A common question I continue to get refers to trailer width requirements. This article appeared a couple of years ago in Tracks and I think it is worth printing again. 

There are many stories, myths, assumptions and controversy. I thought the best thing to do is bring the straight skinny directly from the source. Width is covered in 23 CFR PART 658 – TRUCK SIZE AND WEIGHT, ROUTE DESIGNATIONS – LENGTH, WIDTH AND WEIGHT LIMITATIONS.

The specific sections that apply to our discussion are sections 658.5 Definitions, 658.15 Width, 658.16 Exclusions from Length and Width, and 658 Appendix D. I have provided Section 658.15 Width in its entirety and the width portion of Section 658.16. The reason I include Section 658.5 Definitions is to address “Special Mobile Equipment” which is mentioned in the width section of the regulation. Finally, I have provided paragraph three of Appendix D to Part 658, which is referenced in Section 658.16 Exclusions from length and width determinations.  

Section 658.15 Width

(a) No State shall impose a width limitation of more or less
than 102 inches, or its approximate metric equivalent, 2.6
meters (102.36 inches) on a vehicleoperating on the National
Network, except for the State of Hawaii, which is allowed to
keep the State’s108-inch width maximum by virtue of section
416(a) of the STAA (Surface Transportation Assistance Act).

(b) The provisions of paragraph (a) of this section do not
apply to special mobile equipment as defined in Sec. 658.5.

(c) Notwithstanding the provisions of this section or any
other provision of law, a State may grant special use permits
to motor vehicles, including manufactured housing, that
exceed 102 inches in width.

Definition from Section 658.5 – Special mobile equipment
Every self-propelled vehicle not designed or used
primarily for the transportation of persons or property and
incidentally operated or moved over the highways,
including military equipment, farm equipment,
implements of husbandry, road construction or
maintenance machinery and emergency apparatus which
includes fire and police emergency equipment. This list is
partial and not exclusive of such other vehicles as may fall
within the general terms of this definition.

The above covers the basic width requirements on federal highways for commercial vehicles. The next section discusses exclusions, which I have followed with the relevant portion of Appendix D. The appendix is referred to in the exclusions section and lists specific items excluded from width calculation. I have omitted paragraphs one and two of the appendix, which provide length exclusions on the front and rear of the vehicle. The width portion is covered in paragraph three of Appendix D and is provided after the Section 658.16 in its entirety.

Sec. 658.16 Exclusions from length and width determinations

(a) Vehicle components not excluded by law or regulation
shall be included in the measurement of the length and width
of commercial motor vehicles.

(b) The following shall be excluded from either the measured
length or width of commercial motor vehicles, as applicable:
(1) Rear view mirrors, turn signal lamps, handholds for
cab entry/egress, splash and spray suppressant devices,
load induced tire bulge;
(2) All non-property-carrying devices, or components
thereof—
(i) At the front of a semitrailer or trailer, or
(ii) That do not extend more than three inches beyond
each side or the rear of the vehicle, or
(iii) That do not extend more than 24 inches beyond the
rear of the vehicle and are needed for loading or
unloading, or
(vi) Listed in appendix D to this part;

Appendix D to Part 658
Devices That Are Excluded From Measurement
of the Length or Width of a Commercial Motor Vehicle

The following devices are excluded from measurement of the length or width of a commercial motor vehicle, as long as they do not carry property and do not exceed the dimensional limitations included in §658.16. This list is not exhaustive.
1. Omitted – covers front of the vehicle
2. Omitted – covers rear of the vehicle
3. Devices excluded from width determination, not to exceed
three inches from the side of the vehicle including, but not
limited to, the following:
(a) Corner caps;
(b) Hazardous materials placards and holders;
(c) Lift pads for trailer on flatcar (piggyback) operation;
(d) Rain gutters;
(e) Rear and side door hinges and their protective
hardware;
(f) Side marker lamps;
(g) Structural reinforcement for side doors or intermodal
operation (limited to one inch from the side within the
three inch maximum extension);
(h) Tarping systems for open-top trailers;
(i) Movable devices to enclose the cargo area of flatbed
semitrailers or trailers, usually called tarping systems,
where no component part of the system extends more
than three inches from the sides or back of the vehicle
when the vehicle is in operation. This exclusion applies to
all component parts of tarping systems, including the
transverse structure at the front of the vehicle to which
the sliding walls and roof of the tarp mechanism are
attached, provided the structure is not also intended or
designed to comply with 49 CFR 393.106, which requires
a headerboard strong enough to prevent cargo from
penetrating or crushing the cab; the transverse structure
may be up to 108 inches wide if properly centered so that
neither side extends more than three inches beyond the
structural edge of the vehicle. Also excluded from
measurement are side rails running the length of the
vehicle and rear doors, provided the only function of
the latter, like that of the transverse structure at the front
of the vehicle, is to seal the cargo area and anchor the
sliding walls and roof. On the other hand, a headerboard
designed to comply with 49 CFR 393.106 is load bearing
and thus limited to 102 inches in width. However, the
“wings” designed to close the gap between such a
headerboard and the movable walls and roof of a tarping
system are width exclusive, provided they are add-on
pieces designed to bear only the load of the tarping system
itself and are not integral parts of the load-bearing
headerboard
structure;
(j) Tie-down
assembly on
platform
trailers;
(k) Wall
variation from
true flat; and
(l) Weevil pins and sockets on low-bed trailers.
As mentioned above, this is the regulatory requirements for commercial motor vehicles operated on the National Network of federal highways. Individual states regulate width within their states and on their local and state roads for non-commercial vehicles. State width requirements can be found in the NATM Guidelines for Minimum Manufacturing Practices in section B-1 State Towing Laws. While many states follow the federal requirements on the interstate for non-commercial vehicles, many limit width to 96 in. on their highways and roads. Only two states that I am aware have provisions for over 102 in., they are California and Hawaii.

I believe most trailers that have issues with width tend to be trailers used for commercial purposes and thus fall under the above requirements. Regardless, commercial or not, the burden of non-compliance lies with the operator and the operator (or customer) is responsible for the fines and inconvenience. So while it is legal to build an over width trailer (for special purposes), it is illegal for its operation without special permits. Special permits, or over-width permits, are controlled by the individual states and in some states, such as New Jersey, they only approve over-width of the cargo not the vehicle.

The concern I hear most often is with the design of the trailer to 103 or 104 in. and not falling under any exclusions stated above. Usually, it is the frame with oversized fenders (10 ¾ in. width fender) or the axle with wheel and tires that extends beyond the 102 in. These design issues ultimately place the burden on the consumer, who often is unaware the vehicle does not comply with standard width limitation. 
The skinny on width is to keep your trailer skinny and do not disappoint your customer.