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What Is a Semi? How Big Can it Get? Basic Tractor-Trailer Definitions and Size Limits

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There's lots of names for 18-wheelers, but do they all mean the same thing? And how big can they really get? We’ll cover basic definitions of names and size limitations for these massive machines barreling down our interstates and highways, which often cause tragic accidents.

Names for Tractor-Trailers

What is a tractor-trailer? What is a semi-trailer? Is there a difference? Not really. They’re just names for the same thing. Let me break that down for you.

“Tractor-trailer” may be the best way to picture what these machines really are. The tractor is the engine and the cab where the trucker sits. The trailer carries the cargo. It’s attached to the tractor.

The various names for these vehicles include semi, semi-trailer, semi-truck, tractor-trailer, trailer truck, tractor truck, semi-tractor-trailer truck, semi-tractor truck, transport truck, transfer truck, big rig, and 18-wheeler.

Legally, the most important name is a commercial motor vehicle. That means it’s governed by federal safety regulations, which we discuss a little more below.

Semi-Trailer Size Limitations

How long, wide, tall, and heavy is a semi? We look to federal trucking safety regulations for guidance, namely Federal Motor Carrier Safety Administration regulations, which set safety standards that often play an important role in semi-truck wreck cases.

  • Height. Shockingly, there are no limits under federal law. It’s set by states. Most limits range from 13 ½ feet to 14 feet.
  • Width. Federal law sets this at just over 102 inches, which is about 8 ½ feet. This measurement excludes safety equipment like mirrors.
  • Length. These are set at the minimum allowable lengths. The minimum length is 48 feet or a grandfathered minimum set by state law. In South Carolina, it’s 48 feet. For a double trailer, meaning two trailers connected to the cab or tractor, the minimum length is 28 feet to 28 ½ feet per trailer for a total of at least 56 to 57 feet.
  • Weight. This is governed by a federal definition called the gross vehicle weight rating. It’s the maximum amount for the vehicle to carry set by the manufacturer. To be considered a commercial motor vehicle, a truck must have a gross vehicle weight rating (GVWR) of at least 10,001 pounds. Tractor-trailers easily meet that definition, as most of them can weight up to 80,000 pounds.

If You’ve Been Hit By One of These Monsters, You Need Help

The size of semi-trucks combined with highways speeds causes horrific crashes with devastating impacts, resulting in severe injuries or even death. Worse for victims, these cases are extremely complicated—and from a legal standpoint, you may under a supreme disadvantage almost from the moment of impact.

If you’ve got questions about a semi-trailer accident, call toll-free at 888-230-1841 or fill out a Get Help Now form. You can also download our free book on car accidents, which contains a chapter devoted to semi-truck accidents.

Be sure to check out what other people say it’s like to work with us on websites we don’t own or control, like Avvo.com reviews.

Rob Usry
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Rob is a Spartanburg personal injury lawyer. Rob also practices as a criminal defense attorney.
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