Truck Trailer Manufacturers Association v. EPA

Washington, DC - Complex Civil Litigation lawyer argued the cause for petitioner.

The most widely recognized "tractor-trailer” combination

is what a layperson calls a semitruck. A trailer is the back

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 respondentsintervenors 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 RespondentIntervenors do not attempt to refute what we've already

explained: (a) a motor vehicle, as defined, must be selfpropelled; 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 selfpropelled, 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 heavyduty” and other times it says "commercial medium- and heavyduty.” 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 heavyduty 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 heavyduty 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

definitionssays: "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 tractortrailer 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 heavyduty 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 onhighway 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 selfpropelled, 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 onhighway 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 MEDIUMAND 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 heavyduty 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 onhighway 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 MEDIUMAND 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 "onhighway” 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 nonlawyers, 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-TrailerCorporation/35519 (last accessed Nov. 4, 2021); U.S. Vehicle-Miles,

BUREAU TRANSP. STAT., https://www.bts.gov/content/us-vehiclemiles (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-statisticalproducts-and-data/national-transportationstatistics/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 longestablished 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 statutorilydirected 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 tractortrailers—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

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NATIONAL ACADEMIES, TECHNOLOGIES AND APPROACHES TO

REDUCING THE FUEL CONSUMPTION OF MEDIUM- AND HEAVYDUTY 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: Tractortrailers "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 fueleconomy 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 heavyduty 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 heavyduty on-highway vehicles. And NHTSA did. In the final rule,

NHTSA chose a standard for box trailers not in miles-pergallon 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-gasolinelabel.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 fueleconomy 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 onhighway 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 trailerincluding 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 tractortrailers: 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 on6 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 onhighway 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 * * * .”).

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.