Why Do Trucks Have to Be Weighed?

Large cargo trucks are a seemingly endless source of intrigue for other drivers on the road, who muse about everything from why they have spikes on their wheels to why they have an extra set of tires.

Another mystery to ponder: Why do trucks need to be weighed?

According to HowStuffWorks, forcing drivers to make a stop on a roadside scale has to do with two things: taxation and safety.

States tax goods and products by volume, meaning that a truck’s weight when carrying a load informs the tax due by the carrier, or owner and operator of the truck. This tax is typically collected to cover the expense of maintaining roadways that are subject to the wear of transportation.

Although taxes are part of the reason, the primary concern over weight is safety. Scales make sure a truck is safe to navigate on roads and doesn’t exceed standard weight limits, which can increase risks due to overloading. Some highways may not be able to handle excessive weights, which can prompt road damage or raise the potential for an accident.

For most areas, the maximum weight is 34,000 pounds, though scales are typically rated for much more than that. Some are warrantied to handle trucks up to 80,000 pounds each at 200 times a day for 25 years.

Weight in these systems can measured (either per axle or gross weight) by an electric current which changes depending on the weight, known as a load-cell system; or a bending-plate system, which measures the strain placed by the weight of the vehicle on a strain gauge. Piezoelectric systems measure voltage to determine weight.

Drivers typically stop off at weigh stations (known in trucker slang as a “chicken coop”) operated by a state’s department of motor vehicles to have their truck put on a scale and sometimes get a visual inspection. Some scales can assess weight even while a truck is in motion, making it easier and more efficient for drivers. The stations are staffed to assist drivers and collect data, though they may be unattended on weekends, evenings, or holidays.

So what happens if a driver “flunks” a weigh station visit by exceeding the maximum weight? Consequences can vary by state, but a truck that’s too heavy can invite citations or fines. Some areas calculate the fine by the pound, which can be pennies or even over $1. For a truck that’s significantly over the limit, that can mean tens of thousands of dollars.

If it’s so heavy that there’s concern it may be unsafe, the truck can be held at the station. For problems beyond the scope of weigh station staffers, state troopers can be summoned.

[h/t HowStuffWorks]