The short version: federal weight limits — 20,000 lb single axle, 34,000 lb tandem, 80,000 lb gross — bind on the Interstate System and nowhere else; off it, a state sets its own. Width on the National Network is fixed at 102 inches, a floor and a ceiling at once. There is no federal height limit. And 4 of the states we cover print statutory weights above the federal figures because of grandfather rights or a compliance clause — which is the trap this page exists for. Every passage below is quoted verbatim from FHWA and folded behind its own citation.
Federal weight limits on the Interstate System
Compilation of Existing State Truck Size and Weight Limit Laws, Exhibit 2 — Federal Highway Administration, May 2015. Public domain (17 U.S.C. §105).verbatim
"Federal weight standards apply to commercial vehicle operations only on the Interstate Highway System, which consists of approximately 50,000 miles of limited access, divided highways that span the Nation. Off the Interstate Highway System, States may set their own commercial vehicle weight standards. Federal standards for commercial vehicle maximum weights on the Interstate Highway System are as follows: Single Axle – 20,000 lbs.; Tandem Axle – 34,000 lbs.; GVW – 80,000 lbs."
Bridge Formula Weights, FHWA-HOP-19-028, p. 2 — Federal Highway Administration, August 2019. Public domain.verbatim
"In addition to Bridge Formula weight limits, Federal law states that single axles are limited to 20,000 pounds, and axles spaced more than 40 inches and not more than 96 inches apart (tandem axles) are limited to 34,000 pounds. Gross vehicle weight is limited to 80,000 pounds (23 U.S.C. 127)."
Note the 40-inch lower bound. A tandem is defined by a spacing range, not just a maximum — axles closer together than 40 inches are not a tandem under federal law. Our own engine captures the 96-inch (8 ft) upper bound and does not yet model the lower one; that is a real edge, it is stated here rather than hidden, and it affects only unusually tight axle groups.
Federal size limits on the National Network
Compilation of Existing State Truck Size and Weight Limit Laws, Exhibit 2 — FHWA, May 2015. Public domain.verbatim
Overall length: "No Federal length limit exists for most truck tractor-semitrailers operating on the NN. Exception: … vehicles designed and used specifically to carry automobiles or boats in specially designed racks may not exceed a maximum overall vehicle length of 65 feet, or 75 feet, depending on the type of connection…"
Trailer length: "no State may impose a length limit of less than 48 feet … on a semitrailer … Similarly … no State may impose a length limit of less than 28 feet on a semitrailer or trailer operating in a truck tractor-semitrailer-trailer (twin-trailer) combination."
Width: "On the NN, no State may impose a width limit of more than or less than 102 inches. Safety devices (e.g., mirrors, handholds) necessary for the safe and efficient operation of motor vehicles may not be included in the calculation of width."
Height: "No Federal vehicle height limit exists. State standards range from 13.6 feet to 14.6 feet."
The width rule is the unusual one: it is a floor and a ceiling simultaneously. A state may not go under 102 inches and may not go over it either, which is why 102 inches is the same number in every state column on our legal limits table and height is the column that varies most.
Grandfather rights, and why a state statute can read higher
FHWA-HOP-19-028 (Aug 2019) and Compilation of Existing State Truck Size and Weight Limit Laws, pp. 15–16 — FHWA, May 2015. Public domain.verbatim
"There are three different grandfather clauses in Title 23 USC §127."
"When the Interstate System axle and gross weight limits were adopted in 1956, and amended in 1975, States were allowed to keep or 'grandfather' weight limits that were higher."
"When considered together, the successive grandfather provisions provided by Congress in 1956, 1974, and 1991 and other exceptions results in 37 States and the District of Columbia having allowances to exceed Federal weight limits on their Interstate highways (in many States these exceptions are very limited)."
"For those States that have claimed general exceptions to Federal limits on Interstate highways (such as higher single or tandem axle limits); these higher limits also apply to non-Interstate elements of the NHS."
The compliance-clause trap — eight states, and we cover three of them
Compilation of Existing State Truck Size and Weight Limit Laws, p. 17 — FHWA, May 2015. Public domain.verbatim
"the United States Secretary of Transportation shall withhold 50 percent of appropriated funds from a State that sets weight limits for Interstate travel that are higher or lower than the standard Federal limits for Interstate highways, with some exceptions" (23 U.S.C. §127[a]).
"Although most States explicitly establish two sets of weight limits, one for State highways and one for Interstate highways, others do not explicitly draw out separate limits for Interstates in statute. In at least eight such States (Connecticut, Hawaii, Nebraska, Nevada, New Jersey, New York, Washington, Wyoming), the weight limits given in statute are higher than the standard Federal limits for Interstate highways, but a Federal compliance clause is in place that implies that the standard Federal weight limits would apply to Interstate travel…"
This is the single most actionable paragraph on the page, and it applies directly to our own data. 4 of the eight are in our covered set — Nevada, New Jersey, New York, Washington — and their statutory axle figures read above the federal Interstate limits. A reader taking a state's statutory single-axle figure and applying it to an Interstate lane will misprice the move. Read the state statute for the state system, and 20,000 / 34,000 / 80,000 lb for the Interstate itself.
How the numbers got where they are
Compilation of Existing State Truck Size and Weight Limit Laws, p. 4 — FHWA, May 2015. Public domain.verbatim
"The first laws establishing limits on truck weight in the United States were enacted by several States in 1913. By 1933, all States had established some laws… The Federal Government began regulating truck size and weight in 1956… The first Federal truck size and weight regulations limited combination trucks to an overall gross vehicle weight of 73,280 lbs., limited single axle weights to 18,000 lbs., and restricted tandem axle weights to 32,000 lbs. Trucks were limited to a width of 8 feet (96 inches)…"
"In 1974, Congress passed a bill allowing States to increase weight limits … to a maximum of 80,000 lbs. GVW … 20,000 lbs. on a single axle and 34,000 lbs. on a tandem axle. The increase, however, was not a mandate… In 1982, Congress passed the Surface Transportation Assistance Act (STAA), which imposed the Federal 80,000 lb. limit as a mandate across the entire Interstate Highway System."
The 1956 → 1974 → 1982 sequence is why grandfather rights exist at all: states that were already lawfully running heavier were not made to come down.
Questions
Do federal weight limits apply on every road?
No. The federal single-axle, tandem-axle and gross limits apply on the Interstate System. Off the Interstate, states set their own commercial vehicle weight standards, which is why a state statute can print a higher axle figure and both numbers can be correct.
Why is 102 inches the magic width?
Because on the National Network federal law makes it a floor and a ceiling at once: a state may not impose a width limit of more or less than 102 inches. Safety devices such as mirrors and handholds are excluded from the measurement.
Is there a federal height limit?
No federal vehicle height limit exists. State standards range from about 13 ft 6 in to 14 ft 6 in, which is why height is one of the columns that varies most across the state table.
What is a grandfather right?
An allowance for a state to keep weight limits that were lawful before the federal standards were adopted. Successive provisions in 1956, 1974 and 1991, plus other exceptions, leave 37 states and the District of Columbia with some allowance to exceed federal weight limits on their Interstate highways — in many cases a very limited one.