• 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.