What Are Those Domes on the Side of the Highway Used For?

It's easy to ignore some things you see on car trips, like the black tubes that sometimes stretch across the road, or the brightly colored balls on power lines. The giant storage domes that pop on the sides of highways are harder to miss. Many people pass the unusual roadside architecture every day without knowing its purpose. According to Canada Salt Group Ltd., the structures are used to store massive amounts of road salt for snowstorms.

States and municipalities need a lot of salt to keep roads safe and ice-free during the winter. Road salt doesn't go bad as long as it's kept dry, so it's cost-effective to store the material in bulk on the side of the highway. Domes with 150-foot-diameter, 12-foot-tall concrete walls can store up to 19,000 tons of salt. Whenever a snowy or icy road needs to be cleared, workers have easy access to more salt than they hopefully need.

Storing ice in bulk explains the size of these structures, but not their unusual shape. The reason road salt is stored in domes instead of more conventional four-walled buildings is to maximize the clear space inside. In architecture, the term clear span describes the unobstructed area between two supports. A dome design allows for a large, tall indoor space without support beams, so the trucks that transport the salt can move freely inside it. The materials these domes are made of also serve a purpose. Because salt is corrosive, the storage sites consist of concrete, wooden beams, and roof shingles instead of metal.

No matter how old it gets, salt never loses its ability to melt ice. This is because salt lowers the freezing point of water, which is normally 32°F, to between 20°F and 2°F. So until roads with built-in salt become a thing, storage domes will likely remain a fixture of our roadways.