Ever wonder why this is done. It has nothing to do with traction as many believe. Salt in the chemistry world is reffered to as a colligative property, meaning it can lower and raise the freezing and boiling points of substances it dissolves in. Road crews began adding sand when they realized the dirtier salt was attracting radiant heat from the sun and melting/dissolving faster with the ice.
You’ll note you put salt in water when cooking too. Causes it to boil at a higher temperature.