The first thing to note is that when we talk of “Roman roads”, we almost always mean the viae publicae, roads built by public officials (Initially censors who let out the contracts to build such public works, although later roadways get named after the consuls and praetors who constructed them as the Romans build more of them) and was maintained by the state. But of course these major state highways existed within a wider network of local roads (a via vinciales or actus, pl. actus – everybody loves the Fourth Declension! It’s a step in the right direction) which might or might not be private (a privatum iter). That distinction is important, because it wasn’t that all Roman roads were of the high quality we tend to think of – the roads we’re thinking of were prestige projects undertaken by the state, but a whole [mess] of private lanes and dirt paths existed too.

That said, for the major viae publicae, the combination of archaeologically preserved examples and a few references in literary sources gives us a good sense of what “best practices” for Roman road construction were. To start with, obstructions were cleared and then a trench was dug where the road would go. The trench was then filled with three packed layers: a stone layer, then a gravel layer and finally a sand or cement layer. On this was placed the surface of stone blocks, cambered so that rainwater drained to the sides of the road. The stone surface was then held in place by an umbo (literally a “knob” or “swelling”, the same word is used for shield bosses) on the edges of the road. Within a town, the umbo might in turn border a stone sidewalk, but out in the countryside, that wasn’t present.

On the one hand, we know that the high-quality construction of Roman paved roads impressed in the ancient world; Dionysius of Halicarnassus proclaims them one of Rome’s three most magnificent works (alongside aqueducts and sewers, Dion. Hal. Ant. Rom. 3.67.5; note also in this vein Strabo 5.3.8; Plin. HN 36.125). On the other hand, we also know that not all Roman roads were built to the same high standard. While we know that various Roman legislation spelled out required road widths, for instance, the actual width of Roman roads varied widely. And while we’re here, I should note, the story you have heard that modern roadway width or standard railway gauge being based on Roman road width (or, heaven help us, Roman “war chariots”, a thing the Romans did not have) is rather wrong. Nor were they always well-constructed or maintained; Julian, for instance, bemoans a poorly maintained road in a letter to Libanius in 363.

Also, if I may stop a moment, one thing I hear frequently said about Roman roads is that their survival serves as some sort of indictment on modern road construction, “they don’t build them like they used to!” There are a few layered bad assumptions here. The first, of course, is survivorship bias; that road outside of Chalcis that Julian complained was sinking into a swamp probably isn’t available for us to see and similarly badly maintained or poorly constructed Roman roads are simply gone or only visible with archaeological methods and so unavailable for comparison. At the same time, modern roads are asked to do things which ancient roads were not; I do not suspect many Roman roads would last very long if hundreds of 20-ton (or more) trucks were rumbling over it daily. Indeed, preserved Roman roads outside of the cities often have deep ruts worn several inches deep in the stone from the passage of carts (carts and other vehicles were generally banned inside of Roman cities; city streets were for pedestrians). Finally we know that Roman roads, just like modern ones, required maintenance and reconstruction fairly regularly in the period of their use; Laurence (op. cit., 66) has a neat table of inscribed milestones on the Via Appia, for instance, referring to repair or reconstruction, with sections of the roadway repaired in stages from 97 to 110AD.

Nevertheless, the generally high quality of Roman roads, as noted, was recognized in antiquity and is still apparent in their survival today. In contrast to many unpaved sections of Persian royal roads, for instance, Roman viae publicae were paved as a matter of course and standard widths (c. 3.5m wide; the oldest Roman laws, the Twelve Tables, set a minimum width on straights of c. 2.4m though as noted this was not always followed) were fairly generous. And they tended to be well-engineered, with relatively flat surfaces (which are often quite a bit less flat today due to ground shifting as well as the erosion of concrete between paving stones) and straight lines, though the notion that all Roman roads were ruler-straight is, of course, wrong (though some are!). In particular, the Roman system of road construction, while it demanded considerable up-front labor costs, was seemingly designed to keep long-term maintenance demands low. This was a style of road building which accepted big up-front costs in exchange for lower long-term maintenance, which of course demands a lot of initial state capacity to manage the costs and labor demands (something Rome’s successors would mostly lack).

So as roads go, the Roman ones were uncommonly good (albeit with a wider degree of variation than is often appreciated), but the marvel of the Roman roads is not that they had a few good, paved roads – states had been building paved roads for some time, as noted above – but that they had a massive system of them. Scale, more than quality, was the Roman achievement here, though the quality was also quite high.

Bret Devereaux, “Collections: Roman Roads”, A Collection of Unmitigated Pedantry, 2023-06-02.