To melt the ice on your driveway, you can use two moles of rock salt (#NaCl#) or two moles of calcium chloride (#CaCl_2#) which solute will have the greatest effect and why?
Calcium chloride dissociates in aqueous solution to give 3 particles, sodium chloride yields 2, so the former material will have the greatest effect.
Freezing point depression is a colligative property, which depends on the number of solute particles. Clearly, calcium chloride delivers 3 particles, whereas sodium chloride delivers only 2. As I recall, calcium chloride is a hell of a lot more expensive than rock salt, so this practice would not be too economic.
Incidentally, this practice of salting the roads is one of the reasons why older cars in North America tend to be absolute rust buckets. If you look at the undersides and panels of some of the older vehicles, they have suffered such corrosion from salt splashing on their chassis that the only thing that holds the car together seems to be rust.