Everywhere the Roman Empire went, a priority was the construction of a quality system of roadways. These facilitated rapid movement of the Roman legions, especially on the viae militares (military roads). These were also used for commerce, as were the viae publicae (public roads). The greatest of the Roman roads were the Appian Way (via Appia) and the Flaminian Way (via Flaminia).
Construction of a Roman road began with the agger, a foundation built to ensure drainage, which provided a stable base for the road. Without proper drainage, any roadbed would become uneven over time. The agger consisted of a drainage ditch which was built up again above ground level. The agger was filled with materials that were found to hand, usually large, tightly packed stones. This foundation was topped by one or more layers of gravel, sand and stones, used to give the road some flexibility and resilience. Sometimes the underlying agger was wider than the upper layers, providing a rough "shoulder". The road culminated with the "metalling", a gravelled or pebbled surface mixed with crude concrete, mortar, or even slag from local ironworks. A few major roads were then topped again by stone slabs set in mortar.
This layering meant that the road was typically raised above ground level - from 3 to 6 feet or more, depending on the materials used and the importance of the road.
The road was highest in the center, sloping to the sides to clear runoff and debris to the sides, which were usually lines with curbs and drainage ditches. Another ditch was often dug some ways to the side of the road, providing a cleared area that deprived highwaymen of hiding spots.
Roman roads were usually made arrow straight between point of origin and destination. They would build over swamp and bore through mountains rather than divert the road's course.
For more information, see:
http://www.unc.edu/courses/rometech/public/content/transport/Adam_Pawluk/Contruction_and_Makeup_of_.htm
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