number of Mexican trucks operating in the United
States. Due to these calculations, the EA concluded that
the issuance of the proposed regulations would have no sig-
nificant impact on the environment, and hence FMCSA, on
the same day as it released the EA, issued a FONSI.
On March 19, 2002, FMCSA issued the two interim rules,
delaying their effective date until May 3, 2002, to allow
public comment on provisions that FMCSA added to satisfy
the requirements of § 350. In the regulatory preambles,
FMCSA relied on its EA and its FONSI to demonstrate com-
pliance with NEPA. FMCSA also addressed the CAA in
the preambles, determining that it did not need to perform
a “conformity review” of the proposed regulations under 42
U. S. C. § 7506(c)(1) because the increase in emissions from
these regulations would fall below the Environmental Pro-
tection Agency’s (EPA) threshold levels needed to trigger
such a review.
In November 2002, the President lifted the moratorium on
qualified Mexican motor carriers. Before this action, how-
ever, respondents filed petitions for judicial review of the
Application and Safety Monitoring Rules, arguing that the
rules were promulgated in violation of NEPA and the CAA.
The Court of Appeals agreed with respondents, granted the
petitions, and set aside the rules. 316 F. 3d 1002 (CA9 2003).
The Court of Appeals concluded that the EA was deficient
because it failed to give adequate consideration to the over-
all environmental impact of lifting the moratorium on the
cross-border operation of Mexican motor carriers. Accord-
ing to the Court of Appeals, FMCSA was required to con-
sider the environmental effects of the entry of Mexican
trucks because “the President’s rescission of the moratorium
was ‘reasonably foreseeable’ at the time the EA was pre-

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pared and the decision not to prepare an EIS was made.”
Id., at 1022 (quoting 40 CFR §§ 1508.7, 1508.8(b) (2003)).
Due to this perceived deficiency, the Court of Appeals re-
manded the case for preparation of a full EIS.
The Court of Appeals also directed FMCSA to prepare a
full CAA conformity determination for the challenged reg-
ulations. It concluded that FMCSA’s determination that
emissions attributable to the challenged rules would be
below the threshold levels was not reliable because the
agency’s CAA determination reflected the “illusory distinc-
tion between the effects of the regulations themselves and
the effects of the presidential rescission of the moratorium
on Mexican truck entry.” 316 F. 3d, at 1030.
We granted certiorari, 540 U. S. 1088 (2003), and now
reverse.
II
An agency’s decision not to prepare an EIS can be set
aside only upon a showing that it was “arbitrary, capricious,
an abuse of discretion, or otherwise not in accordance with
law.” 5 U. S. C. § 706(2)(A). See also Marsh v. Oregon Nat-
ural Resources Council, 490 U. S. 360, 375–376 (1989);
Kleppe v. Sierra Club, 427 U. S. 390, 412 (1976). Here,
FMCSA based its FONSI upon the analysis contained within
its EA; respondents argue that the issuance of the FONSI
was arbitrary and capricious because the EA’s analysis was
flawed. In particular, respondents criticize the EA’s failure
to take into account the various environmental effects caused
by the increase in cross-border operations of Mexican
motor carriers.
Under NEPA, an agency is required to provide an EIS
only if it will be undertaking a “major Federal actio[n],”
which “significantly affect[s] the quality of the human envi-
ronment.” 42 U. S. C. § 4332(2)(C). Under applicable CEQ
regulations, “[m]ajor Federal action” is defined to “includ[e]
actions with effects that may be major and which are poten-
tially subject to Federal control and responsibility.” 40

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CFR § 1508.18 (2003). “Effects” is defined to “include: (a)
Direct effects, which are caused by the action and occur at
the same time and place,” and “(b) Indirect effects, which
are caused by the action and are later in time or farther
removed in distance, but are still reasonably foreseeable.”
§ 1508.8. Thus, the relevant question is whether the in-
crease in cross-border operations of Mexican motor carriers,
with the correlative release of emissions by Mexican trucks,
is an “effect” of FMCSA’s issuance of the Application and
Safety Monitoring Rules; if not, FMCSA’s failure to address
these effects in its EA did not violate NEPA, and so
FMCSA’s issuance of a FONSI cannot be arbitrary and
capricious.
A
To answer this question, we begin by explaining what this
case does not involve. What is not properly before us, de-
spite respondents’ argument to the contrary, see Brief for
Respondents 38–41, is any challenge to the EA due to its
failure properly to consider possible alternatives to the pro-
posed action (i. e., the issuance of the challenged rules) that
would mitigate the environmental impact of the authoriza-
tion of cross-border operations by Mexican motor carriers.
Persons challenging an agency’s compliance with NEPA
must “structure their participation so that it . . . alerts the
agency to the [parties’] position and contentions,” in order to
allow the agency to give the issue meaningful consideration.
Vermont Yankee Nuclear Power Corp. v. Natural Resources
Defense Council, Inc., 435 U. S. 519, 553 (1978). None of
the respondents identified in their comments any rulemaking
alternatives beyond those evaluated in the EA, and none
urged FMCSA to consider alternatives. Because respond-
ents did not raise these particular objections to the EA,
FMCSA was not given the opportunity to examine any pro-
posed alternatives to determine if they were reasonably
available. Respondents have therefore forfeited any objec-

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tion to the EA on the ground that it failed adequately to
discuss potential alternatives to the proposed action.
Admittedly, the agency bears the primary responsibility to
ensure that it complies with NEPA, see ibid., and an EA’s
or an EIS’ flaws might be so obvious that there is no need
for a commentator to point them out specifically in order to
preserve its ability to challenge a proposed action. But that
situation is not before us. With respect to FMCSA’s ability
to mitigate, respondents can argue only that FMCSA could
regulate emissions from Mexican trucks indirectly, through
making the safety-registration process more onerous or by
removing older, more polluting trucks through more effec-
tive enforcement of motor carrier safety standards. But re-
spondents fail to identify any evidence that shows that any
effect from these possible actions would be significant, or
even noticeable, for air-quality purposes. The connection
between enforcement of motor carrier safety and the envi-
ronmental harms alleged in this case is also tenuous at best.
Nor is it clear that FMCSA could, consistent with its limited
statutory mandates, reasonably impose on Mexican carriers
standards beyond those already required in its proposed
regulations.
B
With this point aside, respondents have only one complaint
with respect to the EA: It did not take into account the
environmental effects of increased cross-border operations
of Mexican motor carriers. Respondents’ argument that
FMCSA was required to consider these effects is simple.
Under § 350, FMCSA is barred from expending any funds to
process or review any applications by Mexican motor carri-
ers until FMCSA implemented a variety of specific applica-
tion and safety-monitoring requirements for Mexican car-
riers. This expenditure bar makes it impossible for any
Mexican motor carrier to receive authorization to operate
within the United States until FMCSA issued the regula-
tions challenged here. The promulgation of the regulations,

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the argument goes, would “caus[e]” the entry of Mexican
trucks (and hence also cause any emissions such trucks
would produce), and the entry of the trucks is “reasonably
foreseeable.” 40 CFR § 1508.8 (2003). Thus, the argument
concludes, under the relevant CEQ regulations, FMCSA
must take these emissions into account in its EA when evalu-
ating whether to produce an EIS.
Respondents’ argument, however, overlooks a critical fea-
ture of this case: FMCSA has no ability to countermand the
President’s lifting of the moratorium or otherwise categor-
ically to exclude Mexican motor carriers from operating
within the United States. To be sure, § 350 did restrict
the ability of FMCSA to authorize cross-border operations
of Mexican motor carriers, but Congress did not other-
wise modify FMCSA’s statutory mandates. In particular,
FMCSA remains subject to the mandate of 49 U. S. C.
§ 13902(a)(1), that FMCSA “shall register a person to pro-
vide transportation . . . as a motor carrier if [it] finds that
the person is willing and able to comply with” the safety and
financial responsibility requirements established by DOT.
(Emphasis added.) Under FMCSA’s entirely reasonable
reading of this provision, it must certify any motor carrier
that can show that it is willing and able to comply with the
various substantive requirements for safety and financial re-
sponsibility contained in DOT regulations; only the morato-
rium prevented it from doing so for Mexican motor carriers
before 2001. App. 51–55. Thus, upon the lifting of the mor-
atorium, if FMCSA refused to authorize a Mexican motor
carrier for cross-border services, where the Mexican motor
carrier was willing and able to comply with the various sub-
stantive safety and financial responsibilities rules, it would
violate § 13902(a)(1).
If it were truly impossible for FMCSA to comply with both
§ 350 and § 13902(a)(1), then we would be presented with an
irreconcilable conflict of laws. As the later enacted provi-
sion, § 350 would quite possibly win out. See Posadas v. Na-

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Opinion of the Court
tional City Bank, 296 U. S. 497, 503 (1936). But FMCSA
can easily satisfy both mandates: It can issue the application
and safety inspection rules required by § 350, and start proc-
essing applications by Mexican motor carriers and authorize
those that satisfy § 13902(a)(1)’s conditions. Without a con-
flict, then, FMCSA must comply with all of its statutory
mandates.
Respondents must rest, then, on a particularly unyielding
variation of “but for” causation, where an agency’s action is
considered a cause of an environmental effect even when the
agency has no authority to prevent the effect. However, a
“but for” causal relationship is insufficient to make an agency
responsible for a particular effect under NEPA and the rele-
vant regulations. As this Court held in Metropolitan Edi-
son Co. v. People Against Nuclear Energy, 460 U. S. 766, 774
(1983), NEPA requires “a reasonably close causal relation-
ship” between the environmental effect and the alleged
cause. The Court analogized this requirement to the “famil-
iar doctrine of proximate cause from tort law.” Ibid. In
particular, “courts must look to the underlying policies or
legislative intent in order to draw a manageable line between
those causal changes that may make an actor responsible for
an effect and those that do not.” Id., at 774, n. 7. See also
W. Keeton, D. Dobbs, R. Keeton, & D. Owen, Prosser and
Keeton on Law of Torts 264, 274–275 (5th ed. 1984) (proxi-
mate cause analysis turns on policy considerations and con-
siderations of the “legal responsibility” of actors).
Also, inherent in NEPA and its implementing regulations
is a “ ‘rule of reason,’ ” which ensures that agencies deter-
mine whether and to what extent to prepare an EIS based
on the usefulness of any new potential information to the
decisionmaking process. See Marsh, 490 U. S., at 373–374.
Where the preparation of an EIS would serve “no purpose”
in light of NEPA’s regulatory scheme as a whole, no rule of
reason worthy of that title would require an agency to pre-
pare an EIS. See Aberdeen & Rockfish R. Co. v. Students

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Challenging Regulatory Agency Procedures (SCRAP), 422
U. S. 289, 325 (1975); see also 40 CFR §§ 1500.1(b)–(c) (2003).
In these circumstances, the underlying policies behind
NEPA and Congress’ intent, as informed by the “rule of
reason,” make clear that the causal connection between
FMCSA’s issuance of the proposed regulations and the entry
of the Mexican trucks is insufficient to make FMCSA respon-
sible under NEPA to consider the environmental effects of
the entry. The NEPA EIS requirement serves two pur-
poses. First, “[i]t ensures that the agency, in reaching its
decision, will have available, and will carefully consider, de-
tailed information concerning significant environmental im-
pacts.” Robertson, 490 U. S., at 349. Second, it “guaran-
tees that the relevant information will be made available to
the larger audience that may also play a role in both the
decisionmaking process and the implementation of that deci-
sion.” Ibid. Requiring FMCSA to consider the environ-
mental effects of the entry of Mexican trucks would fulfill
neither of these statutory purposes. Since FMCSA has no
ability categorically to prevent the cross-border operations
of Mexican motor carriers, the environmental impact of the
cross-border operations would have no effect on FMCSA’s
decisionmaking—FMCSA simply lacks the power to act on
whatever information might be contained in the EIS.
Similarly, the informational purpose is not served. The
“informational role” of an EIS is to “giv[e] the public the
assurance that the agency ‘has indeed considered environ-
mental concerns in its decisionmaking process,’ Baltimore
Gas & Electric Co. [v. Natural Resources Defense Council,
Inc., 462 U. S. 87, 97 (1983)], and, perhaps more significantly,
provid[e] a springboard for public comment” in the agency
decisionmaking process itself, ibid. The purpose here is to
ensure that the “larger audience,” ibid., can provide input as
necessary to the agency making the relevant decisions. See
40 CFR § 1500.1(c) (2003) (“NEPA’s purpose is not to gener-
ate paperwork—even excellent paperwork—but to foster ex-

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Opinion of the Court
cellent action. The NEPA process is intended to help public
officials make decisions that are based on understanding of
environmental consequences, and take actions that protect,
restore, and enhance the environment”); § 1502.1 (“The pri-
mary purpose of an environmental impact statement is to
serve as an action-forcing device to insure that the policies
and goals defined in the Act are infused into the ongoing
programs and actions of the Federal Government”). But
here, the “larger audience” can have no impact on FMCSA’s
decisionmaking, since, as just noted, FMCSA simply could
not act on whatever input this “larger audience” could
provide.2
It would not, therefore, satisfy NEPA’s “rule of reason” to
require an agency to prepare a full EIS due to the environ-
mental impact of an action it could not refuse to perform.
Put another way, the legally relevant cause of the entry of
the Mexican trucks is not FMCSA’s action, but instead the
actions of the President in lifting the moratorium and those
of Congress in granting the President this authority while
simultaneously limiting FMCSA’s discretion.
Consideration of the CEQ’s “cumulative impact” regula-
tion does not change this analysis. An agency is required
to evaluate the “[c]umulative impact” of its action, which is
defined as “the impact on the environment which results
from the incremental impact of the action when added to
other past, present, and reasonably foreseeable future ac-
tions regardless of what agency (Federal or non-Federal) or
person undertakes such other actions.” § 1508.7. The “cu-
mulative impact” regulation required FMCSA to consider
the “incremental impact” of the safety rules themselves,
in the context of the President’s lifting of the moratorium
2 Respondents are left with arguing that an EIS would be useful for
informational purposes entirely outside FMCSA’s decisionmaking process.
See Brief for Respondents 42. But such an argument overlooks NEPA’s
core focus on improving agency decisionmaking. See 40 CFR §§ 1500.1,
1500.2, 1502.1 (2003).

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and other relevant circumstances. But this is exactly what
FMCSA did in its EA. FMCSA appropriately and reason-
ably examined the incremental impact of its safety rules as-
suming the President’s modification of the moratorium (and,
hence, assuming the increase in cross-border operations of
Mexican motor carriers). The “cumulative impact” regula-
tion does not require FMCSA to treat the lifting of the mora-
torium itself, or consequences from the lifting of the morato-
rium, as an effect of its promulgation of its Application and
Safety Monitoring Rules.3
C
We hold that where an agency has no ability to prevent a
certain effect due to its limited statutory authority over the
relevant actions, the agency cannot be considered a legally
relevant “cause” of the effect. Hence, under NEPA and the
implementing CEQ regulations, the agency need not con-
sider these effects in its EA when determining whether its
action is a “major Federal action.” Because the President,
not FMCSA, could authorize (or not authorize) cross-border
operations from Mexican motor carriers, and because
FMCSA has no discretion to prevent the entry of Mexican
trucks, its EA did not need to consider the environmental
effects arising from the entry.4
3 The Court of Appeals and respondents contend that the EA contained
numerous other errors, but their contentions are premised on the con-
clusion that FMCSA was required to take into account the increased
cross-border operations of Mexican motor carriers.
4 Respondents argue that Congress ratified the Court of Appeals’ deci-
sion when it, after the lower court’s opinion, reenacted § 350 in two appro-
priations bills. The doctrine of ratification states that “Congress is pre-
sumed to be aware of [a] . . . judicial interpretation of a statute and to
adopt that interpretation when it re-enacts a statute without change.”
Lorillard v. Pons, 434 U. S. 575, 580 (1978). But this case involves the
interpretation of NEPA and the CAA, not § 350. Indeed, the precise re-
quirements of § 350 were not below, and are not here, in dispute. Hence,
congressional reenactment of § 350 tells us nothing about Congress’ view

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III
Under the CAA, a federal “department, agency, or instru-
mentality” may not, generally, “engage in, support in any
way or provide financial assistance for, license or permit, or
approve, any activity” that violates an applicable state air-
quality implementation plan. 42 U. S. C. § 7506(c)(1); 40
CFR § 93.150(a) (2003). Federal agencies must, in many cir-
cumstances, undertake a conformity determination with
respect to a proposed action, to ensure that the action
is consistent with § 7506(c)(1). See 40 CFR §§ 93.150(b),
93.153(a)–(b). However, an agency is exempt from the gen-
eral conformity determination under the CAA if its action
would not cause new emissions to exceed certain threshold
emission rates set forth in § 93.153(b). FMCSA determined
that its proposed regulations would not cause emissions to
exceed the relevant threshold amounts and therefore con-
cluded that the issuance of its regulations would comply with
the CAA. App. to Pet. for Cert. 65a–66a, 155a. Critical to
its calculations was its consideration of only those emissions
that would occur from the increased roadside inspections
of Mexican trucks; like its NEPA analysis, FMCSA’s CAA
analysis did not consider any emissions attributable to the
increased presence of Mexican trucks within the United
States.
The EPA’s rules provide that “a conformity determination
is required for each pollutant where the total of direct
and indirect emissions in a nonattainment or maintenance
area caused by a Federal action would equal or exceed”
the threshold levels established by the EPA. 40 CFR
§ 93.153(b) (2003). “Direct emissions” are defined as those
covered emissions “that are caused or initiated by the Fed-
eral action and occur at the same time and place as the
as to the requirements of NEPA and the CAA, and so, on the legal issues
involved in this case, Congress has been entirely silent.

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action.” § 93.152. The term “[i]ndirect emissions” means
covered emissions that
“(1) Are caused by the Federal action, but may occur
later in time and/or may be further removed in distance
from the action itself but are still reasonably foresee-
able; and
“(2) The Federal agency can practicably control and
will maintain control over due to a continuing program
responsibility of the Federal agency.” Ibid.
Unlike the regulations implementing NEPA, the EPA’s
CAA regulations have defined the term “[c]aused by.” Ibid.
In particular, emissions are “[c]aused by” a federal action if
the “emissions . . . would not . . . occur in the absence of the
Federal action.” Ibid. Thus, the EPA has made clear that
for purposes of evaluating causation in the conformity review
process, some sort of “but for” causation is sufficient.
Although arguably FMCSA’s proposed regulations would
be “but for” causes of the entry of Mexican trucks into the
United States, the emissions from these trucks are neither
“direct” nor “indirect” emissions. First, the emissions from
the Mexican trucks are not “direct” because they will not
occur at the same time or at the same place as the promulga-
tion of the regulations.
Second, FMCSA cannot practicably control, nor will it
maintain control, over these emissions. As discussed above,
FMCSA does not have the ability to countermand the Presi-
dent’s decision to lift the moratorium, nor could it act cate-
gorically to prevent Mexican carriers from being registered
or Mexican trucks from entering the United States. Once
the regulations are promulgated, FMCSA would have no
ability to regulate any aspect of vehicle exhaust from these
Mexican trucks. FMCSA could not refuse to register Mexi-
can motor carriers simply on the ground that their trucks
would pollute excessively. FMCSA cannot determine

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whether registered carriers actually will bring trucks into
the United States, cannot control the routes the carriers
take, and cannot determine what the trucks will emit. Any
reduction in emissions that would occur at the hands of
FMCSA would be mere happenstance. It cannot be said
that FMCSA “practicably control[s]” or “will maintain con-
trol” over the vehicle emissions from the Mexican trucks, and
it follows that the emissions from the Mexican trucks are not
“indirect emissions.” Ibid.; see also Determining Conform-
ity of General Federal Actions to State or Federal Implemen-
tation Plans, 58 Fed. Reg. 63214, 63221 (1993) (“The EPA
does not believe that Congress intended to extend the prohi-
bitions and responsibilities to cases where, although licensing
or approving action is a required initial step for a subsequent
activity that causes emissions, the agency has no control over
that subsequent activity”).
The emissions from the Mexican trucks are neither “di-
rect” nor “indirect” emissions caused by the issuance of
FMCSA’s proposed regulations. Thus, FMCSA did not vio-
late the CAA or the applicable regulations by failing to con-
sider them when it evaluated whether it needed to perform
a full “conformity determination.”
IV
FMCSA did not violate NEPA or the relevant CEQ regu-
lations when it did not consider the environmental effect of
the increase in cross-border operations of Mexican motor car-
riers in its EA. Nor did FMCSA act improperly by not per-
forming, pursuant to the CAA and relevant regulations, a
full conformity review analysis for its proposed regulations.
We therefore reject respondents’ challenge to the procedures
used in promulgating these regulations. Accordingly, the
judgment of the Court of Appeals is reversed, and the case
is remanded for further proceedings consistent with this
opinion.
It is so ordered.