Standard Roman roads consisted of a metalled surface (ie gravel or pebbles) on a solid foundation of earth or stone.
A simple yet technologically advanced plan was in place and implemented for the construction of each road.
Where possible, roads were built in the straightest line possible, only avoiding major terrain obstacles where it made practical sense. A Roman road was a multi-layered architectural achievement, but the construction process was fairly simple to define.
First the two parallel trenches were built on either side of the planned road, with the resulting earthworks, stone, etc., being dumped and built up in the space between the two ditches. The Agger, as this was called, could be up to 6 ft. (1.8 m) high and 50 ft. (15 m) wide. Alternatively it could be very slight or almost non-existent as was the case with most minor roads.
Next, the diggers would make a shallow 8 to 10 foot wide depression down the length of the agger, and line the edges with kerb stones to hold the entire construction in place. The bottom of this depression would then be lined with a series of stone fillers. 6 to 8 inch stones would form the foundation layer, with fist sized stones placed on top. In early roads the remaining gap would then be filled in with course sand to fill between the stones and to cover them by approximately 1 ft.
Later roads may have used Roman volcanic concrete to mix the entire mixture together making the whole structure more solid. The road surface was then laid down using large, tight fitting, flat stones that could be found and transported locally. These larger surface stones would be cut to fit when possible to make the surface as smooth and seamless as possible.
Bridle paths were then dug and smoothed, leaving the earth unpaved for horse travel. The roads were built for infantry, and it was easier on horse hooves to walk alongside the stone roads. Though the Romans did use horseshoes, they were tied on to the hooves, not nailed, making them unstable. Additionally, during the construction, forests and obstacles on either side of the road could be cleared to a considerable distance to guard against ambush attempts.