If you may spend sometime to consider this... According to Kinetic Theory. The particles in salt solution should have higher activity than the solid salt since they can move more freely. Thus the cation and anion (in solution) have more possibilty to collide and transfer energy to water molecules in ice to cause the ice melt faster... This should be reasonable explanation :)
I accept the fact that some salt can be used to melt the ice in solid form (Such as CaCl2) since they can easily absorb humidity to form solution itself. However, I do not accept that solid NaCl salt can be used directly as it will result slow melting rate... (I used to learn that in some very cold countries, such as Cananda, use NaCl salt solution with con.~ 20% - 30% to melt the ice on their road).