This winter has been unusually severe across much of the United States, requiring transportation agencies to put down large amounts of salt to melt snow and ice on roadways.

Last year, U.S. officials applied about 17 million tons of salt to roads, says Lori Roman, the president of the Virginia-based Salt Institute, an industry group. Roman says the totals are on track to be higher this year.

Ron Wright, who manages snow and ice removal for the state of Idaho and co-founded the industry group Pacific Northwest Snowfighters, says this winter has left transportation departments scrambling for salt, especially in the East and Midwest. However, Chicago Mayor Rahm Emanuel insists that the Windy City will have enough road salt to meet the challenge of this winter, as has New York Governor Andrew Cuomo.

Much sought after this winter, salt is a fascinating commodity.

View Images Rock salt is loaded at a facility near Detroit, Michigan; the city has its own rock salt mine. PHOTOGRAPH BY PAUL SANCYA, AP

Where Does All That Salt Come From?

The salt used on roads is often called rock salt, because its grains are much coarser than table salt. It's still the same molecule—sodium chloride—but table salt is ground, purified, and often has additives like iodine (in order to decrease the incidence of goiters) and anti-clumping agents.

There are three primary ways to make salt, says Roman. The sea salt often used in gourmet cooking is produced by evaporating seawater, typically using the sun. Table salt used in foods is often captured through "solution mining": Water pumped underground dissolves the salt, which is then recovered in solution. The solution is dried; the salt crystallizes out.

Most rock salt for roads is mined "dry" from underground seams of crystal salt, which formed from the evaporation of ancient seas. Miners follow shafts underground and break out slabs of the salt with dynamite and powered shoveling machines. Trucks or conveyors haul the salt to crushing machines.

Rock salt "comes from all over the world," says Roman. "There are mines all over the United States," notably near Cleveland, under Detroit, and in New York State, Kansas, and Louisiana.

View Images A New Jersey Department of Transportation truck spreads rock salt on southbound Interstate 295. More than 17 million tons of salt will be spread on roads this year. Photograph by Tom Gralish, The Philadelphia Inquirer/AP

Detroit's Natural Salt Stash

The rock salt under Detroit was first discovered in 1895. Development of the resource proceeded slowly until 1914, when the salt mine produced 8,000 tons of rock salt each month, much of it hauled by donkeys, lowered into, but reportedly never brought back up, from the mine. Because of its local resource, Detroit was the first city in the world to apply salt to its roads in 1940.

Detroit's salt mines closed for a time in 1983, due to falling prices for salt, but they were reopened in 1998. The massive complex includes more than 100 miles (160 kilometers) of tunnels about 1,200 feet (365 meters) below the city and stretches from the suburb of Dearborn in the northwest to Allen Park in the southwest.

Managing the Supply

Most public works managers start winter with 125 percent of the road salt they think they'll need, says Ron Wright. They protect the salt with tarps or by storing it in buildings.

If more salt is needed as the season progresses, as has happened this year in many areas, they reorder as they go. It normally takes 10 to 14 days to receive the salt, says John West, director of public works for Maywood, Illinois, a Chicago suburb.

Roman says some towns are experiencing slight delays when reordering, but she says "there isn't a salt shortage."

Still, because of increased demand, rising prices may become a concern. West says his most recent reorders are covered under existing contracts for $54 a ton. If he needs additional supplies, he may be forced to buy salt on the open market, where prices have risen to around $100 a ton.

That could stress municipal budgets.

As for "salt alternatives" like beet juice or cheese brine, Roman doesn't have data on how many cities actually use such substitutes. Wright says substitutes don't eliminate the need for rock salt but can be used in conjunction with them to speed up melting or cut down on the amount of salt needed. But, she adds, "people will still need to use salt."

Most importantly, it saves lives. Roman points out that a study carried out by Marquette University in Milwaukee, Wisconsin, found that road salt reduced crashes by 88 percent, injuries by 85 percent, and accident costs by 85 percent.

"Some say snow fighters may save more lives than firefighters," she avers.