tractortrailer

A tractor-trailer, commonly known as a semi-truck, semi, or tractor-trailer, is a heavy vehicle used for freight transport. It consists of two main parts: a tractor unit that provides propulsion and a separate trailer that carries cargo. The two are connected by a fifth-wheel coupling, allowing the trailer to pivot for steering while the tractor supplies power and braking. Because the trailer is supported at its front by the tractor, the arrangement is described as a semi-trailer.

Tractor units vary in cabin design and engine size, while trailers come in several configurations. The most common trailer is a dry van, a fully enclosed box; refrigerated trailers (reefers) maintain controlled temperatures; flatbeds carry oversized loads; and other types include tankers and lowboys for specialized cargo.

In practice, tractor-trailer combinations in many regions measure roughly 40 to 53 feet in overall length and have a gross vehicle weight rating around 80,000 pounds in the United States (with regional limits). The trailer carries most of the payload, while the tractor provides propulsion and braking. Axles on both units determine payload capacity and turning ability; braking systems on trailers are coordinated with the tractor's brakes.

Regulatory aspects typically require a commercial driver's license and compliance with hours-of-service rules, vehicle weight limits, and safety standards. Regional restrictions vary by country, but the general concept of a two-part, pull-type freight vehicle is common worldwide.

Use and impact: The tractor-trailer is a primary tool for long-haul freight, offering high cargo capacity and flexible routing. Efficiency, maintenance costs, and safety considerations influence its operation; ongoing advances include aerodynamic enhancements, telematics, automatic braking, and more stringent training to reduce accidents.