That depends on the forumla you get. The one I have can be used on top of snow that is up to 2inches (the one or two times I used it last year when it wasn't forecasted to snow but only snowed a inch or so I used it ontop of the snow and worked just fine though like I mentioned above I used it for the sidewalk and the front steps but the bag doesn't say "hey don't used for a driveway" it's just that people generally don't do it). If the snow is more than 2 inches then you shovel first. Granted in the OP's case it's doubtful they would get just 2 inches or less at a time given the amount of snow they get per year but in my area that's entirely possible. Though as I mentioned before it tends to work best if put down before it snows so it can help prevent icing up and helps melt the snow quicker.