The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation (DoT) and was created in 1966. It is responsible for the federal size and weight regulations for commercial motor vehicles Code of Federal Regulations (CFR), 23 CFR Part 658.
The first time regulations were enacted for the size of commercial motor vehicles was in 1956. At that time, the maximum width of a commercial vehicle allowed on the interstate highway system was 96 inches. The Surface Transportation Assistance Act (STAA) of 1982 increased the maximum width of commercial motor vehicles to 102 inches, excluding mirrors and certain safety devices. The only exception to this regulation is in the state of Hawaii where the maximum width is 108 inches. If an individual state wants to allow vehicles on their highway network that are greater than the maximum width, then the state must issue a special over-width permit.
The federal requirements for vehicle lengths are described as minimums that states can follow. The minimum for a semitrailer is 48 feet or the grandfathered limit for a specific state. For example, the grandfathered minimum length in Colorado is 57 feet and four inches, while in New Mexico the grandfathered minimum is 57 feet and 6 inches.
There is no federal vehicle height requirement for commercial motor vehicles, which allows states to set their own height restrictions. Most height limits range from 13 feet, 6 inches, found in Iowa and Massachusetts, to 14 feet in Missouri and California, with exceptions granted for lower clearance on particular roads.
The regulation that describes weight restrictions for commercial motor vehicles is found in 23 CFR Part 658.17. The maximum weight that states must enforce for vehicles on the interstate system is 80,000 pounds for the overall gross weight limit. The regulations also require that the maximum weight for a single axle is 20,000 pounds, and 34,000 pounds for a tandem axle vehicle.
A single axle vehicle can include a vehicle that has two axles but those axles cannot be more than 40 inches apart. If the axles on the vehicle are more than 40 inches apart and less than 96 inches apart, then the vehicle would be considered a tandem axle vehicle.
Each state can issue overweight permits, as it is not a federal responsibility. A state can issue a permit for a vehicle if the load cannot be divided into smaller loads, such as damaging the load or requiring more than eight hours to divide the load.
In 1975 the Bridge Formula was passed by Congress to calculate the limitations on the weight-to-length ratio of a vehicle crossing a bridge. When commercial motor vehicles became heavier in the 1970s, there was a concern that a great number of bridges along the interstate highway system would not stand up to the additional weight. The axle spacing is important to the bridge formula, as a vehicle towing another vehicle or trailer puts more strain on a bridge than a lone vehicle of the same weight. Federal law states that two or more consecutive axles may not exceed the weight calculated by the bridge formula even though single axles, tandem axles, and gross vehicle weights are within legal limits.
Although there are regulations on the weight of vehicles, carriers can send out overweight vehicles which can lead to damaged highways and additional maintenance costs. Research has suggested that the cost of overweight vehicles could range from $265 million to $1.1 billion each year in the U.S. Enforcement of weight restrictions relies heavily on the numerous weigh stations, but with limited hours of operation, the success rate of the weigh stations has been calculated at around one percent.
The federal regulations on the size and weight of commercial motor vehicles are in place for the safety of all vehicles using the interstate highway system. The regulations are important when it relates to the interstate highway system and especially bridges. The damage to the highway system is limited by the current regulations, but enforcement of those regulations is difficult with the current arrangements.