Court Opinion

ID: 5125479
Source: CourtListenerOpinion
Date Created: 2021-11-12 16:01:18.082626+00
Date Added: 2024-06-11T08:22:51.080860
License: Public Domain

United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT

Argued September 15, 2020         Decided November 12, 2021

                        No. 16-1430

   TRUCK TRAILER MANUFACTURERS ASSOCIATION, INC.,
                    PETITIONER

                             v.

       ENVIRONMENTAL PROTECTION AGENCY, ET AL.,
                   RESPONDENTS

        CALIFORNIA AIR RESOURCES BOARD, ET AL.,
                     INTERVENORS

         On Petition for Review of an Action of the
    United States Environmental Protection Agency and
    the National Highway Traffic Safety Administration

    Elisabeth S. Theodore argued the cause for petitioner.
With her on the briefs were S. Zachary Fayne, Jonathan S.
Martel, and Samuel F. Callahan.

     H. Thomas Byron, III, Attorney, U.S. Department of
Justice, argued the cause for respondents. With him on the
brief were Jeffrey Bossert Clark, Assistant Attorney General,
Jonathan Brightbill, Principal Deputy Assistant Attorney
General, and Eric G. Hostetler and Jennifer L. Utrecht,
Attorneys. Sue S. Chen, Attorney, and Douglas N. Letter,
                               2
General Counsel, U.S. House of Representatives, entered
appearances.

     Alice Henderson argued the cause for respondents-
intervenors Public Health and Environmental Organizations.
With her on the brief were Jim Dennison, Vickie Patton, Peter
Zalzal, Benjamin Longstreth, Peter Huffman, Vera Pardee,
Joanne Spalding, Andres Restrepo, Susannah Landes Weaver,
Sean H. Donahue, Clare Lakewood, Katherine Hoff, and Kevin
Bundy.

     Xavier Becerra, Attorney General, Office of the Attorney
General for the State of California, Robert W. Byrne, Senior
Assistant Attorney General, Myung J. Park, Supervising
Deputy Attorney General, M. Elaine Meckenstock, Caitlan
McLoon, and Ryan R. Hoffman, Deputy Attorneys General,
William Tong, Attorney General, Office of the Attorney
General for the State of Connecticut, Matthew I. Levine and
Scott N. Koschwitz, Assistant Attorneys General, Maura
Healey, Attorney General, Office of the Attorney General for
the Commonwealth of Massachusetts, Carol Iancu, Assistant
Attorney General, Tom Miller, Attorney General, Office of the
Attorney General for the State of Iowa, Jacob J. Larson,
Assistant Attorney General, Ellen F. Rosenblum, Attorney
General, Office of the Attorney General for the State of
Oregon, Paul Garrahan, Attorney-in-Charge, Peter F.
Neronha, Attorney General, Office of the Attorney General for
the State of Rhode Island, Gregory S. Schultz, Special Assistant
Attorney General, Bob Ferguson, Attorney General, Office of
the Attorney General for the State of Washington, Thomas J.
Young, Assistant Attorney General, Thomas J. Donovan, Jr.,
Attorney General, Office of the Attorney General for the State
of Vermont, and Nicholas F. Persampieri, Assistant Attorney
General, were on the brief for respondents-intervenors
California Air Resources Board, et al. Kathleen A. Kenealy,
                               3
Chief Assistant Attorney General, Office of the Attorney
General for the State of California, entered an appearance.

    Before: MILLETT, KATSAS*, and WALKER, Circuit Judges.

    Opinion for the Court filed by Circuit Judge WALKER.

     Opinion concurring in the judgment in part and dissenting
in part filed by Circuit Judge Millett.

     WALKER, Circuit Judge: In 2016, the Environmental
Protection Agency issued a rule for trailers pulled by tractors
based on a statute enabling the EPA to regulate “motor
vehicles.” In that same rule, the National Highway Traffic
Safety Administration issued fuel efficiency standards for
trailers based on a statute enabling NHTSA to regulate
“commercial medium-duty or heavy-duty on-highway
vehicles.”

    Trailers, however, have no motor. They are therefore not
“motor vehicles.” Nor are they “vehicles” when that term is
used in the context of a vehicle’s fuel economy, since motorless
vehicles use no fuel.

     We therefore grant the petition and vacate all portions of
the rule that apply to trailers.

                               I.

    The most widely recognized “tractor-trailer” combination
is what a layperson calls a semitruck. A trailer is the back

* Judge Katsas was randomly selected to replace then-Judge
Garland, who was a member of the panel at the time the case was
submitted.
                                4
portion attached to a motorized tractor in the front. Trailers
include tanks, car carriers, logging trailers, and platforms.
Greenhouse Gas Emissions and Fuel Efficiency Standards for
Medium- and Heavy-Duty Engines and Vehicles—Phase 2, 81
Fed. Reg. 73,478, 73,640 (Oct. 25, 2016).

     In 2016, the EPA and NHTSA jointly created a rule called
“Greenhouse Gas Emissions and Fuel Efficiency Standards for
Medium- and Heavy-Duty Engines and Vehicles—Phase 2.”
Id. at 73,478. Under that rule, for the first time, those agencies
set greenhouse gas emissions and fuel efficiency standards for
heavy-duty trailers. The rule requires trailer manufacturers to
adopt some combination of fuel-saving technologies, such as
side skirts and automatic tire pressure systems.

     Truck Trailer Manufacturers Association, Inc. objected to
the rule and timely petitioned for review. In 2017, this court
granted the Association’s motion to stay the EPA’s portion of
the rule to the extent it applies to trailers. In 2020, we stayed
the compliance dates in NHTSA’s portion.

                                II.

     An agency’s rule may not exceed the agency’s statutory
authority. 42 U.S.C. § 7607(d)(9)(C); 5 U.S.C. § 706(2)(C).
To understand that authority, we consider the statute’s text,
structure, and context. We ask “whether Congress has directly
spoken to the precise question at issue.” Chevron, U.S.A., Inc.
v. Natural Resources Defense Council, Inc., 467 U.S. 837, 842
(1984). If Congress has, and the agency acted in accordance
with the statute, our inquiry ends. Id. at 842-43 (“If the intent
of Congress is clear, that is the end of the matter; for the court,
as well as the agency, must give effect to the unambiguously
                                 5
expressed intent of Congress.”).1

                      A. The EPA’s Authority

    According to the EPA, it can regulate (1) trailers as motor
vehicles, 42 U.S.C. § 7521(a)(1), and (2) trailer manufacturers
as motor-vehicle manufacturers, id. § 7550(1). We discuss
each in turn.

                        1. Motor Vehicles

     The EPA primarily relied on § 202(a)(1) of the Clean Air
Act for its authority to regulate trailers’ effects on greenhouse
gas emissions. Id. § 7521(a)(1). That section requires the EPA
to set emissions standards for new motor vehicles and their
engines if they emit harmful air pollutants. It provides:

        The Administrator shall by regulation
        prescribe (and from time to time revise) in
        accordance with the provisions of this section,
        standards applicable to the emission of any
        air pollutant from any class or classes of new
        motor vehicles or new motor vehicle engines,
        which in his judgment cause, or contribute to,
        air pollution which may reasonably be
        anticipated to endanger public health or welfare.
        Such standards shall be applicable to such
        vehicles and engines for their useful life (as
        determined under subsection (d), relating to

1
 The Association argues that the agencies are not entitled to Chevron
deference because (1) the EPA didn’t invoke it, and (2) the agencies
are actively reconsidering the rule. However, we need not decide
whether Chevron deference applies because, when the relevant terms
are read in context, they are unambiguous.
                               6
       useful life of vehicles for purposes of
       certification), whether such vehicles and
       engines are designed as complete systems or
       incorporate devices to prevent or control such
       pollution.

Id. (emphases added).

     The Act defines “motor vehicle” to exclude anything that
does not propel itself. Id. § 7550(2). For the purposes of § 202,
a “motor vehicle” is “any self-propelled vehicle designed for
transporting persons or property on a street or highway.”
Id. (emphases added).

    Because trailers are not “self-propelled,” they are not
motor vehicles under § 202. Therefore, the EPA cannot rely
on § 202 to regulate trailers’ effects on greenhouse gas
emissions.

     The EPA and Respondent-Intervenors ask us to focus on
the second half of motor vehicle’s definition, requiring a motor
vehicle to be “designed for transporting persons or property.”
Id. They say “the tractor-trailer as a whole should be
considered the pertinent vehicle” because a tractor “cannot
accomplish its intended purpose” unless the tractor is pulling
the trailer. Respondents’ Br. 29.

     But tractors can carry people and things without trailers
attached. As anyone who has spent any time on a highway
knows, they often do. So a tractor without a trailer can still
accomplish what it is “designed for.” 42 U.S.C. § 7550(2).
That makes a self-propelled tractor a “motor vehicle” long
before a trailer is ever attached.
                                 7
    The EPA also invites us to focus on the second sentence of
§ 202(a)(1): “Such standards shall be applicable to such
vehicles . . . whether such vehicles . . . are designed as
complete systems or incorporate devices to prevent or control
such pollution.” Id. § 7521(a)(1). The EPA says that sentence
allows it to regulate significant components of the complete
vehicle because Congress suggested that the EPA could create
standards for vehicles or portions of vehicles not “designed as
complete systems.” Id.

     But that’s not what Congress did. Rather, it created two
categories of complete motor vehicles. Category one: motor
vehicles with built-in pollution control. Category two: motor
vehicles with add-in devices for pollution control. Either way,
for both categories, the phrase “such vehicles,” in that sentence,
refers to “motor vehicles,” not components. That takes us back
to where we started. Because trailers are not “motor vehicles,”
no     reference     to    “motor     vehicles” — like      “such
vehicles” — covers trailers.

     The EPA’s theories, taken to their logical conclusion,
mean that the EPA could regulate other non-propelled items
attached to a motorized vehicle. For example: rooftop cargo
carriers. They are “designed for transporting . . . property.” Id.
§ 7550(2). And when they are on a car, they are attached to a
motor vehicle. But they are not themselves motor vehicles.2

2
  The EPA claims that our approach would allow a manufacturer to
avoid EPA emissions standards by making an entire car, except for a
component that makes the car self-propelled, such as the ignition
switch. But under our approach, the EPA can ensure compliance by
regulating the assembler of the car — the company that installs the
ignition switch — thereby making the vehicle self-propelled. See id.
§§ 7521(a)(1), 7550(1). With trailers, the problem for the EPA is
                                 8

     Perhaps sensing that it needs to offer a limiting principle,
the EPA claims that it can regulate only “significant” vehicle
components. Respondents’ Br. 34. But that limit is atextual.
Even if a trailer is a motor-vehicle component — which we
doubt3 — we cannot endorse a hazy line mentioned nowhere in
a statutory provision that allows the EPA to regulate “motor
vehicles,” not motor-vehicle components.

     In addition, the EPA is incorrect that § 202(a)(5), (a)(6),
and (k) of the Clean Air Act provide contextual support for its
alleged authority under § 202(a)(1) to regulate any component
of an entire vehicle — even assuming again that a trailer is a
motor-vehicle component. These provisions address fill pipes,
onboard vapor recovery systems, and control of evaporative
emissions of hydrocarbons. 42 U.S.C. § 7521(a)(5), (a)(6), (k).
Even if they allow for regulation of specific vehicle
components, § 202(a)(1) doesn’t allow for the regulation of all
others. Rather, § 202(a)(1) covers self-propelled units, not
components of self-propelled units. That Congress may have
provided separate provisions about specific components
reinforces the limited scope of § 202(a)(1) — regulation of
motor vehicles designed to transport people or property.

                2. Motor-Vehicle Manufacturers

    According to the EPA and Respondent-Intervenors, the
EPA can require trailer manufacturers to abide by the agency’s
emissions standards because the statute’s definition of

that before any trailer shows up, there’s a vehicle that has already
been made self-propelled — the tractor.
3
  We think the trailer is probably more like a rooftop cargo
carrier — a mere attachment to a motor vehicle, rather than a
component of a motor vehicle.
                                  9
manufacturers allows for multiple manufacturers of one
vehicle. The Act defines “manufacturer” as one “engaged in
the manufacturing or assembling of new motor vehicles”:

        [A]ny person engaged in the manufacturing or
        assembling of new motor vehicles, new motor
        vehicle engines, new nonroad vehicles or new
        nonroad engines, or importing such vehicles or
        engines for resale, or who acts for and is under
        the control of any such person in connection
        with the distribution of new motor vehicles . . . .

Id. § 7550(1).

     In making that argument, the EPA and Respondent-
Intervenors do not attempt to refute what we’ve already
explained: (a) a motor vehicle, as defined, must be self-
propelled; and (b) a trailer, by itself, is not self-propelled. But
rather than deducing that a trailer is therefore not a motor
vehicle, they say that because a trailer attached to a tractor can
be imagined as a “tractor-trailer” (true), a trailer manufacturer
is “engaged in the manufacturing” of a motor vehicle (false).

      The key problem for the EPA here — as it was in the
subsection above — is that a tractor is a motor vehicle before
it’s part of a tractor-trailer. With or without a trailer, the tractor
is self-propelled and designed for transporting people or
property on the road. Trailers (and trailer manufacturers) are
therefore unlike parts of a motor vehicle (and their
manufacturers) required for self-propulsion.

     To be sure, a “tractor-trailer” combination is self-
propelled, in the same sense that a van with a cargo carrier is
self-propelled. But again, a tractor is already a motor vehicle,
whether or not the trailer is attached. Trailer manufacturers are
                                 10
thus only “engaged in the manufacturing . . . of new motor
vehicles” in the way that rooftop cargo carrier manufacturers
would be “engaged in the manufacturing . . . of new motor
vehicles” — that is to say, not in a way that follows the
definition of manufacturer.4

     Moreover, because trailers are rarely, if ever, sold together
with tractors, various statutory requirements would be
impossible for trailer manufacturers to satisfy if the “motor
vehicle” were the tractor-trailer combination. For example, a
trailer manufacturer could not obtain the necessary certificate
of conformity prior to sale, because it could not know to which
tractor its trailer would later be attached. See id. § 7525(a)(1).
Nor could it satisfy the requirement to warrant compliance of
the tractor-trailer combination with applicable regulations. See
id. § 7541(a)(1).

     The EPA attempts to distinguish trailers from attachments
like rooftop cargo carriers by asserting the authority to regulate
only “significant” vehicle components. Cf. id. § 7550(9)
(referring to a vehicle part or component as something
“installed in or on motor vehicles”).5 But as mentioned above,
the statute does not distinguish between “significant” and

4
 Likewise, we wouldn’t, for example, say potters manufacture
potted plants.
5
  When the EPA says a trailer is a component, we presume it means
it’s a component “installed in or on motor vehicles,” id. § 7550(9),
not a part required to actually complete a “self-propelled vehicle
designed for transporting persons or property,” id. § 7550(2). But if
the EPA means the second type, we have already explained why a
trailer is not a required segment necessary for a tractor to meet this
motor vehicle definition. So a trailer would not be a component in
that sense.
                             11
“insignificant” components. The EPA cannot save its theory
by inventing an atextual limit.

    Finally, even if trailer manufacturers make vehicle
components — again, we doubt it — the Act’s definition of
“manufacturer” does not include a maker of vehicle
components. See id. § 7550(1). Perhaps that is why Congress
defined “motor vehicle or engine part manufacturer”
separately. Id. § 7550(9) (emphasis added). And it might
explain why Congress referred to motor vehicle components
and component manufacturers elsewhere. See, e.g., id.
§§ 7541(a)(2), 7542(a).

                         *   *    *

    The objects of the EPA’s § 202 Clean Air Act regulations
must be self-propelled. Trailers are not self-propelled.
Therefore, the EPA cannot use § 202(a)(1) to set emissions
standards for trailers and require trailer manufacturers to
comply with them.

                  B. NHTSA’s Authority

    We turn now to the National Highway Traffic Safety
Administration’s authority. NHTSA relies on the Ten-in-Ten
Fuel Economy Act, which was enacted as part of the Energy
Independence and Security Act of 2007. 81 Fed. Reg. at
73,519; see Pub. L. No. 110-140, 121 Stat. 1492 (2007).

     The Ten-in-Ten Fuel Economy Act requires NHTSA, with
help from the EPA, to establish “fuel economy standards for”
certain vehicles. 49 U.S.C. § 32902(b)(1). It instructs the
agencies to examine the “fuel efficiency” for some of those
vehicles, determine metrics for their “fuel efficiency,” and
                                12
make rules to improve their “fuel efficiency.”                   Id.
§ 32902(k)(1)-(2).

     Congress’s fuel economy statute applies to “passenger
automobiles,” “non-passenger automobiles,” and “work
trucks.” Id. § 32902(b)(1)(A)-(C). No one contends that
trailers are covered by those categories.

     The statute also applies to “commercial medium-duty or
heavy-duty on-highway vehicles.” Id. § 32902(b)(1)(C); see
also id. § 32902(k)(2).6 That is the only type of vehicle the
parties discuss here. Congress defined that type of vehicle as
“an on-highway vehicle with a gross vehicle weight rating of
10,000 pounds or more.” Id. § 32901(a)(7). But here, unlike
in the Clean Air Act, Congress didn’t define “vehicle.”

     Section 32902(k) subjects “work trucks” and “commercial
medium- and heavy-duty on-highway vehicles” to a “fuel
efficiency” study and subsequent rulemaking, which includes
“implement[ing] appropriate . . . fuel economy standards.” Id.
§ 32902(k).7

     In 2016, NHTSA used that statutory directive to regulate
trailers. It claims that the term “vehicles” is ambiguous and

6
 Sometimes the statute says “commercial medium-duty or heavy-
duty” and other times it says “commercial medium- and heavy-
duty.” Compare id. § 32902(b)(1)(C) with id. § 32902(k)(2)
(emphases added). They’re the same, and we use both phrases
interchangeably.
7
  Congress separately outlined the requirements for fuel economy
standards for “passenger automobiles” and “non-passenger
automobiles” in § 32902(b)(2), without requiring any fuel efficiency
studies. Compare id. § 32902(b)(2) with id. § 32902(k)(1).
                               13
that it reasonably interpreted “commercial medium- and heavy-
duty on-highway vehicles” to include trailers. Id. We disagree.

    If you went to law school in the past sixty years, you may
have come across the following question: “A legal rule forbids
you to take a vehicle into the public park. Plainly this forbids
an automobile, but what about bicycles, roller skates, toy
automobiles? What about airplanes?” H.L.A. Hart, Positivism
and the Separation of Law and Morals, 71 HARV. L. REV. 593,
607 (1958).

     That question has been called “the most famous
hypothetical in the common law world.” Frederick Schauer, A
Critical Guide to Vehicles in the Park, 83 N.Y.U. L. REV. 1109,
1109 (2008). It shows that the meaning of a word may be broad
in the abstract, but unambiguously narrower in context. Is a
toy truck a “vehicle”? In the abstract, perhaps. As many
dictionaries will show, “vehicle” can, in isolation, be a broad
term. See, e.g., Vehicle, MERRIAM-WEBSTER’S COLLEGIATE
DICTIONARY (11th ed. 2003) (“a means of carrying or
transporting something”). But in the context of a rule
excluding vehicles from a park, the term is not quite so
expansive: It’s safe to say the rule doesn’t cover a picnicking
child playing with a wooden truck. See Antonin Scalia &
Bryan A. Garner, A Note on the Use of Dictionaries, 16 GREEN
BAG 2D 419, 423 (2013) (“Because common words typically
have more than one meaning, you must use the context in
which a given word appears to determine its aptest, most likely
sense.”).

    In the Energy Independence and Security Act of 2007, as
in the hypothetical about a park, the word “vehicle” is
undefined. But here, as there, its context prescribes its limits.
See FDA v. Brown & Williamson Tobacco Co., 529 U.S. 120,
133 (2000) (“It is a fundamental canon of statutory
                                 14
construction that the words of a statute must be read in their
context and with a view to their place in the overall statutory
scheme.”) (cleaned up); Natural Resources Defense Council v.
EPA, 489 F.3d 1364, 1373 (D.C. Cir. 2007) (“absence of a
statutory definition does not” make a statutory term
ambiguous).

      Here, Congress made “fuel economy” — a measure of
“miles traveled by an automobile for each gallon of
gasoline . . . used,” 49 U.S.C. § 32901(a)(11) — a required
trait of the vehicles subject to the fuel efficiency improvement
program, see id. § 32902(k)(1)-(2). In its statutory subsection
on “commercial medium- and heavy-duty on-highway
vehicles,” Congress referred to “fuel economy” three times. Id.
§ 32902(k). 8 And across § 32902 — which covers those
vehicles as well as automobiles and work trucks — it did so
thirty-seven times.9 Of course, quantity isn’t everything. But
by requiring NHTSA to set fuel economy standards when
establishing its fuel efficiency program and then constantly

8
 Twice if you don’t count its reference to the Ten-in-Ten Fuel
Economy Act. Id. § 32902(k)(1).
9
  See id. § 32902 (title) (once); id. § 32902(a) (twice); id.
§ 32902(b)(1) (once); id. § 32902(b)(2) (eleven times); id.
§ 32902(b)(3) (three times); id. § 32902(b)(4) (once); id. § 32902(c)
(once); id. § 32902(d)(1) (three times); id. § 32902(d)(2) (once); id.
§ 32902(e)(2) (twice); id. § 32902(f) (three times); id. § 32902(g)(1)
(once); id. § 32902(g)(2) (once); id. § 32902(h)(1) (once); id.
§ 32902(h)(3) (once); id. § 32902(j)(1) (once); id. § 32902(k)(1)
(once, in reference to the Ten-in-Ten Fuel Economy Act); id.
§ 32902(k)(2) (once); id. § 32902(k)(3) (once).
                                   15
referring us to fuel economy, Congress put the term “vehicle”
in a context limited to machines that use fuel.10

     That limit unambiguously rules out trailers. They are not
“commercial medium- and heavy-duty on-highway vehicles”
as that term is used in § 32902. 11 That alone is enough to
decide this case. But two additional considerations confirm our
conclusion.

     First, grouped words should be given meanings that are
similar in nature. See Agnew v. Government of the District of
Columbia, 920 F.3d 49, 56 (D.C. Cir. 2019) (a term is “known
by the company it keeps”); Antonin Scalia & Bryan A. Garner,
READING LAW: THE INTERPRETATION OF LEGAL TEXTS 195-98

10
  NHTSA points out that a trailer affects the fuel efficiency and fuel
economy of a tractor. But so do bike racks, rooftop cargo carriers,
and other attachments to vehicles. And not even NHTSA argues that
they are vehicles. That said, we agree with the State Intervenors that
NHTSA cannot regulate bike racks for a different reason — because
bike racks do not have “a gross vehicle weight rating of 10,000
pounds or more.” Id. § 32901(a)(7).
Moreover, NHTSA has the authority to set only “fuel economy
standard[s]” for vehicles, id. § 32902(b)(1), and it is unnatural to
describe the extent to which one vehicle affects another vehicle’s
fuel economy as itself a “fuel economy standard” for the whole
vehicle. So while a trailer may affect the “fuel economy standard”
of a tractor, the trailer does not itself have a “fuel economy standard.”
11
  We read “on-highway vehicles” as describing vehicles designed
for use on a highway.           An alternative reading of that
phrase — covering the form in which vehicles travel on the
highway — is at best a distinction without a difference and at worst
a reading that would permit regulation of commercial medium- and
heavy-duty vehicles like bulldozers and excavators that can be driven
on a highway but are not designed for that purpose.
                               16
(2012) (noscitur a sociis). In § 32902(b)(1), the purportedly
ambiguous term “vehicles” is preceded by three other types of
vehicles which all have engines and burn fuel: “passenger
automobiles,” “non-passenger automobiles,” and “work
trucks.” 49 U.S.C. § 32902(b)(1); see also Yates v. United
States, 574 U.S. 528, 543 (2015) (plurality opinion) (“a word
is given more precise content by the neighboring words with
which it is associated”) (cleaned up). And lest there be any
doubt about the limits of the meaning of “automobile,” the
statute defines it as “a 4-wheeled vehicle that is propelled by
fuel, or by alternative fuel . . . .” Id. § 32901(a)(3).

    Trailers have no meaningful similarity to passenger and
non-passenger automobiles because trailers are not “propelled
by fuel, or by alternative fuel.” Nor are they like work trucks,
which have an engine and burn fuel.

     It’s true that the definitions of work trucks and commercial
vehicles do not say “propelled by fuel,” like the definition of
“automobile” does. Compare id. § 32901(a)(7), (a)(19) with
id. § 32901(a)(3). But even NHTSA does not deny that trucks
use fuel, and in the 2007 Act, Congress at times used “trucks”
and “vehicles” synonymously. Specifically, in §§ 107 and 108
of the Act, Congress referred to a “medium-duty and heavy-
duty truck” instead of a “medium- and heavy-duty on-highway
vehicle,” as it does elsewhere. See Pub. L. No. 110-140,
§§ 107-08, 121 Stat. at 1504-05.12

    Since trucks use fuel, and since Congress used “trucks”
and “vehicles” synonymously, “vehicles” use fuel as well (in

12
   In those two sections, Congress told the Secretary of
Transportation and the National Academy of Sciences to “execute an
agreement” according to which the academy would make a report
analyzing vehicle and truck fuel economy standards. See id.
                               17
the context of § 32902). And because a trailer does not
consume fuel, it’s not a vehicle in the context of § 32902.

    Second, NHTSA’s mistaken reliance on the definition of
“motor vehicle” in NHTSA’s organic act is the nail in its coffin.
See 49 U.S.C. § 30102(a)(7). Congress expressly excluded
from that definition’s application the entire chapter on which
NHTSA relies for its authority to regulate trailers.

    To see why, consider Part A and Part C of the subtitle that
governs NHTSA’s authority.

    Within Part A, “motor vehicle” is defined as “a vehicle
driven or drawn by mechanical power and manufactured
primarily for use on public streets, roads, and highways . . . .”
Id.

     You can see why NHTSA and the State Intervenors cite
that definition in their briefs. A trailer is “drawn by mechanical
power.”

     But Part A doesn’t give NHTSA any authority to regulate
trailers. Its purported authority comes from § 32902 in Chapter
329 of Part C. So now consider Part C.

    Part C includes the same definition of “motor vehicle” as
in Part A. Id. § 32101(7). But the introductory phrase to its
definitions says: “In this part (except chapter 329 . . . )—.” Id.
§ 32101 (emphasis added). That means that the “drawn by
mechanical power” definition of “motor vehicle” applies to all
of Part C except for Chapter 329 — the chapter with the
                                 18
provision (§ 32902) that gives NHTSA its purported authority
to regulate trailers.13

    In other words, NHTSA’s authority to regulate trailers
under § 32902 depends on a definition that by its express terms
does not apply to § 32902.14

     Finally, to the degree NHTSA suggests that a tractor-
trailer unit is the relevant vehicle that it can regulate under
§ 32902, its reasoning is misplaced for the same reason that the
EPA’s reasoning was misplaced: A tractor is the relevant
“vehicle” or “truck” for regulation before the trailer is attached.
That’s because the tractor is a vehicle with fuel economy —
“miles traveled” per “gallon of gasoline . . . used.” Id.
§ 32901(a)(11).15

13
  We discuss the phrase “motor vehicle” here only to rebut NHTSA’s
and the State Intervenors’ use of the phrase. Respondents’ Br. 17
(referring to the definition of “motor vehicle” in § 30102(a)(7));
State Intervenors’ Br. 8 (same).
14
   Moreover, the introductory phrase to the definition section of Part
A also provides some insight into how far the “drawn by mechanical
power” definition stretches there. In § 30102, the section begins with
“[i]n this chapter.” So the definition from § 30102(a)(7) is meant to
apply to Chapter 301 — the chapter that § 30102 is in — which is
not the same chapter that § 32902 is in — Chapter 329. And there is
good reason why “motor vehicle” would be defined differently in
Chapter 301 than in Chapter 329. Chapter 301 concerns safety, and
trailers have safety features such as lights, turn signals, and
reflectors. But trailers do not have fuel economy, which is the focus
of Chapter 329. “In this chapter” is thus further evidence that the
§ 30102(a)(7) definition should not apply to a “vehicle” in § 32902.
15
  What’s more, § 32901 defines “commercial medium- and heavy-
duty on-highway vehicles” by reference to a vehicle’s “gross vehicle
weight rating,” 49 U.S.C. § 32901(a)(7), a common phrase used in
                                  19

     And, as with the EPA’s arguments, there is no principled
limit to NHTSA’s theory. Under it, NHTSA could regulate
bike racks, rooftop cargo carriers, or anything similar that
would impact the fuel efficiency of a vehicle.16 To be sure,
NHTSA can regulate tractors based on the trailers they pull,
as can the EPA. But neither NHTSA nor the EPA can regulate
trailers themselves.

                             *    *    *

     Because a trailer uses no fuel, it doesn’t have fuel
economy. And in the statutory context of § 32902, nothing is
a vehicle unless it has fuel economy — a measure of miles
traveled per gallon of fuel used.

     NHTSA therefore lacked the authority to regulate trailers.

NHTSA parlance to describe the “maximum load that can be carried
by a vehicle, including the weight of the vehicle itself,” 81 Fed. Reg.
at 73,485 n.26. But NHTSA regulations separately refer to the gross
combined weight rating, defined as the “maximum load that the
vehicle can haul, including the weight of a loaded trailer and the
vehicle itself.” Id. If “commercial medium- and heavy-duty on-
highway vehicles” were meant to include tractor-trailers, it would
make little sense to define that phrase by a gross vehicle weight
rating, as opposed to a gross combined weight rating.
16
  Under that theory, the weight requirement in the definition of
“commercial medium- and heavy-duty on-highway vehicle” would
not prevent a bike rack from being regulated because it would allow
NHTSA to regulate any attachment that results in a gross vehicle
weight rating of 10,000 pounds or more. Id. § 32901(a)(7).
                               20
                               III.

     The Final Rule relies on statutes that do not give the EPA
and NHTSA authority to regulate trailers. We grant the
petition for review and vacate all portions of the rule that apply
to trailers.
     MILLETT, Circuit Judge, concurring in the judgment in
part and dissenting in part: From the Pacific Coast Highway to
Route 66 to the Pennsylvania Turnpike, eighteen-wheelers,
also known as semitrucks, have long been a familiar presence
on America’s highways. While these tractor-trailers play a key
role in transporting goods from one coast to the other, they also
consume fuel at high rates and emit large amounts of carbon
dioxide and other greenhouse gases. Recognizing the effect of
these heavy-duty vehicles on climate change, President Obama
directed the National Highway Traffic Safety Administration
(“NHTSA”) and the Environmental Protection Agency
(“EPA”) to issue fuel efficiency and greenhouse gas emission
standards for tractor-trailers’ operations. See THE WHITE
HOUSE, IMPROVING THE FUEL EFFICIENCY OF AMERICAN
TRUCKS—BOLSTERING ENERGY SECURITY, CUTTING CARBON
POLLUTION,        SAVING      MONEY       AND       SUPPORTING
MANUFACTURING INNOVATION (Feb. 2014), at 7–8.

    This case is about the EPA’s and NHTSA’s authority to
regulate the trailer portion of a tractor-trailer to improve fuel
economy. See Greenhouse Gas Emissions and Fuel Efficiency
Standards for Medium- and Heavy-Duty Engines and
Vehicles—Phase 2 (“Phase Two Rule”), 81 Fed. Reg. 73,478
(Oct. 25, 2016). Although the two agencies collaborated in
establishing their respective standards, each issued its own
regulations under independent and distinct grants of statutory
authority.

     The EPA acted under the Clean Air Act, which authorizes
the agency to regulate emissions of air pollutants from “new
motor vehicles[.]” 42 U.S.C. § 7521(a)(1). The Clean Air Act
defines “motor vehicle” as “any self-propelled vehicle
designed for transporting persons or property on a street or
highway.” Id. § 7550(2). The EPA viewed trailers as falling
within that definition.
                               2
     Because the trailers are tractor-propelled rather than self-
propelled, I agree with the majority opinion’s judgment that the
Clean Air Act’s text precludes the particular EPA regulations
at issue here. I further agree with the majority opinion that, in
seeking to reduce emissions, the EPA could instead regulate
the tractors, including the types of trailers they are allowed to
pull. Majority Op. 19 (“To be sure, NHTSA can regulate
tractors based on the trailers they pull, as can the EPA.”); see
also Oral Arg. Tr. 10:16–11:1 (Association counsel answering
“probably, yes” when asked if the EPA could “pass a regulation
that says tractors are banned * * * from traveling on roads and
highways if they’re pulling loads that cause the tractor's
emissions to increase by XX amount”). And nothing in today’s
decision forecloses the EPA from regulating the assembler of
the tractor-trailer to ensure that the assembled tractor-trailers
meet specified emission standards. See Oral Arg. Tr. 9:16–17
(Association counsel explaining that the EPA “definitely can
regulate an assembler” that connects a tractor and a trailer).

     But when it comes to the question of NHTSA’s authority
to issue its separate fuel economy regulations, I part ways with
the majority opinion. NHTSA acted under a provision of the
Energy Independence and Security Act of 2007 (“Energy
Independence Act”) that directed NHTSA to establish fuel
efficiency standards for commercial medium- and heavy-duty
“on-highway vehicles[.]” 49 U.S.C. § 32902(k)(2). Unlike the
Clean Air Act, the Energy Independence Act contains no
definition of the term “vehicle” other than regulating it in its
on-highway operation and status. Given that focal point,
NHTSA quite reasonably applied a long-established definition
of vehicles that includes commercial trailers. The majority
opinion’s view that NHTSA’s interpretation somehow runs
afoul of “plain” non-existent text does not stand up.
                              3
                              I

                              A

     Congress first established national fuel economy standards
in 1975. See Energy Policy and Conservation Act, Pub. L. No.
94-163, § 301, 89 Stat. 871, 901 (1975). These initial
standards, enacted to lessen dependence on foreign oil
following the 1973–1974 oil embargo, applied only to
passenger automobiles and light trucks.            RICHARD K.
LATTANZIO ET AL., CONG. RESEARCH SERV., IF10871, VEHICLE
FUEL ECONOMY AND GREENHOUSE GAS STANDARDS 1 (2021);
see also BRENT D. YACOBUCCI & ROBERT BAMBERGER, CONG.
RESEARCH SERV., RL33413, AUTOMOBILE AND LIGHT TRUCK
FUEL ECONOMY: THE CAFE STANDARDS 3 (2007).

     Three decades later, as part of the Energy Independence
Act, Congress extended fuel economy standards to include
“work trucks and commercial medium-duty or heavy-duty on-
highway vehicles[.]” Pub. L. No. 110-140, § 102(a)(2), 121
Stat. 1492, 1499 (2007) (codified at 49 U.S.C.
§ 32902(b)(1)(C)). Those fuel economy standards were just
one piece of a comprehensive fuel efficiency program for work
trucks and medium- and heavy-duty vehicles. See 49 U.S.C.
§ 32902(k)(1).     Congress charged the Secretary of
Transportation with implementing the program through
rulemaking. Id. § 32902(k)(2).

     Before the regulatory process could start, though, the
Energy Independence Act required that the National Academy
of Sciences first complete a study that assessed the fuel
efficiency of medium- and heavy-duty on-highway vehicles.
See 49 U.S.C. § 32902(k)(1); Energy Independence Act,
§§ 107–108, 121 Stat. at 1505. Within a year of receiving the
study, the Secretary of Transportation had to consult with the
Secretary of Energy and the EPA Administrator both to
                               4
determine how to test and measure fuel efficiency in those
vehicles and to identify what factors would affect and improve
their fuel efficiency. 49 U.S.C. § 32902(k)(1)(A)–(D). The
Secretary of Transportation delegated authority over the fuel
efficiency program and the fuel economy standards to NHTSA.
49 C.F.R. §§ 1.94(c), 1.95(j)(3).

    The Energy Independence Act directed NHTSA, again in
consultation with the Secretary of Energy and the EPA
Administrator, to implement the fuel efficiency program
through rulemaking no more than two years after the study’s
completion. 49 U.S.C. § 32092(k)(2). The fuel efficiency
program was to include “appropriate test methods,
measurement metrics, fuel economy standards, and compliance
and enforcement protocols that are appropriate, cost-effective,
and technologically feasible[.]” Id.

     In 2014, the National Academy of Sciences released the
statutorily required fuel economy study that directly preceded
the regulations at issue in this case. See NATIONAL RESEARCH
COUNCIL OF THE NATIONAL ACADEMIES, REDUCING THE FUEL
CONSUMPTION AND GREENHOUSE GAS EMISSIONS OF MEDIUM-
AND HEAVY-DUTY VEHICLES, PHASE TWO: FIRST REPORT
(2014). That study specifically addressed at length the effect
that tractor-trailers (combined tractors and heavy-duty trailers)
have on fuel consumption and efficiency. Id. at 67–87. The
study determined that the largest tractor-trailers account for
60% of the fuel consumption of all heavy-duty on-highway
vehicles, and that regulating the trailer’s design and equipment
“could substantially increase overall fuel savings” for the
tractor-trailer on the highway. J.A. 309–310; see also J.A. 292
(2010 National Academy of Sciences study) (“Trailers, which
present an important opportunity for fuel consumption
reduction, can benefit from improvements in aerodynamics and
tires.”).
                                 5
                                 B

     As required by the Energy Independence Act, NHTSA
then proposed and, after a full notice-and-comment process,
adopted final regulations aimed at improving the fuel
efficiency and economy of on-highway medium- and heavy-
duty vehicles, including tractor-trailers. See Greenhouse Gas
Emissions and Fuel Efficiency Standards for Medium- and
Heavy-Duty Engines and Vehicles—Phase 2, 80 Fed. Reg.
40,138, 40,141, 40,161–40,162 (proposed July 13, 2015);
Phase Two Rule, 81 Fed. Reg. 73,478, 73,504–73,505 (final
rule).

     The final rule explained the importance of regulating the
fuel efficiency of the trailer portion of commercial on-highway
tractor-trailers because large tractor-trailers account for 60% of
the fuel consumption and carbon dioxide emissions from
heavy-duty vehicles. Phase Two Rule, 81 Fed. Reg. at 73,485,
73,639. The EPA calculated that as much as one-third of the
potential reduction in tractor-trailer fuel consumption and
emissions could be achieved through regulation of the trailer’s
equipment and design alone. Id. at 73,516 n.89.1

    1
        Because there are “no available technologies [that] reduce
tailpipe greenhouse gas emissions per gallon of fuel combusted,” a
“rule that limits tailpipe greenhouse gas emissions is effectively
identical to a rule that limits fuel consumption.” Delta Constr. Co.
v. EPA, 783 F.3d 1291, 1294 (D.C. Cir. 2015) (per curiam) (brackets
omitted) (quoting Greenhouse Gas Emissions Standards and Fuel
Efficiency Standards for Medium- and Heavy-Duty Engines and
Vehicles, 76 Fed. Reg. 57,106, 57,124–57,125 (Sept. 15, 2011)); cf.
Reduce Climate           Change,      U.S. DEP’T OF ENERGY,
https://www.fueleconomy.gov/feg/climate.shtml (“Pollution control
devices cannot reduce your car’s CO2 emissions.”) (last accessed
                                  6
     The regulations apply to two types of trailers: box vans
and non-box trailers. See Phase Two Rule, 81 Fed. Reg. at
73,645–73,646. Box vans are the most common kind of trailer.
Id. at 73,640. They have a large, enclosed cargo space that is
permanently attached to the trailer’s frame, as well as fixed
sides and a roof, and are generally between 28 and 53 feet in
length. Id. at 73,640, 73,645. Anyone who has driven on a
highway has seen countless box vans—the trailer portion of
eighteen-wheeler semitrucks—passing by.

     Non-box trailers, on the other hand, generally carry freight
that will not easily fit in a box van. See Phase Two Rule, 81
Fed. Reg. at 73,640. Only three types of non-box trailers are
subject to the regulations: flatbed trailers, tank trailers, and
container chassis. Id. at 73,646–73,647. Think of trailers that
haul construction vehicles, containers of liquids, or enormous
pipes. Id.; see also 40 C.F.R. § 1037.5(g).

     Trailer manufacturers can use a combination of
technologies to improve fuel efficiency and reduce greenhouse
gas emissions. For most box vans, the regulations require the
use of some combination of (i) aerodynamic devices such as
side skirts and gap-closing devices, (ii) weight-reduction
strategies, (iii) low-rolling resistance tires, and (iv) tire pressure
systems. See Phase Two Rule, 81 Fed. Reg. at 73,505, table I–
7; id. at 73,643, 73,647–73,648, 73,665–73,667; see also
40 C.F.R. §§ 1037.107(a)(2), (3), 1037.515; 49 C.F.R.
§§ 535.5(e)(1), 535.6(e)(3), (4).

Nov. 4, 2021); Revised 2023 and Later Model Year Light-Duty
Vehicle Greenhouse Gas Emissions Standards, 86 Fed. Reg. 43,726,
43,788 (proposed Aug. 10, 2021) (“Reducing fuel consumption is a
significant means of reducing [greenhouse gas] emissions from the
transportation fleet.”).
                                7
     For non-box trailers, the regulations require only the use
of certain low-rolling resistance tires and tire pressure systems.
See Phase Two Rule, 81 Fed. Reg. at 73,648, 73,667; see also
40 C.F.R. § 1037.107(a)(4); 49 C.F.R. § 535.5(e)(2).

     The fuel efficiency measures chosen by the final rule draw
on those already adopted by a number of trailer manufacturers
as part of the voluntary SmartWay Transport Partnership
program established by the EPA in 2004. That program
encourages transportation companies, including trailer
manufacturers, to take steps to improve fuel efficiency and to
reduce greenhouse gas emissions. Between 2004 and 2016
more than 3,000 firms, most of them trucking fleets,
participated in the program. See Greenhouse Gas Emissions
Standards and Fuel Efficiency Standards for Medium- and
Heavy-Duty Engines and Vehicles (“Phase One Proposed
Rule”), 75 Fed. Reg. 74,152, 74,159 (proposed Nov. 30, 2010);
Phase Two Rule, 81 Fed. Reg. at 73,640–73,641; NATIONAL
RESEARCH COUNCIL, REDUCING THE FUEL CONSUMPTION AND
GREENHOUSE GAS EMISSIONS OF MEDIUM- AND HEAVY-DUTY
VEHICLES, at 72.

                               II

     NHTSA acted well within its delegated regulatory
authority in establishing fuel efficiency requirements for the
trailer portion of tractor-trailers that regularly travel the
Nation’s highways.

                                A

     To begin with, because the Energy Independence Act does
not define the term “vehicle” other than requiring that (as
relevant here) it be a medium- or heavy-duty on-highway
commercial vehicle, the familiar Chevron framework applies.
See Cuozzo Speed Techs., LLC v. Lee, 136 S. Ct. 2131, 2142
                               8
(2016); Cigar Ass’n of America v. FDA, 5 F.4th 68, 77 (D.C.
Cir. 2021). That is because Congress expressly “delegated
authority to the agency generally to make rules carrying the
force of law,” United States v. Mead Corp., 533 U.S. 218, 226–
227 (2001), and, in fact, the statute requires NHTSA to
implement the fuel economy standards “by regulation,” 49
U.S.C. § 32902(k)(2).          That “express congressional
authorization[] to engage in the process of rulemaking” is a
“very good indicator” that Congress intended the resulting
regulations to carry the force of law and to be reviewed under
Chevron’s deferential standard. Mead Corp., 533 U.S. at 229;
accord Encino Motorcars, LLC v. Navarro, 136 S. Ct. 2117,
2125 (2016).

     In addition, NHTSA’s regulation was promulgated as a
legislative rule through full notice-and-comment rulemaking
for the specific purpose of fulfilling that statutory obligation.
See Mead Corp., 533 U.S. at 226–227 (“[A]dministrative
implementation of a particular statutory provision qualifies for
Chevron deference when it appears that Congress delegated
authority to the agency generally to make rules carrying the
force of law” and “the agency interpretation claiming deference
was promulgated in the exercise of that authority”). NHTSA
expressly invoked its statutorily delegated authority in
outlining the “mandatory standards” its regulations established.
See Long Island Care at Home, Ltd. v. Coke, 551 U.S. 158, 172
(2007); Anna Jacques Hosp. v. Burwell, 797 F.3d 1155, 1166
(D.C. Cir. 2015).

    Tellingly, neither the Association nor the majority opinion
dispute that the regulations established for trailers’ on-highway
operation are the type of legislative rule that qualifies for
Chevron deference if there is ambiguity in the term “vehicle.”
                                9
     The Chevron framework has two steps. First, we
determine “whether Congress has directly spoken to the precise
question at issue” because, “[i]f the intent of Congress is clear,
that is the end of the matter.” Chevron U.S.A. Inc. v. Natural
Res. Def. Council, Inc., 467 U.S. 837, 842–843 (1984). But if
the statute is “silent or ambiguous,” then we will uphold an
agency’s interpretation as long as it is reasonable. See City of
Arlington v. FCC, 569 U.S. 290, 296 (2013); Chevron, 467
U.S. at 843; see also HollyFrontier Cheyenne Refin., LLC v.
Renewable Fuels Ass’n, 141 S. Ct. 2172, 2184 n.1 (2021)
(Barrett, J., dissenting).

    Because the Energy Independence Act does not define
“vehicle” and because NHTSA’s definition is reasonable and
consistent with statutory text and structure, as well as common
usage, NHTSA was well within its lane in regulating how
commercial trailers are designed for use on highways.

                                B

                                1

     The Energy Independence Act contains no definition of
the term “vehicle” beyond requiring (as relevant here) that the
vehicles, when operating “on [the] highway,” qualify as
“commercial medium- [or] heavy-duty vehicles[.]” 49 U.S.C.
§ 32902(b)(1)(C). The Act then defines the term “commercial
medium- and heavy-duty on-highway vehicle” to mean “an on-
highway vehicle with a gross vehicle weight rating of 10,000
pounds or more.” Id. § 32901(7). There is no dispute that,
whether alone or combined with a tractor, the trailers regulated
by NHTSA are “commercial” and satisfy that gross weight
criterion. See, e.g., EPA & NHTSA, GREENHOUSE GAS
EMISSIONS AND FUEL EFFICIENCY STANDARDS FOR MEDIUM-
AND HEAVY-DUTY ENGINES AND VEHICLES - PHASE 2:
REGULATORY IMPACT ANALYSIS (2016), at 3-72–3-73 (finding
                               10
that a typical large box trailer “has an empty weight ranging
between 13,500 and 14,000 pounds[,]” flatbed and tanker
trailers weigh approximately 10,000 pounds empty, and
medium- and heavy-duty semi-trailers have average payloads
ranging between 26,343 pounds and 37,190 pounds).

    By defining the regulated vehicle in terms of its “on-
highway” status, the statute focuses on vehicles in their
highway-using and highway-operating form. So their character
when rolling off the assembly line or otherwise prior to that
highway function does not textually cabin what qualifies as a
vehicle for purposes of Section 32902(b)(1)(C). That stands in
sharp contrast to the Clean Air Act definition on which the EPA
attempted to rely in regulating trailers, which defines “motor
vehicle” as “any self-propelled vehicle designed for
transporting persons or property on a street or highway.” 42
U.S.C. § 7550(2) (emphasis added); compare 49 U.S.C.
§ 32901(a)(3) (defining “automobile” as, inter alia, “a 4-
wheeled vehicle that is propelled by fuel, or by alternative fuel,
manufactured primarily for use on public streets, roads, and
highways”) (emphasis added).

     Notably, at the time the Energy Independence Act was
adopted—and long before—Congress had statutorily defined
“vehicle” in other portions of Title 49 administered by NHTSA
to include the trailer portion of tractor-trailers. The Motor
Vehicle Information and Cost Savings Act defines “motor
vehicle” to include “vehicle[s]” that are “driven or drawn by
mechanical power” on public streets, roads, or highways. Pub.
L. No. 92-513, § 2(15), 86 Stat. 947, 948 (1972) (emphasis
added). In fact, Congress first defined a regulated vehicle in
language that includes trailers as far back as 1966 in NHTSA’s
organic statute, the National Traffic and Motor Vehicle Safety
Act of 1966. That definition provided: “‘Motor vehicle’
means any vehicle driven or drawn by mechanical power[.]”
                                11
Pub. L. No. 89-563, § 102(3), 80 Stat. 718, 718.2 Soon
thereafter, NHTSA promulgated a regulation that specifically
placed trailers within that definition of motor vehicle. See 49
C.F.R. § 571.3 (“Trailer means a motor vehicle with or without
motive power, designed for carrying persons or property and
for being drawn by another motor vehicle.”) (emphasis in
original).

     Other provisions in Title 49 have, since the 1980s,
explicitly included trailers and semitrailers within their “motor
vehicle” definitions. See National Driver Register Act of 1982,
Pub. L. No. 97-364, § 202(5), 96 Stat. 1740, 1741 (codified as
amended at 49 U.S.C. § 30301(4)) (“[A] vehicle, machine,
tractor, trailer, or semitrailer propelled or drawn by mechanical
power and used on a highway * * * [.]”); Commercial Motor
Vehicle Safety Act of 1986, Pub. L. No. 99-570, § 12019(5),
100 Stat. 3207-170, 3207-188 (codified as amended at 49
U.S.C. § 31301(12)) (“[A] vehicle, machine, tractor, trailer, or
semitrailer propelled or drawn by mechanical power * * * [.]”).

     “Vehicle” had a similarly capacious meaning in common
usage in 2007, when Congress passed the Energy
Independence Act. Black’s Law Dictionary, for example,
defined “vehicle” as encompassing “[a]ny conveyance used in
transporting passengers or things by land, water, or air.”
BLACK’S LAW DICTIONARY 1589 (8th ed. 2004). For non-
lawyers, Webster’s defined “vehicle” to include “a means of
carrying or transporting something: Conveyance[,]” “a carrier
of goods or passengers[,]” “a container in which something is

    2
       The Motor Vehicle Safety Act of 1966 created the National
Traffic Safety Agency, see Pub. L. No. 89-563, § 115, 80 Stat. 718,
727, which became part of NHTSA after NHTSA was established in
the Highway Safety Act of 1970, see Pub. L. No. 91-605, § 202(a),
84 Stat. 1713, 1739–1740.
                               12
conveyed[,]” and “a piece of mechanized equipment[.]”
WEBSTER’S NEW INT’L DICTIONARY 2538 (def. 5) (3d ed.
2002); see also 19 OXFORD ENGLISH DICTIONARY 480 (defs. 6,
7a) (2d ed. 1989) (defining “vehicle” as “[a] means of
conveyance provided with wheels or runners and used for the
carriage of persons or goods; a carriage, cart, wagon, sledge, or
similar contrivance” and “[a]ny means of carriage,
conveyance, or transport; a receptacle in which anything is
placed in order to be moved”); WEBSTER’S NEW INT’L
DICTIONARY 2824 (def. 1) (2d ed. 1941) (“That in or on which
a person or thing is or may be carried from one place to another,
esp. along the ground, also through the air; any moving support
or container fitted or used for the conveyance of bulky objects;
a means of conveyance.”).

     Also, as the majority opinion notes, in defining the
meaning of statutory terms, context matters. See, e.g.,
Wisconsin Cent. Ltd. v. United States, 138 S. Ct. 2067, 2071–
2072 (2018); FDA v. Brown & Williamson Tobacco Corp., 529
U.S. 120, 132–133 (2000); Majority Op. 13. The context in
which the Energy Independence Act uses “vehicle” is not an
abstract one; the statute refers to a “commercial” “on-highway
vehicle[.]” 49 U.S.C. § 32902(b)(1)(C). So the types of trailers
subject to regulation are only commercial trailers to be used on
highways, which means a trailer combined with a tractor—that
is, tractor-trailers. Trailers, after all, only go on highways
joined with a tractor or truck.

     And at the time of the Energy Independence Act’s
adoption, Congress would have understood that, when driving
on the highways and roadways, a conjoined tractor-trailer had
its own singular vehicular identity as a work vehicle: in
common parlance, a semitruck or semitrailer. Webster’s, for
example, defined “semi-trailer” as a “trucking rig made up of
a tractor and a semitrailer—called also semi[.]” WEBSTER’S
                                 13
NEW INT’L DICTIONARY 2065 (def. 2) (3d. ed. 2002). This
meaning traces back to well before Congress founded NHTSA.
In 1941, Webster’s defined a semitrailer as “[a] highway
vehicle which when running is supported at its forward end by
a fifth wheel mounted on a motor truck or tractor[.]”
WEBSTER’S NEW INT’L DICTIONARY 2276 (def. 1) (2d ed.
1941) (emphasis added).

     Indeed, since the invention of the semitrailer by a Detroit
blacksmith in 1914, tractors and trailers have functioned
together as work vehicles to drive trillions of miles—and
consume hundreds of billions of gallons of fuel—across the
United States.3 In fact, it is largely in semitruck form that
tractor-trailers fulfill their commercial raison d’être—their
vital transportation function of moving supplies and goods
across the Nation. See FREIGHT FACTS AND FIGURES 2017, at
3-23     (truck-trailer     combinations    “accounted       for
approximately 60.8 percent of commercial truck travel” in
the United States in 2015).

     3
        See Fruehauf Trailer Corporation, in Britannica Academic,
Encyclopedia         Britannica,       (May         17,       2016),
academic.eb.com/levels/collegiate/article/Fruehauf-Trailer-
Corporation/35519 (last accessed Nov. 4, 2021); U.S. Vehicle-Miles,
BUREAU TRANSP. STAT., https://www.bts.gov/content/us-vehicle-
miles (last accessed Nov. 4, 2021); BUREAU TRANSP. STAT.,
FREIGHT FACTS AND FIGURES 2017, at 6–7 (June 2018) (truck-trailer
combinations consumed more than 86 billion gallons of fuel between
2013 and 2015 alone); BUREAU TRANSP. STAT., APPENDIX B –
GLOSSARY,                             at                            4,
https://www.bts.gov/sites/bts.dot.gov/files/docs/browse-statistical-
products-and-data/national-transportation-
statistics/217556/appendix-b-glossary.pdf (last accessed Nov. 4,
2021) (defining “combination truck”).
                              14
     In short, the Energy Independence Act does not textually
constrain the meaning of vehicle in a way that excludes
commercial trailers operated on a highway as tractor-trailers.
Quite the opposite: Ample preexisting and contemporary
statutory provisions, regulations, dictionaries, and common
understanding firmly embrace trailers in their on-highway role
within the meaning of “vehicle.” So NHTSA’s regulatory
choice was not statutorily foreclosed under Chevron’s first
step.

                              2

     The majority opinion openly recognizes the ambiguity in
the term “vehicle.” After all, the “No Vehicles in the Park”
example that the opinion invokes would not have become “the
most famous hypothetical in the common law world[,]” or have
puzzled first-year law students for six decades if the meaning
of vehicle were not ambiguous. See Majority Op. 13 (quoting
H.L.A. Hart, Positivism and Separation of Law and Morals, 71
HARV. L. REV. 593, 607 (1958), and Frederick Schauer, A
Critical Guide to Vehicles in the Park, 83 N.Y.U. L. REV. 1109,
1109 (2008)). In defining what Chevron leaves to agency
discretion, it would be hard to improve on what Hart believed
his thought experiment illustrated: The existence of a
“penumbra of debatable cases in which words are neither
obviously applicable nor obviously ruled out.”            Hart,
Positivism, supra, at 607; see also Schauer, A Critical Guide,
supra, at 1109 (“Hart used the example to maintain that rules
have a core of clear applications surrounded by a penumbra of
uncertainty[.]”). Such debatable cases “will be resolved,”
under Chevron, “within the bounds of reasonable
interpretation, not by the courts but by the administering
agency.” City of Arlington, 569 U.S. at 296.
                               15
     Nevertheless, despite the term’s famous ambiguity, and
the absence of any definition of “vehicle” in the Energy
Independence Act at all, let alone one that forecloses the long-
established inclusion of semitrailers, the majority opinion
offers a laundry list of reasons that it believes “rule[] out
trailers.” Majority Op. 15. But all of those reasons wash out.

    First, the majority opinion reasons that the statute is about
“fuel economy,” and trailers do not “use fuel[,]” Majority Op.
15 (emphasis in original).

     That argument blinks away the statutory focus on the fuel
economy of “on-highway” vehicles. On the highway, trailers
unite with tractors into a single vehicle—a tractor-trailer or,
colloquially, a semitruck. And it is an undisputed fact on the
record before us that the trailer part of the semitruck barreling
down a highway “contribute[s] substantially” to the fuel
consumption and sharply diminished fuel economy of the
tractor-trailer. Phase Two Rule, 81 Fed. Reg. at 73,639. As
the National Academy of Sciences found in its statutorily-
directed study, when it comes to fuel economy “[t]he tractor
and trailer are fundamentally inseparable in addressing
aerodynamic drag[.]”         NATIONAL RESEARCH COUNCIL,
REDUCING THE FUEL CONSUMPTION AND GREENHOUSE GAS
EMISSIONS OF MEDIUM- AND HEAVY-DUTY VEHICLES, at 38; cf.
NATIONAL ACADEMIES OF SCIENCES, ENGINEERING, AND
MEDICINE, REDUCING FUEL CONSUMPTION AND GREENHOUSE
GAS EMISSIONS OF MEDIUM- AND HEAVY-DUTY VEHICLES,
PHASE TWO: FINAL REPORT 166–167 (2020) (relating that, in
a Daimler project, “trailer improvements delivered 72 percent
of the total vehicle aerodynamic improvement, but requir[ed]
only one-third of the aerodynamic engineering effort”).

   In other words, tractor-trailers consume substantially
more fuel than the tractor alone. Phase Two Rule, 81 Fed. Reg.
                               16
at 73,516. So the “average number of miles traveled by” a
tractor-trailer “for each gallon of gasoline[,]” as well as the
additional amount of fuel per mile caused by the trailer portion
itself, present distinct fuel-economy questions that are readily
measurable and just as readily regulable under the statutory
definition of “fuel economy[.]” 49 U.S.C. § 32901(11); see
also, e.g., NATIONAL RESEARCH COUNCIL, REDUCING THE
FUEL CONSUMPTION AND GREENHOUSE GAS EMISSIONS OF
MEDIUM- AND HEAVY-DUTY VEHICLES, at 83 (finding that
regulating non-box trailers “could substantially increase
overall fuel savings”). Nothing in the statutory text or
references suggests that Congress wanted to avert its eyes to
this most commonplace presence on the highways.

     Second, the majority opinion seizes on Congress’s
reference to a “heavy-duty truck” rather than a “heavy-duty
vehicle” in different provisions—Sections 107 and 108 of the
Act. See Majority Op. 16–17; see also Association Br. 43–45.
Those Sections call on the National Academy of Sciences to,
inter alia, study “medium-duty and heavy-duty truck fuel
economy standards” Pub. L. No. 110-140, § 108(a), 121 Stat.
at 1505, and to update its report at five-year intervals until
2025. Id. at § 107(c), 121 Stat. at 1504. In the majority
opinion’s view, the Act’s two references to “medium-duty and
heavy-duty truck[s]” rather than “vehicles” unambiguously
shows that Congress treats trucks and vehicles synonymously.
And because the majority opinion believes all trucks consume
fuel, it thinks all vehicles must consume fuel as well.

     The problem is that the term “truck” does not exclude
trailers, especially in their on-highway form as tractor-
trailers—a.k.a. semitrucks. In fact, the National Academy of
Sciences itself, in the very study that Section 108 ordered, read
the term “truck” to encompass tractor-trailers as a distinct
vehicular entity. See NATIONAL RESEARCH COUNCIL OF THE
                               17
NATIONAL ACADEMIES, TECHNOLOGIES AND APPROACHES TO
REDUCING THE FUEL CONSUMPTION OF MEDIUM- AND HEAVY-
DUTY VEHICLES 2 (2010) (“There are literally thousands of
different configurations for vehicles, including bucket trucks,
pickup trucks, garbage trucks, delivery vehicles, and long-haul
tractor trailers.”) (emphasis added).

     So much for a clear statutory preclusion of on-highway
trailers.

      Third, the majority opinion tries the noscitur a sociis
canon. See Majority Op. 15–17; see also Association Br. 42–
43. In English, that canon says that words are known by the
company they keep. Yates v. United States, 574 U.S. 528, 543
(2015) (plurality opinion). The majority opinion points to
Section 32902(b)’s command that the Secretary of
Transportation issue fuel economy standards for (i) “passenger
automobiles[,]” (ii) “non-passenger automobiles[,]” and
(iii) “work trucks and commercial medium-duty or heavy-duty
on-highway vehicles[,]” 49 U.S.C. § 32902(b)(1), and reasons
that because passenger automobiles, non-passenger
automobiles, and work trucks “use fuel,” the same must be true
of “vehicles.” Majority Op. 15–17.

     No dice. For starters, since Congress’s concern was fuel
efficiency and fuel economy, the common denominator is just
as reasonably that each category uses up fuel when on the road.
Trailers certainly do that in their on-highway status: Tractor-
trailers “use fuel” (Majority Op. 15) (emphasis in original) and,
because of the trailer portion, they use up fuel at a markedly
higher rate, with less fuel efficiency and less fuel economy than
the tractor portion alone. That measurable difference can be
remediated through the types of fuel efficiency and fuel
economy measures for which the Energy Independence Act
calls.
                                18
     In addition, the noscitur canon is generally applied when
the statutory context indicates that Congress intended for
different words to share similar meanings—as indicated, for
example, by the statute applying a single rule to a string of
related terms. See, e.g., McDonnell v. United States, 136 S. Ct.
2355, 2365, 2368–2369 (2016) (interpreting “question” and
“matter” in statute barring officials from soliciting or accepting
bribes in return for “any decision or action on any question,
matter, cause, suit, proceeding or controversy * * * .”) (quoting
18 U.S.C. § 201(a)(3)); United States v. Bronstein, 849 F.3d
1101, 1104, 1108–1109 (D.C. Cir. 2017) (interpreting
“harangue” and “oration” in a provision making it unlawful to
“make a harangue or oration, or utter loud, threatening, or
abusive language in the Supreme Court Building or grounds”)
(quoting 40 U.S.C. § 6134).

     Here, by contrast, the statutory definitions show that, other
than the ability to improve fuel economy, Congress thought of
passenger vehicles, non-passenger vehicles, work trucks, and
medium-or heavy-duty commercial vehicles as each capturing
different types of vehicles. For example, the definitions of
“automobile” and “non-passenger automobile” both exclude
“work truck[s].” 49 U.S.C. § 32901(a)(3), (a)(3)(C), (a)(17).
The statutory definition of automobile expressly requires that
it be “propelled by fuel,” while the definitions of “work truck”
and of “commercial medium- and heavy-duty on-highway
vehicle” do not. 49 U.S.C. § 32901(a)(3), (a)(7), (a)(19); cf.
Salinas v. United States R.R. Retirement Board, 141 S. Ct. 691,
698 (2021) (Absence of limiting language is telling because
“[w]here Congress includes particular language in one section
of a statute but omits it in another section of the same Act, it is
generally presumed that Congress acts intentionally and
purposely in the disparate inclusion or exclusion”) (internal
quotation marks and citation omitted).
                              19
    Even more to the point, unlike in the noscitur cases cited
above, the Energy Independence Act applies a different fuel-
economy rule to work trucks and medium- and heavy-duty
vehicles than it does to automobiles. For work trucks and
medium- and heavy-duty vehicles—but not for automobiles—
Congress tasked the Secretary of Transportation (NHTSA)
with creating a comprehensive fuel efficiency program, of
which fuel economy standards are just a part. See 49 U.S.C.
§ 32902(b)(1)(C), (k). So for work trucks and medium- and
heavy-duty vehicles, Congress expressly undertook a broader
regulatory approach that considered more than just the
consumption of fuel per mile.

     More specifically, Congress instructed the Secretary of
Transportation, in consultation with the Department of Energy
and the EPA, to determine “the appropriate metric for
measuring and expressing commercial medium- and heavy-
duty on-highway vehicle and work truck fuel efficiency
performance[.]” 49 U.S.C. § 32902(k)(1)(B). Congress relied
on the standard “fuel economy” metric of miles-per-gallon for
automobiles, see id. § 32902(b)(2), but instructed the agencies
to think differently about work trucks and medium- or heavy-
duty on-highway vehicles. And NHTSA did. In the final rule,
NHTSA chose a standard for box trailers not in miles-per-
gallon but in gallons-per-1,000-ton-mile. See Phase Two Rule,
81 Fed. Reg. at 73,647–73,648; cf. Gasoline Vehicles: Learn
More About the Label, U.S. DEP’T OF ENERGY,
https://www.fueleconomy.gov/feg/label/learn-more-gasoline-
label.shtml (last accessed Nov. 4, 2021) (“[I]t may be more
meaningful to express fuel efficiency in terms of consumption
(e.g., gallons per mile * * *) rather than in terms of economy
(miles per gallon).”).

    By applying this broader approach and different fuel-
economy metric specifically to work trucks and highway
                               20
vehicles, the statute categorized them as horses of a different
color from “passenger automobiles” and “non-passenger
automobiles.” So the noscitur a sociis tool does not fit here.

    Fourth, the majority opinion declares it a “nail in
[NHTSA’s] coffin” that the definition of “motor vehicle” as “a
vehicle driven or drawn by mechanical power,” see 49 U.S.C.
§ 32101(7), does not apply to Section 32902 of the Energy
Independence Act. Majority Op. 17–18.

     Of course that definition is not statutorily made applicable
to Section 32902. The whole reason we are here is because
there is no governing definition of “vehicle” in Section 32902.

    More to the point, the relevant phrase in Section 32902 is
not “motor vehicle,” but “medium-duty or heavy-duty on-
highway vehicles,” 49 U.S.C. § 32902(b)(1)(C). So the only
question before this court is whether anything in the text of
Sections 32902 or 32901, or even in Section 32101, precludes
NHTSA from reading the undefined term “vehicle” in the
phrase “medium-duty or heavy-duty on-highway vehicles” in
the same way that other parts of Title 49 expressly read
“vehicle”—as including trailers.

     There is not. Section 32101’s introductory clause limiting
the required application of its “motor vehicle” definition
simply means that definitional symmetry was not mandated; it
does not mean that adopting a similar meaning of “vehicle” was
prohibited. See Fisher v. Pension Benefit Guar. Corp., 994
F.3d 664, 671 (D.C. Cir. 2021) (When considering statutes
administered by agencies, silence “may signal permission
rather than proscription”) (internal quotation marks omitted)
(quoting Catawba County v. EPA, 571 F.3d 20, 36 (D.C. Cir.
2009) (per curiam)); Van Hollen, Jr. v. Federal Election
Comm’n, 811 F.3d 486, 493–494 (D.C. Cir. 2016) (“[A]
congressional mandate in one section and silence in another
                                21
often suggests not a prohibition but simply a decision not to
mandate any solution in the second context, i.e., to leave the
question to agency discretion.”) (internal quotation marks
omitted) (quoting Catawba County, 571 F.3d at 36).4

     Also, Section 32101 is not the only place that the trailer-
including definition of “motor vehicle” appears. It also appears
in Section 30102 of Title 49, which governs Chapter 301 on
Motor Vehicle Safety. Congress’s pattern of usage, then,
textually supports NHTSA’s judgment in this case; it certainly
does not textually foreclose it. “[T]hat Congress spoke in one
place but remained silent in another, as it did here, ‘rarely if
ever’ suffices for the ‘direct answer’ that Chevron step one
requires.” Catawba County, 571 F.3d at 36 (quoting Cheney
R.R. Co. v. ICC, 902 F.2d 66, 69 (D.C. Cir. 1990)).

     The majority opinion responds that it makes sense to
define motor vehicle as encompassing trailers in Chapter 301,
which addresses Motor Vehicle Safety, because trailers have
safety features such as lights, turn signals, and reflectors.
Majority Op. 18 n.14. But as the regulations before us show,
trailers also have features that significantly affect fuel economy
and efficiency—the concerns of Chapter 329.5

    4
      Chapter 329 does separately use the phrase “motor vehicle,”
see 49 U.S.C. § 32902(d)(3), (f); see also id. § 32913 (mentioning
“the motor vehicle industry”), so those are the provisions for which
the exclusionary phrase that the majority opinion seizes upon would
seem to have actual relevance.
    5
         The majority opinion’s premise is that Congress used the
“motor vehicle” definition only in chapters that were relevant to
trailers. Not so. The same definition also applies, for example, to
Chapter 327, which prohibits tampering with odometers—a feature
                                22
     Fifth, the majority opinion argues that by defining
“commercial medium- and heavy-duty on-highway vehicle” in
terms of the vehicle’s “gross vehicle weight rating[,]” the
statute excludes trailers. Majority Op. 18 n.15 (quoting 49
U.S.C. § 32901(a)(7)). That is so, according to the majority
opinion, because while NHTSA regulations define gross
vehicle weight rating to mean “the maximum load that can be
carried by a vehicle, including the weight of the vehicle
itself[,]” the regulations also define a similar metric for tractor-
trailers: gross combined weight rating. Phase Two Rule, 81
Fed. Reg. at 73,845 n.26 (explaining that gross combined
weight rating “describes the maximum load that the vehicle can
haul, including the weight of a loaded trailer and the vehicle
itself”). The majority sees Congress’s choice to define the
vehicles at issue here in terms of the more general weight rating
category as excluding trailers.

    That is a non sequitur. For one thing, by using the more
general weight category, Congress swept in more vehicles, not
fewer. If Congress had defined “commercial medium- and
heavy-duty on-highway vehicle[s]” in reference to gross
combined weight rating, it would have limited that term just to
semitrailers. Congress plainly covered more medium- and
heavy-duty vehicles than semitrailers.

    For another thing, as the regulatory definition of gross
combination weight rating makes clear, trailers can also have
gross vehicle weight ratings. See 49 C.F.R. § 383.5 (defining
gross combination weight rating in terms of, inter alia, “[t]he
sum of the gross vehicle weight ratings * * * or the gross
vehicle weights * * * of the power unit and the towed unit(s)”)

that the majority opinion does not claim trailers themselves have.
See 49 U.S.C. § 32701.
                                23
(emphasis added).6 It is no surprise, then, that other regulations
apply to certain trailers based on their gross vehicle weight
rating. See, e.g., 49 C.F.R. § 571.224 (establishing certain
requirements for “trailers and semitrailers with a gross vehicle
weight rating * * * of 4,536 kg or more”); see also NHTSA,
THE EFFECTIVENESS OF UNDERRIDE GUARDS FOR HEAVY
TRAILERS (2010), at ii (explaining related NHTSA mandates
for “all trailers with [gross vehicle weight ratings] of 10,000
pounds or greater”). So the majority opinion’s focus on how
vehicle weight is calculated at the very least underscores the
statute’s ambiguity, and does much to reinforce NHTSA’s
conclusion.

     Lastly, the majority opinion resorts to the slippery slope.
It says that trailers cannot be vehicles because tractors are
already vehicles before the trailers are hooked on. Otherwise,
the majority opinion worries, the combination of a truck and
anything that affects its fuel economy, including heavy
suitcases or bike racks, would qualify. Majority Op. 19, 19
n.16; see also Association Br. 39.

      That tack might work if tractor-suitcases or tractor-bike
racks (i) could even conceivably be thought of as identifiable
“commercial medium- [or] heavy-duty on-highway” vehicles
in a way distinct from the tractor itself, 49 U.S.C.
§ 32902(b)(1)(C); (ii) have long been statutorily defined as
vehicles, including in the relevant agency’s organic statute; and
(iii) fit naturally within dictionary definitions and the common
understanding of a distinct on-highway vehicle. It was
textually permissible for the agency to determine that on-

    6
      The regulations double down on trailers’ status as vehicles by
providing that the gross combination weight rating of a truck “will
not be used to define a commercial motor vehicle when the [truck] is
not towing another vehicle.” 49 C.F.R. § 383.5 (emphasis added).
                                   24
highway tractor-trailers are themselves vehicles. That does not
make tractor-suitcases or tractor-bike racks distinct types of
vehicles, and thereby give NHTSA the power to regulate
suitcases or bike racks. There is neither slope nor slip.

     In a similar vein the majority opinion argues that on-
highway vehicles must be designed for use on a highway,
because otherwise NHTSA could regulate vehicles “like
bulldozers and excavators[.]” Majority Op. 15 n.11. But if
Congress wanted to require that regulated “on-highway
vehicle[s]” be designed for use on a highway, it could have said
as much, just as it did elsewhere in the same subsection. See
49 U.S.C. § 32901(a)(3) (“automobile” means, inter alia, “a 4-
wheeled vehicle that is propelled by fuel, or by alternative fuel,
manufactured primarily for use on public streets, roads, and
highways”) (emphasis added); Maine Cmty. Health Options v.
United States, 140 S. Ct. 1308, 1323 (2020) (Courts “generally
presume[] that when Congress includes particular language in
one section of a statute but omits it in another, Congress
intended a difference in meaning”) (internal quotation marks
and citations omitted).

     Anyhow, commercial trailers are designed specifically for
highway transportation use.7 It is, after all, commercial trailers
that transport bulldozers down highways to their off-highway
sites.

     7
       That does not mean NHTSA can prescribe fuel economy
standards for just any vehicle that happens on rare occasion to wind
up briefly on a highway. See Wisconsin Dep’t of Revenue v. William
Wrigley, Jr., Co., 505 U.S. 214, 231 (1992) (“[T]he venerable maxim
de minimis non curat lex (‘the law cares not for trifles’) is part of the
established background of legal principles against which all
enactments are adopted * * * .”).
                               25
                           *****

      At the end of the day, the clear prohibition on trailers
falling within NHTSA’s regulatory authority that the majority
opinion tries to wring out of statutory silence is illusory.

                              III

                               A

     Because nothing in the statutory text or structure precludes
NHTSA’s interpretation of “vehicles” from including the
trailer portion of tractor-trailers that routinely travel the
highways, the last question under Chevron is whether
NHTSA’s reading is reasonable. See 467 U.S. at 843–844; City
of Arlington, 569 U.S. at 296; see also Dada v. Mukasey, 554
U.S. 1, 29 n.1 (2008) (Scalia, J., dissenting). For many of the
reasons already discussed regarding the statutory text,
established definitions, the nature of on-highway trailers, and
their considerable impact on the fuel economy and efficiency
of tractor-trailers, NHTSA’s interpretation passes muster.

     To start, NHTSA’s interpretation is consistent with the text
of the Energy Independence Act. As noted before, including
these on-highway trailers—which take the form of tractor-
trailers (or, in common parlance, semitrucks)—in the definition
of vehicle is consistent with statutory, dictionary, and common
usage. See pages 9–25, supra.

     NHTSA’s interpretation also maps onto the statute’s
purpose. The central purpose of the Energy Independence Act
in general, and Section 32902 in particular, is to increase the
fuel efficiency of the vehicles and conveyances that travel the
Nation’s highways. See Pub. L. No. 110-140, § 102(a)(2), 121
Stat. at 1498; id. (“Title I—Energy Security Through Improved
Vehicle Fuel Economy”). Section 32902(k) separately requires
                                26
the Secretary of Transportation to develop a fuel efficiency
program for medium- and heavy-duty vehicles “designed to
achieve the maximum feasible improvement” in fuel efficiency
for those vehicles. 49 U.S.C. § 32902(k)(2) (emphasis added).

     Including commercial on-highway trailers within the
scope of medium- and heavy-duty vehicles contributes
considerably to that goal, while omitting them would
materially impede its achievement. Large tractor-trailers
account for 60% of the fuel consumption of heavy-duty
vehicles, and the trailer “contribute[s] substantially” to the fuel
consumption of the tractor-trailer as a whole. Phase Two Rule,
81 Fed. Reg. at 73,639. The National Academy of Sciences
estimated that “a given percentage reduction” in fuel
consumption for tractor-trailers “will save more fuel than a
matching percent improvement in any other vehicle
category[,]” and that “about half of the total possible fuel
savings” in all medium- and heavy-duty vehicles comes from
tractor-trailers. J.A. 306. Notably, one-third of that potential
savings comes from regulating the trailer’s design and
equipment. Phase Two Rule, 81 Fed. Reg. at 73,516 n.89. So
regulating the trailer portion “present[s] an important
opportunity” for fuel efficiency improvements. J.A. 292 (2010
National Academy of Sciences study).

    For all of those reasons, this court should hold that
NHTSA acted reasonably and within its statutory authority in
regulating trailers.

                                B

     The Association (but not the majority opinion) argues that
deference to NHTSA is inapplicable because, in August 2017,
NHTSA and the EPA wrote to the Association indicating their
intent to reconsider the trailer provisions through new notice-
                               27
and-comment rulemaking.         See Association Br. 48.      The
Association is incorrect.

     First, nothing ever came of the agency’s plans to revisit the
topic, and, in December 2019, NHTSA did not object to this
case going forward. Our duty is to decide the case before us,
not some case that might have been. Cf. Hüls America Inc. v.
Browner, 83 F.3d 445, 450 n.6 (D.C. Cir. 1996) (regulation
“merit[ed] traditional Chevron deference” where the agency
“ha[d] never adopted any different interpretation” of the
statutory provision at issue, although the agency had
“suggested it could reconsider” the regulation).

     Second, NHTSA’s regulations were adopted through
notice-and-comment rulemaking and have the force of law.
That status is not lost just because an agency writes in a letter
about its intent to reconsider the regulation. Under the
Administrative Procedure Act, an agency can only “amend or
repeal” a regulation “us[ing] the same procedures * * * as [it]
used to issue the rule in the first instance.” Perez v. Mortgage
Bankers Ass’n, 575 U.S. 92, 101 (2015).

      The Association relies on Global Tel*Link v. FCC, 866
F.3d 397 (D.C. Cir. 2017). That case has no application here.
In that case, the Federal Communications Commission issued
an order that used industry-wide averages to set caps on the
interstate and intrastate rates and fees that payphone providers
could charge for phone calls made by inmates in correctional
facilities. Id. at 405. But after the Commission “experienced
‘significant changes in [its] composition,’” it formally advised
the court that it “was ‘abandoning * * * the contention[s] * * *
[both] that the Commission had authority to cap intrastate
rates” and “‘that the Commission [had] lawfully considered
industry-wide averages in setting the rate caps.’” Id. at 406
(first alteration and omissions in original; citations omitted).
                               28
     In deciding the case, this court emphasized the “oddity” of
determining whether the parts of the order addressing intrastate
rates and industry-wide costs warranted Chevron deference
“when the agency ha[d] abandoned those positions.” Global
Tel*Link, 866 F.3d at 407–408. As a result, the court assessed
the “best reading” of the statute at issue, id. at 408 (quoting
Miller v. Clinton, 687 F.3d 1332, 1342 (D.C. Cir. 2012)), and
vacated the order’s provisions related to caps on intrastate rates
and the use of industry-averaged cost data, id. at 416.

      This case is quite different. NHTSA has not “abandoned”
its trailer regulations. It has continued to defend them actively
in this court and to claim Chevron deference. See Agencies Br.
14, 25–27.

                                C

    Lastly, the Association argues that, if the EPA’s
regulations are vacated—as the court today orders—then
NHTSA’s fuel efficiency regulations must also be vacated. See
Association Br. 27. But the agencies intended the two sets of
regulations to stand on their own, and any overlap in the
compliance process could be resolved on remand.

The Association views the question as one of severability. See
Association Br. 27–36. That seems an ill fit. While
severability principles apply within a single statute, see, e.g.,
Free Enterprise Fund v. Public Co. Acct. Oversight Board, 561
U.S. 477, 508 (2010), or regulation, see, e.g., K Mart Corp. v.
Cartier, Inc., 486 U.S. 281, 294 (1988), that is not the practice
for separate agency regulations issued by distinct agencies
under different statutory schemes. Nevertheless, because both
parties encouraged the court to look at the issue through the
lens of severability, see Association Br. 27–36; Agencies Br.
40–52; Oral Arg. Tr. 36:23–25, I will do so for argument’s
sake.
                               29
     Under a severability analysis, this court asks (i) whether
the agencies intended the regulations to be severable, and
(ii) whether the remaining regulations could function without
the stricken portion. See Verizon v. FCC, 740 F.3d 623, 659
(D.C. Cir. 2014) (citing MD/DC/DE Broadcasters Ass’n v.
FCC, 236 F.3d 13, 22 (D.C. Cir. 2001)). Courts presume that
regulations are severable, see Community for Creative Non-
Violence v. Turner, 893 F.2d 1387, 1394 (D.C. Cir. 1990), and
that presumption holds here.

     The driving factor in severability analysis is agency intent.
See Sierra Club v. FERC, 867 F.3d 1357, 1366 (D.C. Cir. 2017)
(“The question whether an agency order is severable turns on
agency intent.”). Courts should only decline to sever
regulations if “there is substantial doubt that the agency would
have adopted the same disposition regarding the unchallenged
portion if the challenged portion were subtracted[.]” Sierra
Club, 867 F.3d at 1366 (quoting Epsilon Elecs., Inc. v. United
States Dep’t of Treasury, 857 F.3d 913, 929 (D.C. Cir. 2017)).

     I need look no further than the agencies’ own words to
dispel any doubt. In response to comments, the agencies said
explicitly that “the NHTSA fuel consumption standards are
independent of the EPA greenhouse gas standards and vice
versa,” and that the agencies “regard each of these standards as
legally severable.” J.A. 421.

     That makes sense. The agencies were each acting pursuant
to different sources of authority and distinct congressional
directives, J.A. 421; Phase Two Rule, 81 Fed. Reg. at 73,512,
73,521, and they promulgated their own sets of regulations that
were published in separate parts of the Code of Federal
Regulations, see 40 C.F.R. part 1037; 49 C.F.R. part 535.
Although the fuel efficiency and emission standards
                               30
accomplish similar outcomes, the agencies were clear during
the rulemaking process that one could stand without the other.

     To be sure, the agencies “worked in close partnership” to
“create a single, effective set of national standards[.]” Phase
Two Rule, 81 Fed. Reg. at 73,479. But that does not mean that
neither one would have adopted their own standards—pursuant
to their own statutory authority and directives—without the
other’s. Their coordination was not sua sponte, but pursuant to
both statutory mandate, see 49 U.S.C. § 32902(b)(1), (k), and
White House directive; see also Phase Two Rule, 81 Fed. Reg.
at 73,479; THE WHITE HOUSE, IMPROVING THE FUEL
EFFICIENCY OF AMERICAN TRUCKS. Importantly, the goal of
that coordination was not to create standards inextricably
dependent on one another, but to “avoid unnecessarily
duplicative testing and compliance burdens” for
manufacturers. Phase Two Rule, 81 Fed. Reg. at 73,487.

     Nevertheless, the Association insists that the rule’s lack of
a severability clause is fatal. See Association Br. 29. But the
law is settled that “[t]he absence of a severability clause cuts
neither against nor in favor of severance; the presumption of
severability remains intact.”       Association of American
Railroads v. United States Dep’t of Transp., 896 F.3d 539, 549
(D.C. Cir. 2018).

    NHTSA’s regulations also can function sensibly without
the EPA’s because NHTSA’s fuel efficiency limits do not
substantively depend in any way on the EPA’s emission limits.
Compare 49 C.F.R. § 535.5(e) with 40 C.F.R. § 1037.107; see
Delta Constr., 783 F.3d at 1296–1297 (“[N]othing in
NHTSA’s standards even suggests that they are dependent on
EPA’s standards * * * .”).

    Having said that, the regulations currently have an
overlapping compliance process. In the rulemaking, NHTSA
                              31
and the EPA attempted to reduce compliance burdens and to
simplify the process for trailer manufacturers. See Phase Two
Rule, 81 Fed. Reg. at 73,487, 73,495, 73,664; Phase One
Proposed Rule, 75 Fed. Reg. at 74,159. To that end, the
regulations are designed so that compliance with NHTSA’s
regulations also constitutes compliance with the EPA’s
regulations. See Phase Two Rule, 81 Fed. Reg. at 73,640.

     The greatest points of overlap include references by the
NHTSA regulations to the EPA regulations concerning (i) the
compliance equation, see 49 C.F.R. § 535.6(e)(3); (ii) the
certificate of conformity, see id. §§ 535.9(a)(1), 535.10(a)(5);
and (iii) the submittal and verification of compliance data, see
id. §§ 535.8(a)(2), 535.10(c)(4).

     Yet NHTSA’s regulations could function independently
despite this overlap. Trailer manufacturers are already
instructed to send compliance information to NHTSA if the
EPA’s database is unavailable. 49 C.F.R. § 535.8(a)(6). And
NHTSA assesses the manufacturers’ compliance with its fuel
efficiency standards each year. Id. § 535.9(a). NHTSA can
also seek more information and conduct audits, field testing,
and verification testing as it sees fit. Id. §§ 535.8(j),
535.9(a)(1); see also Delta Constr., 783 F.3d at 1297
(concluding during the Phase One regulatory process that the
NHTSA and EPA regulations could be “bifurcated”). Any
lingering uncertainty could be addressed on remand. As for the
content of its regulations, NHTSA would need to clean up the
cross-references in its own regulations to the EPA’s regulations
and explain how the compliance equation, certificate of
conformity process, and reporting and verification of data
would work without the EPA regulations in place.

                           *****
                               32
     NHTSA’s inclusion of commercial trailers in its fuel
economy and efficiency regulations was a reasonable
interpretive judgment that falls squarely within its statutory
delegation. It is consistent with the Energy Independence Act’s
statutory text, structure, context, and purpose. It also comports
with statutory and dictionary definitions of the term “vehicle,”
as well as common usage. For all of the majority opinion’s
insistence that Congress’s silence can be turned into an express
prohibition on the inclusion of trailers, there is simply no there
there.

    For all of these reasons, I respectfully dissent.