Minecraft wiki entry for Building_a_metropolis, associated text: Recommended material, gray/black concrete and yellow terracotta.
Should have clear lanes.
Should have a lot of intersections and traffic lights.
Should be 3-9 blocks wide.
Should have many branch-offs.
Should cut through any obstacle that can be removed in less than a minute.
Should not be too steep.
Should wind around any other obstinate that is too big to remove.