Road salt is made from rock salt that is derived from underground mines and crushed. The rock salt is treated with an anti-caking agent for optimal effectiveness. Rock salt is coarser than table salt, but is still made from sodium chloride.
Road salt is purified after it is mined, and is also infused with iodine. The underground mines where it is derived from were formed by the evaporation of ancient seas. The mines can be found all over the country, and all over the world.
During particularly bad winters, salt may become more scarce and in turn, more expensive. Salt alternatives have been used at these times, including beet juice and cheese brine.