> ... because salt is mostly usable for light to moderate snowfall and ...

Perhaps more important - salt's effectiveness fades as the temperature decreases. Sand and gravel do not have that problem. So if you're running the Road Dept. in an area where serious cold ain't some rare event - why would you bother with salt?

EDIT: I know the "melt to pavement, solar heating finishes the job" tactic. Which can work with heavier snowfall, if you plow/shovel before salting. Colder weather inhibits both halves of the melt-&-heat. (Plus the further north you are, the shorter & slantier the sun's rays get, even on clear days.)