Each state legislature has passed regulations that set the legal vehicle sizes and weights for their particular jurisdiction. The limitations cover the legal limits allowed for width, height, length, and weight for various vehicles and/or trailer configurations. The regulations also address the limitations for vehicles traveling on interstate or designated highways within the state and/or other state or supplemental highways.

The Federal Highway Administration (FHWA) is responsible for setting size limits that must be allowed on the National Network and weight limits for the Interstate Highway System and Defense highways.

The federal bridge formula is used for roads that are designated as being part of the National Network, and while most states use the federal bridge formula for all roads in their state, some have a separate state formula they use for other state or county roads.

• Requirements for CMV size and weight limits are essential for maintaining the condition of highway systems and keeping their users safe.
• The STAA established minimum weights that states must allow for motor vehicles operating on the Interstate Highway System.

Commercial motor vehicle (CMV) size and weight limit requirements help preserve the physical condition of the highway transportation system and the safety of its users. A vehicle’s weight plays a major role in damage to roadways and related infrastructure.

Not every roadway or bridge is created equal. Some are designed to handle heavy vehicles while others, such as secondary roadways, are not.

Are there seasonal restrictions?

As the ground becomes saturated from the spring thaw, roads need time to adjust to the changing seasons. Seasonal weight restrictions may change from year to year based on the weather. Typically, effective dates are set based on:

• Frost depths measured along state highways,
• Observed road conditions, and
• Weather forecasts.

Once a state or province feels its roads have regained stability, it will remove the weight restrictions. This occurs when the frost depth has decreased enough to allow moisture to escape.

What must a carrier do to operate oversize or overweight?

Besides having regulations pertaining to legal limitations on vehicle width, height, length, or weight, each state has regulations on handling oversize or overweight vehicle movements. These regulations indicate when an overdimensional or overweight permit is required; the types of permits available and their fees; permit insurance requirements; maximum permitted vehicle limitations; sign, flag, light, and escort requirements; and permit travel restrictions.

See the state limit topics for additional information.

Where are the federal limits found?

The Federal Highway Administration (FHWA) is responsible for setting size limits that must be allowed on the National Network and weight limits for the Interstate Highway System.

Federal definitions, limits, and exceptions are found in 23 CFR Part 658.

What is the National Network?

The National Network is a list of interstate highways and state-designated highways, or portions of highways, for which federal size and weight requirements are enforced. Appendix A of Part 658 includes a state-by-state list of designated highways.

Appendix A to Part 658 — National network — Federally designated routes

Also see the state limit topics for additional information.

What is the STAA?

The Surface Transportation Assistance Act of 1982 (STAA) addressed a situation where states restricted weights by setting lower weights than allowed by neighboring states. The STAA set minimums that states must allow on the Interstate Highway System. This act also expanded federal regulation of commercial vehicle size by requiring the FHWA to designate a National Network of highways, including interstates, where states must allow commercial vehicles of certain dimensions and configurations to operate.

States were prohibited from denying reasonable access for these vehicle combinations to terminals; facilities for food, fuel, repairs, and rest; and points of loading and unloading for household good carriers.

• With a gross vehicle weight of more than 80,000 pounds, an LCV must be operated by a specially trained driver whose CDL includes a “double/triple trailer” endorsement.

A “longer combination vehicle” (LCV) is any combination of a truck-tractor and two or more trailers or semitrailers that operate on specifically allowed highways at a gross vehicle weight greater than 80,000 pounds.

What are the requirements regarding LCVs?

Because of LCV length and special characteristics, drivers require special training as well as a “double/triple trailer” endorsement on their commercial driver’s licenses (CDLs). There are also state-based permitting, operational, and equipment requirements drivers and carriers must follow.

Regulatory citations:

• 49 CFR 380.301 — General requirements
• 49 CFR Part 380 — Special training requirements

Employers are prohibited from allowing drivers to operate LCVs unless those drivers can produce either:

• An LCV Driver-Training Certificate as evidence of successful completion of an LCV driver training course; or
• An LCV Driver-Training Certificate of Grandfathering, showing that the driver was exempt from training requirements based on experience, when training was first required in 2004.

LCV driver training was required on and after June 1, 2004. The LCV Driver-Training Certificate of Grandfathering could only be issued during the year following that compliance deadline.

Motor carriers must retain all LCV training certificates or certificates of grandfathering in their employees’ Driver Qualification files.

Carriers are allowed to conduct the required training, provided the training program meets requirements in Part 380, Special Training Requirements, and any state or local requirements. Training consists of classroom training and behind-the-wheel practice. All LCV driver-instructors must be qualified according to 380.301, and proof of their qualifications must be retained in the driver-instructor’s Driver Qualification file.

A list of allowable LCV combinations can be found in Appendix C to 23 CFR Part 658.

Also see the state limit topics for additional information.

• Several factors dictate the vehicle lengths permitted on highways, with 49 of 50 states allowing 53-foot trailers to be operated.
• Kingpin to rear axle (KPRA) distance limits are enforced by many states to control the wheelbase and turning radius of combination vehicles.

The “length allowed” varies by state, by route, by the number of trailing units, whether the unit(s) is a trailer or a semitrailer, whether it is pulled by a straight truck or a truck-tractor, and by the kingpin-to-rear-axle (KPRA) length. For these reasons, the answer is dependent on the variables.

Federal size and weight limits are not determined by the Federal Motor Carrier Safety Administration (FMCSA), but by the Federal Highway Administration (FHWA) in 23 CFR, Part 658. What states must allow is found in Section 13.

The Surface Transportation Assistance Act (STAA) pre-empted states from enforcing laws and regulations that would impose on the National Network a trailer length limit of less than 48 feet for truck tractor-semitrailer combinations, less than 28 feet for truck tractor semitrailer-trailer combinations (doubles), or an overall length limit of less than 45 feet for buses.

The act grandfathered certain trailer dimensions in actual and lawful use in 1982 and authorized the FHWA to adopt regulations to accommodate specialized equipment, such as automobile transporters.

For trailers more than 48 feet long, the STAA grandfathered any “trailers or semitrailers of such dimensions as those that were in actual and lawful use.” Almost half of the states already allowed at least 53-foot trailers, but the other states remained somewhere less than 53 feet. Since that time, all states but Hawaii allow 53-foot trailers at least on some routes.

What is excluded from the federal length limit?

For most vehicles, length is measured from bumper to bumper, plus any part of a load that is overhanging the vehicle. However, many non-property-carrying devices are not included in the length calculation.

These exclusions are found in:

• 23 CFR 658.16
• Appendix D of Part 658

The FHWA allows overhang on some “specialized equipment” such as automobile and boat transporters. For automobile and boat transporters, the FHWA’s regulations allow a 3-foot front and a 4-foot rear overhang on the National Network. These overhangs are not included when considering the vehicle’s overall length.

The Fixing America’s Surface Transportation Act (FAST Act), provided that a state could not enforce a regulation that imposed a vehicle length limitation of less than 80 feet on a stinger-steered automobile transporter with a front overhang of less than 4 feet and a rear overhang of less than 6 feet.

What are the KPRA requirements?

All states other than Hawaii allow 53-foot trailers at least on some routes; however, many states have a KPRA distance limit when trailers more than 48 feet long are used. States regulate this distance to help control the wheelbase and turning radius of the combination.

Kingpin to rear axle matrix

How must length overhangs be identified?

The FMCSA permits the allowed size and weight, but has regulations regarding how overhangs must be flagged or illuminated.

Rear overhangs of more than 4 feet

• When headlamps are required:
  • One at each side of the overhang — a red side marker light visible from the side placed to indicate maximum overhang.
  • On the rear of the overhang — two red lights visible from the rear, one at each side. In addition to a pair of red reflectors also visible from the rear, one at each side, placed to indicate maximum width.

• When headlamps are not required:
  • If the overhang is 2 feet or less wide, a single 18-inch red or orange fluorescent warning flag placed at the extreme rear.
  • If the overhang is more than 2 feet wide, two 18-inch red or orange fluorescent warning flags placed at extreme rear indicating maximum width.

The FMCSA makes no assurance in its rules that these overhangs are allowed on any given roadway, only how they must be marked if allowed.

• Vehicles operating on the National Network must comply with a width limit of 102 inches.
• FMCSA regulations specify precisely how width overhangs are to be flagged or illuminated.

The “width allowed” varies by state and the route within a state.

Federal size and weight limits are not determined by the Federal Motor Carrier Safety Administration (FMCSA), but by the Federal Highway Administration (FHWA) in 23 CFR, Part 658. What states must allow is found in Section 15.

The Surface Transportation Assistance Act (STAA) established a maximum/minimum width limit of 102 inches, or 8 feet, 6 inches, for the National Network.

What is excluded from the federal width limit?

For most vehicles, width is measured from a point on one side or end of a commercial motor vehicle (CMV) to the same point on the opposite side or end of the vehicle, plus any part of a load that is overhanging the vehicle. However, many non-property-carrying devices are not included in the length calculation.

These exclusions are found in:

• 23 CFR 658.16
• Appendix D of Part 658

How must width overhangs be identified?

The FMCSA permits the allowed size and weight, but has regulations regarding how overhangs must be flagged or illuminated.

Side overhangs of more than 4 inches

• When headlamps are required:
  • On the front edge of the overhang — an amber light visible from both the front and side.
  • On the outermost extremity’s rear edge of the overhang — a red light visible from both the rear and side.
  • If the overhang’s length is 3 feet or less, the overhang can be marked with a single amber light visible from the front, both sides, and rear. However, if the overhang is located near or at the rear of the vehicle, a red light visible from the front, side, and rear is used.

• When headlamps are not required, the maximum width is marked with an 18-inch red or orange fluorescent warning flag.

• The two main factors limiting vehicle weight are the “rated” gross value and the “allowed” gross value.
• Maximum gross weight allowed is seldom achieved because of the direct influence of other considerations.

The Federal Highway Administration (FHWA) sets weights that states must allow on interstate highways, including the maximum gross vehicle weight at 80,000 pounds (subject to the Federal Bridge Law), the maximum gross weight upon any one axle at 20,000 pounds, and the maximum gross weight on tandem axles at 34,000 pounds.

The FHWA also covers the relationship of tire width and axle ratings: “States may not limit tire loads to less than 500 pounds per inch of tire or tread width, except that such limits may not be applied to tires on the steering axle. States may not limit steering axle weights to less than 20,000 pounds or the axle rating established by the manufacturer, whichever is lower.” Adding additional uncertainty with the “mays” and “may nots” are grandfathering dates and other exclusions.

There are two primary limiters of weight: the first is the “rated” gross value, and the second is the “allowed” gross value. These two are not just applied to the vehicle(s); they are also applied to axle groups, axles, and tires.

Usually, the maximum gross weight allowed will be the state’s maximum allowed gross. Carriers must be careful, though, because unless a load is perfectly balanced across the axles and groups, the maximum is rarely met due to one of the other limits, or strikes, being reached first. These limits include:

• The allowed axle weight(s) based on the state or local statutes,
• The allowed axle group weight based on the bridge law (each combination of axles and distance is determined separately),
• The allowed weight per tire or inch of tire width,
• Each tire’s rating, and
• The registered vehicle weight.

• Deciding whether or not a driver should enter a scale comes down to heeding this adage: “Better to be safe than sorry.”
• Providers of vehicle bypass solutions can lighten the inspection load for carriers and drivers, as well as enforcement personnel.

There is no single answer to determine at what weight or what type of vehicle must enter a scale. The answer varies from state to state, and even from scale to scale within a state. There are, of course, federal size and weight limits for interstate routes, but also separate sets of rules for state and local roads. Enforcement for all types of highways generally falls on the state’s Department of Transportation (DOT) officers.

If a scale is not clearly marked indicating the vehicles that must enter, then to answer the question of “Should the driver pull in?” is that it’s far better to pull in and be waived or green-lighted through than to be chased down.

States have a secondary “line of defense” on the flanks or perimeter so vehicles “taking the scenic route” do not have a free pass to evade enforcement. This secondary effort has multiple assets, but portable scales play a large role. States are required by 23 CFR 675.9 to develop a plan for the maintenance of an effective enforcement process. The plan needs to describe the procedures, resources, and facilities that the state intends to devote to enforcement of its vehicle size and weight laws.

For a plan to be accepted by the Federal Highway Administration (FHWA), the facilities and resources portion of the plan must utilize a combination of two devices to deter the “evasion of size and weight measurement.” Options for a state’s plan include fixed platform scales, portable wheel scales, semiportable or ramp scales, or weigh in motion (WIM) equipment.

WIM systems are used to pre-screen vehicles on the highway or that are entering a fixed scale. The systems can measure axle spacing, determine the weight of each axle, calculate the weight on a series of axles, and provide the gross weight. WIM systems are not used in the United States for direct enforcement as traffic cameras sometimes are. The WIM system identifies vehicles that should be selected for a closer look or brought to a fixed or portable scale.

Since stopping for inspections takes up valuable and finite time, most drivers and carriers would prefer to avoid as many such stops as possible. Drivers and carriers are not alone in preferring to steer clear of unnecessary weighing and inspection of vehicles. Enforcement personnel would also rather concentrate on vehicles that need to be inspected.

Several providers of vehicle bypass solutions can assist in focusing the available inspection bandwidth on likely candidates while allowing others to bypass the scale altogether. They work on similar principles — change the carrier or its vehicle approaching on the horizon from an unknown to a known. Safe carriers tend to operate safely as a matter of culture.

The Federal Motor Carrier Safety Administration (FMCSA) has an Inspection Selection System (ISS) used at roadside inspection stations to help inspectors identify and prioritize carriers for inspections. Carriers with a low score of 1 to 49 receive a “pass” recommendation in the ISS. The bypass systems communicate with the ISS to let inspectors know that the carrier for the vehicle approaching has a low ISS score.

Systems may send additional information to help the enforcement system make the right decision. There are circumstances, such as traveling under an oversize or overweight permit, that the driver would still need to pull in even if a system indicates the vehicle may be able to bypass the scale.

Also see the state limit topics for additional information regarding scale locations and vehicles that are required to enter.

• The greater the number of axles for any length of vehicle, the greater the gross weight permitted in the entire combination.
• Each state has established regulations that govern the handling of oversize or overweight vehicle movements.

The Federal Bridge Law was established in 1975 by the U.S. Congress. It is codified in 23 CFR 658.17. The bridge law was established to set a ratio of length to width for when a heavy vehicle crosses a bridge ― hence the name. It doesn’t take a mathematician to realize that weight over a small area will create more stress than the same weight placed over a greater area.

Since truck wheels provide a relatively small area of contact with a surface, an effective way to increase surface area is to add extra wheels. In the bridge law and formula, extra wheel surface is accomplished by adding axles. The more axles in any length, the more gross weight is allowed in the span.

The Formula:

W = 500 (LN / (N-1) + 12N + 36)

As in any formula, the letters stand for variables. There is only one unknown variable in the formula, which is represented by “W.” The “W” stands for “weight,” or the allowed gross weight on the axles that are being considered. “L” stands for “length” in feet between the outermost axles under consideration, and the “N” stands for the number of axles.

The bridge allowance is “maxed” out once the axle or total gross weight is reached, unless allowed additional weight by an overweight permit. When overweight permits are issued, many states still expect the axles to “scale out” to the bridge formula.

The bridge rule applies to any, and all, possible axle groups.

Can a vehicle exceed the legal limits?

Along with regulations pertaining to legal limitations on a vehicle’s width, height, length, or weight, each state has instituted regulations on handling oversize or overweight vehicle movements. These regulations indicate when an over dimensional permit is required; the type of permits available and their fees; permit insurance requirements; maximum permitted vehicle limitations; sign, flag, light, and escort requirements; and permit travel restrictions.

Also see the state limit topics for additional information.

Like most Canadian motor carrier regulations, the commercial vehicle size and weight limits are enforced at the province or territory level.

In order to create uniformity in vehicle dimensions and weights throughout Canada, the Canadian jurisdictions have agreed to a “Memorandum of Understanding” (MOU) on vehicle size and weights.

Created by the Council of Ministers of Transportation and Highway Safety, the MOU establishes standard height, width, length, and weight limits for eight common vehicle configurations. A task force within the Council, the Task Force on Vehicle Weights and Dimensions Policy, periodically meets to discuss possible areas for improvement and to decide on updates to the MOU, if warranted.

Find more information about Canada’s vehicle weights and dimensions requirements.