Which people united their empire with a road system that extended more than 12 000 miles?

The Romans are famous for having built a great network of roads around their empire and for having invented the stone-paved roads, which had a military purpose (they speeded up the movement of troops and made the transport of supplies to the soldiers at the front or stationed in garrisons quicker and easier).

The Romans built far more than 12,000 miles of roads, The network of roads in the empire totalled 400,000 kilometres (249,000 miles). The famous Roman stone-paved roads constitute 20% of this network; that is, 80,500 kilometres (50,000 miles). The other roads were either paved with gravel or were levelled earth roads.