Compliance with the law is checked when vehicles pass through a weigh station, often located at the borders between states or on the outskirts of major cities. There is one exception to the formula which allows the common five-axle semi-truck configuration to weigh the maximum legal gross weight without violating the bridge formula law.
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 gross weight limits still in use today.
A division of the DOT, the Federal Motor Carrier Safety Administration (FMCSA) regulates all safety-related aspects of the trucking industry. The FMCSA regulates the length, width, and weight limits of CMVs for interstate commercial traffic. Interstate commercial traffic is generally limited to a network of Interstate Highways, U.S. highways, and state highways known as the National Network (NN). Provided the truck remains on the NN, they are not subject to the state limits. State limits (which can be lower or higher than federal limits) come into effect for intrastate commercial traffic, provided the vehicle is not on the NN.)
CMVs are defined by the FMCSA as vehicles engaged in interstate commerce, used to transport passengers or property; with a gross vehicle weight of or more; designed or used to transport more than 8 passengers (including the driver) for compensation; designed or used to transport more than 15 passengers (including the driver) without compensation; or is used to transport hazardous materials in quantities requiring the vehicle to be marked or placarded under hazardous materials regulations.
The weight and size of CMVs are restricted for practical and safety reasons. CMVs are restricted by gross weight (total weight of vehicle and cargo), and by axle weight (i.e., the weight carried by each tire). The federal weight limits for CMVs are for gross weight (unless the bridge formula dictates a lower limit), for a tandem axle, and for a single axle. A tandem axle is defined as two or more consecutive axles whose centers are spaced more than but not more than apart. Axles spaced less than apart are considered a single axle.
In effect, the formula reduces the legal weight limit for shorter trucks with fewer axles (see table below). For example, a three-axle dump truck would have a gross weight limit of , instead of , which is the normal weight limit for a five-axle semi-truck.
The August 2007 catastrophic collapse of the Interstate 35W Mississippi River bridge in Minneapolis has brought renewed attention to the issue of truck weights and their relation to bridge stress. As of January 2008, the National Transportation Safety Board has not yet determined the official reason for the bridge collapse, although design failures are blamed in initial assessments. Reports suggest that as early as 1998, 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.
Federal estimates suggest truck traffic has increased over 200% since 1970, shortly before the federal gross weight limit for trucks was increased by . This is also the period from when many of today's interstate bridges were built. Research clearly shows that increased truck traffic (and therefore, increased stress) shortens the life of bridges. Estimates indicate that a single truck does as much damage to roads as 750 cars.
Some smaller bridges have a weight limit (or gross weight load rating) usually indicated by a sign posted in front of the bridge and visible to anyone driving over the bridge. These are necessary when the weight limit of the bridge is lower than the federal or state gross weight limit for trucks. Driving a truck over a bridge that is too weak to support it usually does not result in an immediate collapse. The bridge may develop cracks, which over time can weaken the bridge and cause it to collapse. Most of these cracks are caught during mandated inspections of bridges. Most bridge collapses occur in rural areas, result in few injuries or deaths, and receive relatively little media attention. As many as 150 bridges collapse each year, and most of those are the result of soil erosion around bridge supports.
In special cases involving unusually overweight trucks (which require special permits), not observing a bridge weight limit can lead to disastrous consequences. Just fifteen days after the collapse of the Minneapolis bridge, a heavy truck collapsed a small bridge in Oakville, Washington. It was determined that the truck was over the weight limit of the bridge. However, there was no indication that the truck was in violation of the bridge formula.
FMCSA regulation §658.17 states:
Two or more consecutive axles may not exceed the weight computed by the bridge formula, even if the gross weight of the truck (or the weight on one axle) is below otherwise legal limits. Although this means that any two axles must comply with the formula, experience has shown that axles 1 through 3, 1 through 5, and 2 through 5 are critical and must be checked. This means that the axle group which comprises the entire truck, known as the outer group; and the interior axle groups, known as the tractor group and trailer group groups, must also comply with the bridge formula. If these combinations are found to be satisfactory, then all of the other axle groups on this type of vehicle normally will be satisfactory.
This exception allows for the standard 5-axle semi-truck configuration to weigh up to (the legal limit) without being in violation of the bridge formula law. Without this exception, the bridge formula would allow an actual weight of only to on tandems spaced to apart. This exception was sought by the American Trucking Associations specifically so trucking companies could use trailers and gross . It was the only way tank truck operators could reach 80,000 pounds without adding axles.
| Distance in feet between any|
group of two or more axles 1
|Gross weight in pounds 2|
|2 axles||3 axles||4 axles||5 axles||6 axles||7 axles|
|Less than 8 3||34,000||34,000|
|More than 8 4||38,000||42,000|