To a lot of people, tractor trailer trucks, or semis, are speeding behemoths that you don’t want to be driving next to. To the kids in your back seat, they are a source of entertainment on long car rides—pump your elbow and the driver might honk his horn. But to all of us, they are the lifeblood of our existence, the source of the products on supermarket shelves, Amazon deliveries, and the gas in our cars. You might fly by them on the highway without thinking much about all that, but there’s a lot worth knowing about them.
Why Are They Called Semis?
There are about 2.8 million semitrucks (also called tractors) registered in the U.S. as of 2016. Their trailers—known as tractor-trailers—have no front wheels and can be used only when connected to the tractor part of the truck. (Their brakes are automatically applied when the trailer is standing unattached.) Only when a truck is connected, and pressure from the truck’s engine-powered air pump releases the brakes, can the trailer roll. Thus: “semi-trailer.” The terms “semi” and “semitruck” evolved from there.
How Much Weight Can They Haul?
The maximum weight for a U.S. semitruck and full trailer is 80,000 pounds spread over 18 conventional wheels. Out in the wide-open spaces of Australia, however, “road trains” can have four trailers and weigh in excess of 300,000 pounds.
How Much Do We Rely On Them, Exactly?
In 2016, semitrucks drove 175 billion miles. Recent estimates suggest 68 percent of all goods in the U.S. are delivered by semitruck, which works out to about 60,000 pounds per American every year. The two largest commodities by weight are agricultural and building materials.