The resulting upward transport of water vapor is particularly important in sandy or gravely soils. These soils have large pore spaces through which the water vapor can move easily. Where the pore spaces are large, the forces that move liquid water upward are less effective. Being something akin to capillary forces, they operate best when the pore spaces are small. Even after the roadbed is frozen, there still is upward migration of moisture through the frozen soil toward the cold road surface.
Frozen soil is funny stuff since it still contains liquid water.
As winter progresses, more and more water moves upward.