Commercial-size car carrying trailers are commonly used to ship new cars from the manufacturer to auto dealerships; in the U.S., shipping of used vehicles is also a big industry, employed by car owners who are relocating and choose to ship their cars instead of driving, as well as consumers who have just purchased a vehicle on the second-hand market (particularly online) and need it delivered to their location.
Like other semi-trailers, most commercial car carrier trailers attach to the tractor using a fifth wheel coupling. Trailers can either be enclosed, possessing walls like a conventional box trailer, which affords the shipped vehicles more protection at the cost of lower capacity; or open, as in the commonly seen skeletal tube steel design, which exposes the vehicles to the elements but allows for greater carrying capacity. An American commercial car carrier typically fits between 5 and 9 cars, depending on the car size and trailer model (capacity is limited by an 80,000 lb... See More weight cap that a road vehicle is subject to under U.S. law.). Significantly higher-capacity vehicles have been observed around the world, such as a side-by-side loading Chinese model.
Open commercial car carrier trailers typically have a double-decker design, with both decks subdivided into a number of loading and storage ramps that can be tilted and lifted independently of one another with hydraulics. Unlike flatbed tow trucks, which often need to transport non-running vehicles, car carrier trailers aren't equipped with loaders or winches, instead relying on the vehicles to be loaded under their own power. The trailer hydraulics allow the ramps to be aligned on a slope so cars can be driven up and secured to the ramp floor with chains, tie-down ratchets or wheel straps, after which the ramp can be tilted in any direction to optimize stacking.
To load vehicles on the top deck of a double-decker commercial trailer, the rear half of the deck can tilt and be lowered hydraulically, forming a drive-up ramp to the upper deck. The top deck is usually loaded first and off-loaded last, since the presence of cars on the lower deck can make it impossible to lower the top deck ramp.
Trailer hydraulics are operated using a control box mounted on the trailer itself.