From a tiny door set into their main door to wheels that magically float above the road without touching the pavement, there are some downright strange things about semi truck trailers. Here are the most bizarre features of big rig trailers, and an explanation of each.

Why do some semi truck trailers have a tiny door set into their main door?

There’s no clear consensus on the purpose of the tiny door set into some semi truck trailers’ main door. We do know they are only on refrigerated “reefer” trailers. When Jalopnik interviewed truckers, some said the truck’s tiny doors are called “fruit doors” and were engineered to slide a single crate in or out. Others called them “inspection doors” and say they’re so checkpoint officials–such as at the produce checkpoint on the California border–can confirm the trucker is carrying what they say they are. The weirdest part is that some trailer manufacturers decline to comment on the purpose of the tiny doors. But the truth is that reefer trucks have one at the rear and one high on the trailer, at the front, because they’re used to vent out a trailer, especially when the inside is wet and the refrigeration unit is turned off on a back haul. Read more about the tiny doors in semi truck trailers.

Why do some semi truck trailer doors have a quilted pattern?

Many semi truck trailer manufacturers offer a stainless steel door skin upgrade. Why? The material is easier to clean and better resists corrosion than a regular painted door skin. But there’s only one problem with this shiny stainless steel: It forms a huge mirror and can hit drivers following the truck with deadly glare. The Utility Trailer company explains, “The high gloss quilted design presents a unique appearance and reduces glare to following vehicles.” The multi-pane pattern prevents any one driver from getting hit with all the sun’s glare. Jalopnik tested this theory and found that driving behind a semi truck with a quilted door–even with the sun directly behind you–was less bright than getting hit by an oncoming car’s high beams. Read more about quilted semi truck trailer doors.

Why do some semi truck tires not touch the ground?

Another bizarre feature you might notice on some semi trucks is one or more axles of wheels not spinning because they’re hovering above the road. These are “drop axles” the trucker can press into action by hitting a button. Many are on air bag suspension, so as the onboard compressor inflates the air springs, these wheels touch the road and then take weight off the rest of the wheels. The trucker may just use them when the rig is carrying a heavier load. Or, they may use the drop axles on certain roads for a bizarre regulatory reason.

What’s the big silver keg hanging beneath some semi truck trailers?

No, these shiny round tanks hanging beneath semi truck trailers aren’t actually beer kegs. But that doesn’t mean these trailers aren’t hauling beer. They are auxiliary fuel tanks, often to run a generator that operates the “reefer” trailers’ refrigeration unit. Find out why reefer trailers need a standalone refrigeration system that can operate when their truck abandons them.

Why do some semi-truck trailers have a red, diamond-shaped sign?

Semi trucks may be carrying all kinds of things. But in the case of a crash, emergency crews will be especially interested in whether a given trailer’s contents is flammable, poisonous, or an inhalation hazard. The DOT requires semi truck trailers have durable, diamond-shaped signs warning of dangerous cargo. So many trailers get a diamond-shaped placard with metal pages that turn–much like a book–to reveal various warning symbols. See all the DOT hazmat placards.

Why do some semi truck trailers have a huge plastic kiddie slide underneath?

This recent development on some semi truck trailers looks bizarre indeed. But they make much more sense when you realize how important aerodynamics is to a big rig. Modern trucks are trying all kinds of things to break the mythical 10 mpg, thus driving down shipping costs and increasing profits. The space beneath a semi truck trailer creates a bit of vacuum, pulling the truck backward. Then, when fresh air tumbles in to fill the space, that air hits the rear axles and slows the truck down more. The “UnderTray” system is an increasingly popular plastic aerodynamic element engineered to increase MPG. See the rest of the components of SmartTruck’s UnderTray system.

Why do some semi truck trailers have fins?

Just like the above UnderTray system, fins attached to the back of a semi truck are engineered to improve its MPG. They improve airflow behind the truck by allowing air in to fill the space behind the trailer gradually, instead of tumbling in all at once. While long tail cones might be more effective, they would make semi truck trailers too long for certain states or municipalities. The “TrailerTail” system can fold away when necessary.

Why do some semi truck trailers have bars hanging from the back?

Rear-ending a semi truck could be deadly. If your car slides beneath the bed of the trailer, the shape of it could go right over your hood and smash through your windshield. For this reason, trucks must have some kind of crash structure below a certain, legislated height. Enter the “Mansfield bar” which is attached to semi truck trailers to protect motorists following behind. See the study that reveals it’s more dangerous to sideswipe a Mansfield bar than hit it head on.

Why do so many semi truck trailers have Maine license plates?

It turns out “Vacationland” is one of the most popular places to register a semi truck trailer. No, it’s not because truckers love lobster. Many of these trailers have never visited Maine. The state’s low registration fees and lax laws make it an ideal place to register a trailer–even if your trucking company is headquartered elsewhere. The result is that Maine has more semi truck trailers registered than tractors to pull them.

So there you have it: From zany to controversial to surprising safety features. Knowing more about the strangest features of semi truck trailers will make the highways less confusing. And arm you with some interesting trivia for your next roadtrip.