Key takeaways
- On the Interstate System, federal rules limit gross vehicle weight to 80,000 pounds, a single axle to 20,000 pounds and a tandem axle to 34,000 pounds.
- The Federal Bridge Formula can lower the allowed weight on a group of axles, depending on how many axles the group has and how far apart they are.
- Federal weight limits include all enforcement tolerances, except a scale allowance of no more than 5 percent when officers weigh with portable scales.
- Each state decides which trucks must enter an open weigh station, so the posted signs before the scale are the rule that applies.
- A certified public scale at a truck stop gives you a printed ticket with your axle and gross weights before you reach an enforcement scale.
A weigh station is a roadside site where state officers check a truck's weight and, when they choose, its driver, paperwork and equipment. When a station is open, posted signs tell you whether your truck must exit; you roll across or stop on the scale, then follow the signal back to the highway or into the inspection area. Know your weights before you get there, because federal limits leave no built-in tolerance.
What happens at the scale
Most stations use some combination of three tools.
Static scales
A static scale weighs your truck while it's stopped or creeping across at a walking pace. Officers see the weight on each axle or axle group and the gross weight, and compare them against the limits. If everything is within limits and nothing else catches their attention, a signal sends you back to the highway.
Weigh-in-motion
Weigh-in-motion sensors are built into the pavement, on the station's entrance ramp or on the highway itself. They weigh trucks as they roll and help officers decide which trucks need the static scale. A truck that looks legal may be signaled to bypass the static scale; one that reads heavy is sent across it.
Bypass programs
Several bypass and preclearance programs exist, some run by states and some by private companies, using a transponder or a phone. As you approach, the system checks your carrier's credentials and safety record, and at sites with weigh-in-motion, your weight. If you're cleared, you get a signal to keep going; if not, you pull in. A bypass signal only covers the station that gave it, and states keep exceptions. Oregon, for example, requires oversize and heavy-haul loads to stop at open scales even when the transponder shows a green light.
Signals at the station
Inside the station, a light or message board tells you what to do next: proceed back to the highway, stop on the scale, or pull into the inspection area and bring your paperwork. Follow the signal and any instructions from the officer. If you aren't sure what a signal means, stop and ask rather than leave.
Open and closed signs
Signs before each station tell you it's coming and whether it's open. When the sign reads open, the trucks the state requires must take the exit. When it reads closed, you continue, though officers can still stop you on the road.
Which trucks must enter is a state rule, not a federal one. Oregon's version shows how specific it gets: where a sign reads "All Trucks Over 20,000# GVW Next Right," vehicles over 20,000 pounds must stop; where there's no such sign, commercial vehicles of 10,000 pounds or more must stop. Read every sign on the approach, especially in a state you don't run often.
Federal weight limits
The federal limits in 23 CFR 658.17 apply on the Interstate System and reasonable access to it. States set limits on other roads and can issue special permits for loads that exceed them.
Limit | Federal maximum | Notes |
|---|---|---|
Gross vehicle weight | 80,000 lb | Less if the bridge formula gives a lower number |
Single axle | 20,000 lb | Applies to any one axle, including one in a group |
Tandem axle | 34,000 lb | Axles more than 40 and not more than 96 inches apart |
Axle groups | Bridge formula | Depends on the number of axles and their spacing |
The bridge formula
The Federal Bridge Formula protects bridges by tying allowed weight to how weight is spread out. It reads W = 500 × (LN ÷ (N − 1) + 12N + 36), where:
- W is the maximum weight on any group of two or more consecutive axles, to the nearest 500 pounds.
- L is the distance in feet between the outer axles of the group.
- N is the number of axles in the group.
In practice, more axles spread farther apart can carry more weight, up to the 80,000-pound cap. One exception is written into the rule: two consecutive sets of tandem axles may carry 34,000 pounds each if the distance between the first and last axles of those tandems is 36 feet or more.
No tolerance to lean on
Under 658.17(g), the federal limits include all enforcement tolerances. The one exception is a scale allowance for portable scales (wheel-load weighers), capped at 5 percent. Fine schedules that skip a fine below a threshold, such as 1,000 pounds, are treated as tolerances the law doesn't authorize. If your ticket says 80,400 pounds, plan on being overweight.
Check your weight on a certified scale first
Many truck stops have certified public scales; drivers often call any of them a CAT scale, after the best-known network. You pull on with your steer axle, drive axles and trailer axles each on their own platform, and you get a printed ticket with each group's weight and the gross.
Weigh right after loading, before the first enforcement scale. A five-axle tractor-trailer at 12,000 pounds on the steer, 34,000 on the drives and 34,000 on the trailer tandems is right at 80,000 gross, with nothing to spare. If the drives are heavy, sliding the fifth wheel shifts weight between steer and drives; if the drives or trailer tandems are heavy, sliding the trailer tandems shifts weight between them. Keep the ticket with your paperwork.
Inspections at the scale
A weigh station is also where many roadside inspections happen. The Commercial Vehicle Safety Alliance defines the inspection levels officers use. A Level I inspection is the full North American Standard Inspection of the driver and the vehicle, including brakes, coupling devices, tires and cargo securement. Level II is a walk-around of the same driver and vehicle items, limited to what can be checked without getting under the truck. Level III covers the driver and credentials: license, medical examiner's certificate, record of duty status and hours of service, seat belt, vehicle inspection reports, and carrier identification and status. Other levels cover special one-item checks, vehicle-only inspections without the driver, and certain radioactive shipments.
How Fort Route helps
Fort Route shows weigh stations and truck scales on the map, and while you navigate, "what's ahead" names the next scale on your route. Each weigh station shows its posted hours or "Hours unknown," and where hours are known, whether it's usually open or closed now. That's a schedule, not a live status: the open or closed sign on the road is what counts.
Truck-safe routing checks your truck's gross and axle weight against road restrictions when it builds the route. Map data can be wrong, and some roadside scales aren't mapped, so always obey posted signs.
Sources
Frequently asked questions
Do I have to stop at every open weigh station?
You must enter when the state's rules and the posted signs require it for your truck. Thresholds vary by state: Oregon, for example, requires vehicles over 20,000 pounds to stop where its sign says so, and commercial vehicles of 10,000 pounds or more where there's no such sign.
What is the maximum legal weight for a semi?
Federal rules allow 80,000 pounds gross on the Interstate System, with 20,000 pounds on a single axle and 34,000 pounds on a tandem. The bridge formula can lower the allowed weight for a given axle layout, and loads over these limits need a state permit.
Is there a weight tolerance at the scale?
No, the federal limits include all enforcement tolerances. The only exception is an allowance of no more than 5 percent when officers weigh with portable scales.
What does weigh-in-motion do?
Weigh-in-motion sensors in the pavement weigh trucks as they roll. Officers use the readings to decide which trucks can bypass the static scale and which need to cross it.
Where can I check my weight before a weigh station?
Use a certified public scale, found at many truck stops. You get a printed ticket with your steer, drive and trailer axle weights and your gross, so you can adjust before you meet an enforcement scale.