I read in the run up to the Big Blizzard that Mayor Bloomberg has 250,000 tons of salt ready to spread on the streets of New York. The blizzard is happening on a late Friday-early Saturday morning, so the impact on the New York City economy is not as significant as it would be if the blizzard were occurring on a Monday morning, thereby disrupting office workers' commute.
From the work I did at the NYC Comptroller's Office in 1992-2006, when I served as chief economist, I know that the crucial variables in a snowstorm for determining economic impact are the timing, the precipitation and the temperature (TPT). The impact is reduced if the snow is on a weekend, if the precipitation is low (two inches is where trouble can start) and if the temperature is above 32 degrees Fahrenheit.
Putting down salt allows the City of New York to reduce the impact of freezing temperature on the ability of commuters to get to work, or shoppers to get to stores. One of the worst scenarios is a slushy snowfall and then a deep freeze, causing icy roads.
In the laboratory, adding salt (sodium chloride) to water can bring down the freezing point from 32 degrees Fahrenheit to several degrees lower. It is hard to define this lower number because it depends on how much salt one puts down on the roads (the more salt that is added, the higher the salinity percentage and the lower the freezing point of the water). In a lab, the freezing point can be brought down lower than in storm conditions.
So salt is only useful to add when the temperature is below the freezing point for salty water - otherwise the salt water will simply run off into the ground or into sewers. By the time temperatures drop, if they do, the salt water is gone.
Similarly, below some temperature, adding salt is a waste because it is below the freezing point of water, unless temperatures are expected to rise above freezing, in which case the salt will speed up melting.
Finally, many people with ecological concerns argue against using any salt at all. Salt is corrosive of vehicles and the roads themselves. The runoff is terrible for plants and marine life. Pets that play outside get the salt on their paws and suffer from the abrasion. Read about this in "Why You Shouldn't Use Salt to Melt Ice."