Minecraft wiki entry for Building_a_metropolis, associated text: Small rural stone-material road:
Recommended material: Stone, cobblestone, stone bricks
Should wind around any natural structure or well-established construction. Feel free to gouge out rock-formations, place support-beams, etc as needed for road to be placed on.
Should be 3 to 7 blocks wide.
Should generally be under 1/3 slope. But avoid sloping in general as long as it doesn't mean tunneling through mountain-ranges or bridging valleys.