Sugar water freezes faster than salt water, because salt has more molecules than sugar. Normally, water freezes at 32°F, however, when a substance is added to the water, it lowers its freezing point. This is not because of the chemical nature of the substance being added, but rather the number of molecules. This is referred to as a colligative property.
While sugar lowers the freezing point of water, salt lowers it even further. If 0.35 ounces of salt is put in 3.53 ounces of water, the freezing point drops to 21.4°F. If the same amount of sugar is added to the same amount of water, the freezing point still drops, but only by one degree to 31°F.
There can be up to six times as many molecules in salt as there are in sugar, which accounts for the difference in freezing points. Because of this, and because of the general availability of salt, it is used to treat roads that are frozen. This helps to lower the freezing point, and therefore prevents ice from forming on the roads.