Trucks are often used in combination with other modes of transportation. For example, a loaded trailer can be separated from a truck tractor and loaded onto a railroad flatcar. The trailer is then transported to a railroad terminal near its final destination, where it is reconnected to another truck tractor and driven to the destination. Standardized shipping containers are transported by ship or train and then loaded onto tractor-semitrailers for delivery to their final destination.
Containerization is a method of transporting cargo by packing it into large containers. This cargo then can be moved as a unit by ship, truck, airplane, or railroad, and from one type of transportation to another. Containerization reduces the time and cost required to load and unload cargo.
Most shipping containers are made of steel. They can carry general cargo--that is, anything that can be packaged or that forms a package in itself. Containers also handle hazardous goods and cargo requiring refrigeration or tanks. Most major ports have special cranes that lift containers on and off ships. A container's size depends on its cargo. But most containers measure from 20 to 40 feet (6.1 to 12.2 meters) long and 8 to 9-1/2 feet (2.4 to 2.9 meters) high. Containers transported by air may be smaller.
The large trailers pulled by semi tractors have their own rear suspension and wheels, with the front of the trailer being supported by the fifth wheel on the tractor. Semi trailers also have folding supports under the front that are lowered when the trailer is detached from the tractor and parked. The brakes on the trailer’s axles have air hoses that attach to the tractor’s brake system, so the tractor and trailer brakes work together. Trailers have their own signal, tail, and brake lights, all of which are powered by the tractor’s electrical system.
Trailers come in many different designs, depending on the intended cargo. Enclosed, or standard box-type trailers are used to haul a wide variety of goods and merchandise. Double trailers are often used on roads that have sharp turns. Double trailers resemble two smaller trailers linked together and can maneuver through tight turns more easily than standard trailers can. Size and weight restrictions apply and vary from state to state. In the United States, tractor and single trailer combinations generally must be less than 16 m (53 ft) in length and are limited to a maximum weight of 36,000 kg (80,000 lb).
Separate weight limits apply to trailers with single or tandem axles and to double trailers. Maximum trailer height and width are dictated by state law and vary from state to state. For most states, the maximum height is 4.11 m (13.5 ft), and the maximum width is 2.6 m (8.5 ft). In some states, trailers may be equipped with additional wheels and axles to carry heavier loads.
In the late 1950s container ships set the pattern for technological change in cargo handling and linked the trucking industry to deep-Sea shipping. These highly specialized ships carry large truck bodies and can discharge and load in one day, in contrast to the ten days required by conventional ships of the same size.
The container concept is that of fitting cargo into standard-size truck containers and building specialized ships to handle them. The worldwide acceptance of this new handling and transporting method has had far-reaching effects. It has meant a virtual abandonment on major routes of old-style, break-bulk cargo ships. Break-bulk ships carried bales, drums, boxes, bags, and all sorts of piece cargo. Whole fleets of such vessels, some virtually new and with years to go under the old rules, became nearly worthless and were sold to small lines for service on low freight-rate routes.
