Federal Bridge Gross Weight Formula
CMVs are most often tractor-trailers or buses, but the formula is of most interest to truck drivers due to the heavy loads their vehicles often carry.
Early 20th-century weight limits were enacted to protect dirt and gravel roads from damage caused by the solid wheels of heavy trucks.
The formula effectively lowers the legal weight limit for shorter trucks, preventing them from causing premature deterioration of bridges and highway infrastructure.
The one exception to the formula allows a standard five-axle semi-truck configuration to weigh the maximum legal gross weight.
These laws were passed to protect earth and gravel-surfaced roads from damage caused by the steel and solid rubber wheels of early heavy trucks.
[2] In the late 1950s, the American Association of State Highway and Transportation Officials (AASHTO) conducted a series of extensive field tests of roads and bridges to determine how traffic contributed to the deterioration of pavement materials.
In 1964, the AASHTO recommended to Congress that a bridge formula table be used instead of a single gross weight limit for trucks.
The Federal-Aid Highway Act Amendments of 1974 established the bridge formula as law, along with the gross weight limit of 80,000 pounds (36,000 kg).
[2] The formula was enacted as law to limit the weight-to-length ratio of a commercial motor vehicle (CMV).
[11] In effect, the formula reduces the legal weight limit for shorter trucks with fewer axles (see table below).
[12] In November 2008, the National Transportation Safety Board determined there had been several reasons for the bridge's collapse, including (but not limited to): faulty gusset plates, inadequate inspections, and the extra weight of heavy construction equipment combined with the weight of rush hour traffic.
[13] The I-35 Trade Corridor Study reported that the Federal Highway Administration (FHWA) expressed concern over bridges on the I-35 corridor due to an expected increase of international truck traffic from Canada and Mexico, with the FHWA listing it as "high-priority" in 2005.
Most bridge collapses occur in rural areas, result in few injuries or deaths, and receive relatively little media attention.
[21] CMVs are required to pass through weigh stations at the borders of most states and some large cities.
These weigh stations are run by state DOTs, and CMV weight and size enforcement is overseen by the FHWA.
Florida also includes a scale tolerance, which allows for violations of less than 10% to be forgiven, and no fine issued.
[1] This exception allows for the standard 5-axle semi-truck configuration to gross up to 80,000 pounds (36,000 kg) (the legal limit)[9] without being in violation of the bridge formula law.
[26] A CMV may exceed the bridge formula limits (or gross weight and its axle weight limits) by up to 550 pounds (249 kg) if the vehicle is equipped with an auxiliary power unit (APU) or idle reduction technology.
To be eligible, the vehicle's operator must prove the weight of the APU with written certification, or—by demonstration or certification—that the idle reduction technology is fully functional at all times.
