Minecraft wiki entry for Building_a_metropolis, associated text: Small rural gravel or partial-stone road:
Recommended material: Gravel or stone.
Should wind around any natural structure or well-established construction. Feel free to cut away absolutely any plant life, but limit pickaxes to flattening hillsides for road to be placed on. (This doesn't mean flattening mountains — just small hills.)
Should be 3 to 7 blocks wide.
Should generally be under 1/3 slope.