You've probably seen the videos -- too many, unfortunately -- of trucks that do not have enough clearance to go under a bridge. That is one way (not recommended) to lighten a trailer.
Or the videos, less frequent but no less dangerous, of vehicles that are too heavy to cross a bridge and don't make it all the way across. Bad for the bridge. Worse for the driver.
There is a reason height limit signs and weight limit signs exist.
National Bridge Law Standards
The U. S. Department of Transportation (USDOT) has rules, lots of them. And very specific ones regarding bridges and vehicles crossing them on interstate highways.
The basics from the DOT website are a good starting point:
Congress enacted the Bridge Formula in 1975 to limit the weight-to-length ratio of a vehicle crossing a bridge. This is accomplished either by spreading weight over additional axles or by increasing the distance between axles.
Compliance with Bridge Formula weight limits is determined by using the following formula:
W = the overall gross weight on any group of two or more consecutive axles to the nearest 500 pounds.
L = the distance in feet between the outer axles of any group of two or more consecutive axles.
N = the number of axles in the group under consideration.
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).
Who likes math? Me, neither. But to stay in compliance, some is required. Here's an online calculator for the above formula, check this out.
The basic concept is that a vehicle cannot weigh too much overall and it cannot weigh too much on any axle or set of axles. The gross weight limitation of 80,000 pounds is simple. Any violations are easily identified. The over-axle weight is a bit more complicated. And remember, the weight (result) is rounded to the nearest 500 pounds.
The below chart is from the same Federal Highway Administration (FHWA) website. Using the below chart is simpler than calculating the weight with the formula.
Exceptions can occur but most states follow the above for bridges on interstate highways.
State Bridge Laws Differ
Every state has its own regulations associated with weights on roads and bridges. In most cases, the states have adopted the above USDOT standards for interstate highways because otherwise they might lose funding. There are some exceptions, however. For information and links to the rules for each state check out these two websites: Federal Highway Administration and Ovesize.io.
For state highways, not part of the interstate highway network, the states have much more variation. No loss of federal funds is involved if a state has a standard different from the federal bridge formula. There is no simple guidance to provide except that the requirements for each state are different and need to be reviewed on a state-by-state basis. (See the above links.)
Signs
There are general rules, and there are exceptions. Look for signs prior to going over or under a bridge. If in doubt, don't go. Stop. Ask.
You know the weight. You know your height clearance. If something does not seem right, it probably is not right. If a sign should be present but is not, get more information before going over or under.
Other Factors
The standards for state roads and bridges differ greatly. Many bridges are old. Too many are not well maintained. Common sense should apply. If it appears to be unsafe‚ find another route.
Often the shipper or receiver is aware of restrictions associated with bridges between the interstate highway and the facility. Ask if in doubt. If going to a new area, ask anyway.
Weather, especially in the winter, can impact bridge travel. Ice on the bridge is dangerous to travel on. Ice or snow on the road under the bridge can reduce actual height clearances.
Ultimately, it is the driver's responsibility to know if the vehicle's weight is within the bridge limits or if the vehicle's height is under the clearance level. Sometimes a longer route is the safest (and, ultimately, the quickest) way to the desired destination.
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