When you’re on a road trip, some things are easy to overlook: those black tubes stretched across the road, those colored balls on power lines. But what about those giant domes on the sides of highways? Ever noticed those? Most folks just pass right on by those storage domes each day, never knowing why they’re there or what they’re for. But, in case you’ve ever been curious (as I was), they’re actually used to store mounds of road salt!

Cities & states have to have a plenty of salt on-hand to keep roads de-iced & drivable in winter. It’s more cost-effective to store the salt in bulk in those domes on the side of the highway, since road salt doesn’t go bad as long as it’s kept dry. Just how much salt can they contain? Well, a typical dome with a 150-foot-diameter & 12-foot-tall concrete walls can store up to 19,000 tons of salt. So, whenever roads get covered in snow and/or ice & need to be cleared, workers have easy access to more salt than they’ll likely need.

So, that explains the size of the domes, but why do they have that unusual shape? Well, the reason it’s a dome rather than a four-walled building is because the structure of the dome maximizes the available space inside. There’s an architectural term, “clear span”, that describes the unobstructed area between supports. That’s what a dome provides: a large, tall indoor space without support beams, so salt trucks can freely move in & out. Plus, they’re also made of specific materials which serve a specific purpose: the domes are made of concrete, wood beams, and roof shingles, instead of metal, since salt is corrosive.

The way de-icing with salt works is that salt lowers the freezing point of water, which is normally 32°F, to between 20°F and 2°F. And no matter how old it gets, salt never loses its ability to melt ice. So, until roads start being made with built-in salt, salt storage domes are probably going to be a fixture on your car trips for a long time to come.

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