The extraordinary greatness of the Roman Empire manifests itself above all in three things: the aqueducts, the paved roads, and the construction of the drains.
Dionysius of Halicarnassus, Ant. Rom. 3.67.5
At the zenith of the Roman Empire’s power and prosperity, over 400,000 km of roadways (viae) connected its cities, 85,000 km of which were paved. In the 2nd century, 29 military roads led out of Rome alone. In 20 BC, Augustus set up the miliarium aureum (golden milestone), near the temple of Saturn, in Rome. All roads were considered to begin from this gilded bronze monument, on which were listed the distances to all the major cities in the empire. Constantine called it the umbilicus Romae, and built a similar—although more complex—monument in Constantinople, called the Milion. Milestones along the roadside marked distances; the modern word “mile” derives from the Latin milia passum, which amounted to 1,000 paces.
Initially designed for the movement of troops, the main roads (viae munitae) were wide, had gutters and walkways on both sides, and a smooth surface of concrete poured over paving stones (the concrete has eroded over the millenia, leaving only the stone, giving the false impression of a bumpy ride).
The primary sources concerning Roman road laying are Vitruvius’s directions for making pavements and a passage in Statius’s epic poem, the Thebiad (1st c.), in which he describes the repairs of the Via Domitiana, a branch road of the Via Appia, leading to Naples. The following paraphrases both texts:
After the civil engineer looked over the site of the proposed road and determined roughly where it should go, the agrimensores went to work surveying the road bed. They used two main devices, the rod and a device called agroma, which helped them obtain right angles. The gromatici placed rods and put down a line called the rigor, trying to achieve straightness by looking along the rods and commanding the gromatici to move them as required. Using the gromae they then laid out a grid on the plan of the road. Using ploughs and spades, the laboratores excavated a fossa (trench) down to bedrock. The depth varied according to terrain. The road was then constructed by filling the ditch with layers of rubble, gravel, sand and stone. When within one meter of the surface, the bed was covered with gravel and compressed. This flattened pavimentum could be used as the road, or it could be completed with a statumen of flat stones, set in cement and crowned for drainage.