Adding salt lowers the melting point of ice, so that it stays a liquid down to lower temperatures. Obviously having liquid water on the road is much less dangerous than ice!
Salt lowers the freezing point of water, due to the addition of salt ions which form bonds with the water atoms. By adding salt to ice, it is able to dissolve in the liquid water within the ice, and hence melt the ice on the road.
Sometimes, however, if there is really thick ice on the road below a certain temperature, the salt cannot dissolve in the water around it and destruct the solid structure of the ice. In this case, sand is often spread over the ice, to provide a form of friction. (I saw this a lot when I lived and studied in Finland!)
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