Having reviewed all of the ideas, I think that using the roofing paper is the best. I always have a roll of #30 weight roofing paper in the garage. This would be better than the lighter #15 weight, since it is thicker.
With the sticky back side up, (the side with no lines in it), the tar paper gradually gathers dust and bits of dirt and would look like a real road.
If you have a basement with slight dampness, or even if you think it has a totally dry floor, you should buy a roll and unroll it along the edges of the floor, against the walls, in the areas where you stack or store cardboard boxes. The paper seals down on the concrete, and your boxes will never get damp, soft or mildew-y. Every time somebody comes into my basement and sees it, they say, "Wow, I should do that."
Mannyrock