There are about 5.6 million semi trailers (or tractor trailers) registered for use in the U.S., almost three times the number of semi trucks (also called tractors). It was the trailer that gave rise to the name semi truck. Since the trailer has no front wheels and can be used only when connected to the tractor part of the truck, it's called a semi-trailer. The terms "semi" and "semi truck" evolved from that.
Trailers are typically 53 feet long. They have brakes that are automatically applied when the trailer is standing unattached to the truck. When the truck is connected to the trailer, pressure from the truck's engine-powered air pump releases the brakes so that it can roll.
Source: http://www.popularmechanics.com/cars/news/pictures/10-things-you-didnt-know-about-semi-trucks?src=rss
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