The Romans built more roads than any other pre-modern civilization.
As mentioned in a previous article two of the main types of Roman roads were the viae publicae and the via militaire or the public roads and the military roads. There were also actus, or local roads, and viae privatae or private roads. The viae publicae are generally the most important, but one should note that what were viae militaire at one point often became viae publicae as the empire expanded. This being the case, many if not most of the roads of Rome were built by the Legions. There are accounts of how this was done.
Plows to break the ground and teams of Legionaries with shovels to remove the loose earth were the normal method for excavating the ditches which are the basis of most Roman roads. Drainage and stability were the key factors in Roman road construction engineering practices. Wherever practical they would excavate down to bedrock, and refill the ditch with three layers of fill. If the bedrock was too deep they would pack the bottom layer, and then cover that with sand and gravel. A middle layer of gravel or rubble often in clay or lime to “cement” it together was then added, and a top layer of metaling consisting of gravel, or in iron producing regions iron slag or other iron waste. This later provided the added benefit of rusting together to form a solid road surface, and is likely why this top strata is generally called metaling even when it is gravel. The metaling is generally laid between two rows of kerb or curb stones, and larger curb stones sometimes surround the wider intermediate layer. The ditches were generally wider than the finished road surface, with the width of the layers diminishing as you get closer to the top.
These are all rough generalizations because if Roman roads have one definitive unifying characteristic it is that there is no uniform universal construction, but rather a basic plan adapted to circumstances and readily available materials. In some places large slabs of stone were used on the upper surface. In places where the ground is flat, level, and solid; kerbs with gravel between them were all that were necessary. Wherever they were built, however, Roman roads are generally laid out in straight lines without rounded curves. Roman surveyors could easily plot very long straight lines and level surfaces from high ground to high ground, but their curves tend to be angular rather than gentle and rounded like those of most modern roads. Not surprisingly, these angular turns tend to occur on high ground where the surveyors plotted a new line..
So, generalizing in a field where generalizations are often misleading we can say Roman roads were well drained and stable. They had three layers constructed in a dug trench or ditch, and were generally paved (metaled), curbed, and straight. Crushed flint and crushed limestone were both frequently used as the upper metaling layer where iron waste was not readily available.
