The Federal Weight Baseline, and Why Meeting It Is Not Enough
Date Published
Three numbers govern weight on the Interstate System, and most drivers know them. The fourth rule, which can override all three, is the one that produces unexpected citations.
The three figures
On the Interstate System the federal baseline is 80,000 lb gross, 20,000 lb on a single axle, and 34,000 lb on a tandem axle.
Those figures are the starting point for every state comparison. A state may sit at them, above them where a grandfathered exception applies, or occasionally below them on particular routes.
The rule that overrides them
The bridge formula, at 23 CFR 658.17, relates the weight permitted on a group of axles to the distance between them. Its purpose is to protect bridge spans: the same weight concentrated over a short wheelbase imposes a greater load on a structure than the same weight spread over a long one.
The consequence is that meeting the single and tandem maximums is not sufficient. A vehicle can be under 20,000 lb on every single axle, under 34,000 lb on every tandem, under 80,000 lb gross, and still exceed what the formula permits for a particular axle group at a particular spacing.
That is why axle spacing appears on permit applications and why sliding a tandem can bring a legal-on-paper load into compliance without removing any freight.
Tolerances are not extra capacity
Several states publish enforcement tolerances, and some fold them into the figure they state. A tolerance is a margin above the posted limit that enforcement will not cite. It is not an increase in the legal limit, it is not uniform between jurisdictions, and it does not travel with you across a state line.
Where states diverge
Because the federal figures bind on the Interstate System specifically, state roads can carry different limits, sometimes higher ones set by a spacing table of the state's own. A carrier planning a route that leaves the Interstate needs the state figure rather than the federal one.
Our per-state tables give the gross, single-axle and tandem-axle figure for each jurisdiction with the statute that establishes it, so the divergence can be read directly rather than assumed.