Salt water freezes slower than regular water because it has a lower freezing point. Unsalted water freezes when water molecules slow down from the removal of heat. The addition of salt creates more heat in the mixture, resulting in a lower freezing point.
The additional heat in salt water results from the process of separating salt from water molecules. This principle has many practical applications. For example, salt is spread over melting snow to prevent it from freezing into ice. Because salt can be corrosive to cars and building materials, it is usually mixed with sand or gravel for de-icing purposes.