Truck Weight, Roads, and Bridges

A legally loaded truck can weigh 80,000 pounds, and a truck at 80,000 pounds can still be over the limit. That is not a contradiction, because federal law constrains Interstate truck weight in three separate ways: a gross limit for the whole vehicle, a limit for each axle or tandem, and the Bridge Formula, which sets a maximum for any group of consecutive axles based on how far apart the outer two are. A bridge does not experience a truck as a total. It experiences it as a quantity of weight concentrated over a length.

Three limits, applied at the same time

FHWA's guidance states the federal figures for the Interstate System: 20,000 pounds on a single axle, 34,000 pounds on a tandem, and 80,000 pounds gross, the last subject to the Bridge Formula dictating something lower. A single axle is one or more axles whose centers are not more than 40 inches apart. A tandem is two or more consecutive axles spaced more than 40 but not more than 96 inches apart. Gross weight means the vehicle or combination plus any load on it.

FHWA is explicit that the pamphlet paraphrases 23 U.S.C. 127 and 23 C.F.R. 658 for illustration, and that in a dispute the statute and the regulation control. The August 2019 edition supersedes the May 2015 one, which is a useful reminder that a figure quoted from an older printing may no longer be the guidance in force.

Why spacing does the work

Congress enacted the Bridge Formula in 1975 to limit the weight-to-length ratio of a vehicle crossing a bridge. Compliance is achieved either by spreading the same weight over more axles or by increasing the distance between them. The reason FHWA gives is structural: as trucks grew heavier through the 1950s and 1960s, allowable weights had to be tied to the number and spacing of axles to protect bridges built for a mixed traffic stream.

The comparison FHWA draws is between two trucks of identical total weight and identical individual axle weights, one long and one short. The stress on the bridge members as the longer truck rolls across is much less, because its weight is spread out while the shorter vehicle concentrates the same load over a smaller area. Axle spacing, in FHWA's phrasing, is as important as axle weight in designing bridges.

The formula itself takes three inputs: W, the gross weight on any group of two or more consecutive axles rounded to the nearest 500 pounds; L, the distance in feet between the outer axles of that group; and N, the number of axles in the group. Below 40 inches the single-axle limit replaces the formula result, and between 40 and 96 inches the tandem limit does.

How a check actually runs

The formula applies to every combination of two or more axles, but FHWA says experience shows three groups are the ones that decide a five-axle tractor-semitrailer: axles 1 through 3, the tractor bridge; axles 1 through 5, the outer bridge; and axles 2 through 5, the trailer bridge. The worked example in the guidance shows why the last of those is the one that catches people.

Every preliminary check on that vehicle passes. The single axle is under 20,000 pounds, both tandems are at 34,000 or under, the gross is exactly 80,000. The trailer bridge still fails by 2,500 pounds, and FHWA's remedy is to remove load or increase the 35-foot spacing.

Single-unit trucks are not exempt from the problem. In FHWA's second example, a four-axle truck at 57,000 pounds gross passes the single-axle, tandem and gross tests and passes axles 1 through 4, then fails axles 2 through 4 at 45,000 pounds against a 42,500-pound maximum for a 9-foot group. Closely spaced axles produce the critical case.

The exceptions that move the answer

Two written exceptions matter. The first is grandfather rights: when Interstate axle and gross limits were adopted in 1956 and amended in 1975, states were allowed to keep higher limits they already had, so Interstate limits in some states are above the federal figures. The second is the tandem exception in 23 U.S.C. 127, under which two consecutive sets of tandem axles may each carry 34,000 pounds provided the first and last axles of those tandems are at least 36 feet apart. FHWA notes that without it, the formula alone would allow only 66,000 to 67,500 pounds on tandems spaced 36 to 38 feet apart, which is the ordinary five-axle configuration.

What the driver carries out of this

Commercial driver training compresses the whole apparatus into a working rule: states set maximums for gross vehicle weight ratings, gross combination weight ratings and axle weights, maximum axle weights are often set by a bridge formula that permits less weight for axles that are closer together, and the purpose is to avoid overloading bridges and roadways. The consequences it lists are handling ones. Overloading affects steering, braking and speed control, overloaded trucks crawl on upgrades and can gain too much speed on downgrades, stopping distance increases, and brakes can fail when forced to work too hard. It adds that in bad weather or in mountains it may not be safe to run at legal maximum weight at all.

Two limits on what this page covers are worth stating. These are Interstate figures, and state and local roads can be posted lower or zoned separately. And the sources here are about bridges; how axle loads wear pavement is a separate body of engineering literature that is not cited above and is not summarized here. For what the numbers mean on a specific vehicle, see how much a semi truck weighs and semi, box and straight trucks compared; for the road system these limits attach to, see how heavy trucks use Interstate highways.