Thinking back to a visit to Pompeii a couple years ago I learned that the pavements and crossings were raised off the road so pedestrians could walk without having to tread in mud and poo and whatnot, and the “stepping stones” for the crossings were placed to allow for the wheels of carriages to pass through unhindered. My question is; does this mean there would have been a widely accepted and used standard for the making of carriages to pass through the crossings in Roman cities? I can’t imagine it’d be a particularly good way of making crossings if any cart outside of that specific city was built to have a wheel base and wheel size that didn’t comply with the local crossings. If so how would such a standard be managed when the only ways to communicate were by letter or by travelling yourself.
Roman street width was regulated in order to allow wheeled traffic to pass through early on in Rome and, later, in the provinces, and we can see this through both legal and archaeological sources. Legal regulation of Roman road width goes as far back as the fifth century BCE Twelve Tables, which give a required road width on a straight track and a bend. Legally, by definition a via had to be eight Roman feet, a width which could accommodate the wheeled traffic we have evidence for. Anything narrower was an actus, which was wide enough for a pack animal, or a semita or iter, both types of narrower paths. And while the small distinction between being called a via and not might seem trivial, that eight feet threshold is reflected in archaeological evidence, meaning that the eight feet width was important for keeping Roman street networks functional.
On the archaeological side, vehicular traffic in Pompeii has attracted quite a bit of attention, and it lines up with what we get from the legal sources. Wheel ruts have been used to look at patterns of movement and use through the city, and we can see one-way and two-way streets there. Notably, certain wheel ruts there may have been intentionally placed, not the result of regular wear, in order to create a ‘railway’ of sorts to guide carts. Since this railway functioned by giving cart wheels guideline tracks to keep them moving straight along the road, there’s a definite assumption that the carts its guiding are all of roughly the same size and will fit into the track.
In addition to our evidence from Pompeii, one place where we can really see the importance of this size (even where it may not be related to vehicular traffic) is in the evolution of urban streets in Sagalassos, where a lot of work has been done on road and water infrastructure. Leaving aside the North-South Colonnaded Street, the grand, monumental entrance to the city center which while much wider than other streets seems not to have been built for wheeled traffic (it included staircases), Sagalassos had a network of main and secondary roads whose widths varied depending on the importance of the road. Most of the main roads were between 5 and 6.7 m (this is pretty typical for main streets in Asia Minor and so in that Sagalassos fits with the larger region), while secondary streets were between 2 and 4 m (2.4 m is the legal threshold for a via). It’s important to note here that Sagalassos was situated at a very mountainous site, and the steeper streets would have likely been inaccessible to vehicular traffic because of their grade. Streets that followed the contours of the terrain would not have had this problem, however, but the question of whether Sagalassos had regular vehicular traffic is uncertain. That said, streets at Sagalassos accommodate the same width threshold we see in Pompeii and other cities. Even as streets underwent a process of encroachment in later Antiquity, a narrowing we can see from stratigraphical evidence and changes in the water infrastructure going along the roads, the streets in Sagalassos largely continue to follow the same eight feet width. They narrow to eight feet, but then no further. That streets undergoing narrowing stop at this threshold precisely indicates pretty heavily that this standard width was important to meet.
Given the strong evidence that Roman streets were built to standard widths that would facilitate the two-way passage of vehicular traffic for the wheeled traffic we have evidence for, it would seem that there was indeed a roughly standard size for vehicular traffic. Since the roads were known, standard, legally-set widths, anyone making a cart would know the dimensions they needed to make their cart in order to fit on those roads — a cart that’s too big and doesn’t fit the roads isn’t particularly helpful — and would end up with a cart roughly the same size as any other cart intended to be used on a standard eight foot Roman road.
Jacobs, Ine and Marc Waelkens. “Five Centuries of Glory: The North-South Colonnaded Street of Sagalassos in the First and the Sixth Century AD.” Istanbuler Mitteilungen 63 (2013): 219-266.
Kaiser, Alan. Roman Urban Street Networks: Streets and the Organization of Space in Four Cities. New York: Routledge, 2011.
Martens, Femke. “Urban Traffic in the Hills of the Eastern Mediterranean: The Development, Maintenance, and Usage of the Street System at Sagalassos in South-Western Turkey.” In La Rue Dans L’Antiquité, edited by Pascale Ballet, Nadine Dieudonné-Glad, and Catherine Saliou. Rennes: Presses universitaires de Rennes, 2008.
Tilburg, Cornelis van. “Traffic Policy and Circulation in Roman Cities.” Acta Classica 54 (2011): 149-171.