Minecraft wiki entry for Building_a_metropolis, associated text: Intercity Highway:
Recommended material: Stone, obsidian, or other road-like materials.
Curves around things, but avoids sharp curves at all costs. Generally stays flat and direct with no stopping areas. Only becomes steep to go through mountain passes. Does not dodge light construction, simply tramples straight through it. Bridges anything but moderate-sized oceans. Typically 15 to 23 blocks wide, but may be smaller in pre-industrial societies.