A lot of us take roads for granted, at least until they are icy or torn up by construction. The concept of creating fixed paths seems to be in our firmware. Finding the shortest distance to food or water and marking a trail to it from home base has always been its own reward.

Roads have come a long way from the simple paths beaten by man and beast. But the basic configuration of paved roads hasn’t changed all that much since the Roman empire. Whatever they’re made of, they need to be able to drain water and support heavy loads.

New issues arose as modes of transportation shifted in favor of the automobile. Road surfaces needed to provide friction against tires. But how did we get from the stone-paved roads of Rome to the asphalt and concrete roads of today?

All Roads Lead to Rome

There is evidence that the Mesopotamians were laying stone roads as early as 4000BC. The Bronze Age greatly advanced tool making, which made it much easier to cut stone. Around 2400BC, the Egyptians were building roads in order to transport the huge blocks of limestone that make up the pyramids. It was clear from the beginning that roads must be able to support heavy loads without buckling or shifting. Long-lasting, quality roads would require ingenuity and craftsmanship in equal parts.

The oldest existing paved road was built in Crete around 2000BC. It’s about twelve feet wide and is made of sandstone mixed with a clay-gypsum binding material. Two rows of basalt slabs run down the middle of it. The road slopes downward from the center on both sides to promote drainage, and some sections even have gutters.

The Romans drastically changed the course of transportation engineering because they built connected systems of sturdy roads. This idea was crucial to the spread of their control because they were able to move armies around and collect taxes.

The vast network of roads centered on the empire’s capital and drastically increased the capacity for trade with neighboring countries such as Greece. Over the course of their rule, the Romans laid some 50,000 miles of hard-surfaced roadways. These roads fostered communication throughout the empire and helped to create an extensive culture that informed much of Western civilization.

Regular Roman roadways were all created in roughly the same way. They averaged around fifteen feet wide and four feet thick, and were built using several layers of supporting material. Every new road project began just as they do today—with earthwork engineering. Workers dug a deep, level trench for the roadway with a ditch along each side for drainage. They would cover the trench with a layer of sand or small broken stones. These would be spread out evenly and then tamped down with rollers.

On top of this was a layer of large stones set in a mortar made of lime. Shards of bricks and pottery were often distributed on top of this stone layer and set with clay-based cement. On the surface was a course of large volcanic rock cobbles stones arranged as tightly as possible.

Important military roads radiating from Rome were wider and more elaborately built. The most famous one, the Appian Way, ran between Rome and the port of Brindisi and spanned about 350 miles. The Appian Way was broad enough that two carriages could pass each other with ease. Roads like this one were paved with thick blocks of silex arranged closely together, the gaps between them filled with gravel and sand.

Paving the Way for Modern Roads

Roads were built the Roman way for centuries. The solid, well-draining model provided traction for early wheeled vehicles and the horses that pulled them. Roads began to change in 18th century France when a transportation engineer named Pierre-Marie-Jérôme Trésaguet suggested that deep foundation courses of stone and gravel were unnecessary. He believed that solid subsoil would suffice to support the surface material.

Trésaguet proposed that roads be built with a course of thick stones laid on their edges at the bottom of a crowned trench. These vertical foundation stones would be pounded together to ensure a tight fit. A two-inch layer of broken stones sat atop the large foundation stones, followed by an inch-thick layer of gravel.

Around the same time, a Scottish stonemason named Thomas Telford began building roads similar to Trésaguet’s, though he put special emphasis on supporting a lot of weight. Telford’s foundations also used thick stones placed on edge, but they were not as tall as Trésaguet’s. These stones were carefully laid by hand. They varied in height, sloping downward from the middle toward the edges.

No More Than a Mouthful

The biggest paradigm shift since the Roman systems of roads began with John Loudon McAdam, another Scottish road builder. His legacy lives on in the “macadam” roads of today.

McAdam looked for cheaper methods that required less labor. He believed that the thick stone foundations of Roman engineering were completely unnecessary, and was certain that the issues of load support and drainage could be addressed with a bed of sharp-edged broken stones packed together.

McAdam insisted that these stones not exceed one inch in any dimension. His own rule of thumb was that a properly-sized stone would fit in the road worker’s mouth. He supplied roadside stone inspectors with one-inch rings so that they could keep tabs on size as workers broke up stones with small hammers.

McAdam built his roads as level as possible. The average rise from the edges to the center was only three inches. He didn’t believe in rolling the stones to tamp them down. Instead, he let traffic and time do the work. This made his road-building methods cheaper and therefore more attractive to government officials, although the settling period was hard on horses’ hooves.

Warming Up to Other Methods

McAdam’s roads were revolutionary. But the rise of the automobile forced certain changes in road construction. As tires sucked at the road surface, they kicked up a lot of dust and dismantled the road. An attempt to solve this problem was made by spraying the surface with a mixture of coal tar and ironworks slag. This material was patented under the name tarmac. Beginning in the 1920s, the stone aggregate and binding material were mixed together in advance. Coal tar was replaced by bitumen, as substance that occurs naturally and as a by-product of distilling petroleum.

Rubberized asphalt and snowmelt systems are among the most modern advances. Rubberized asphalt contains finely ground crumb rubber from old tires. This significantly reduces traffic noise and the number of tires in landfills, but the material can only be applied to roads within a certain temperature range.

Snowmelt systems use either electrical elements to heat the surface or a hydronic tube system to deliver a water and antifreeze mixture. These systems can be activated manually or start automatically when well-placed sensors detect moisture and freezing temperatures.

Sources

Benson, Fred and Maxwell Gordon Lay. “roads and highways.” Encyclopaedia Britannica. Britannica Academic. Encyclopædia Britannica Inc., 2015. Web. 8 Nov. 2015.

Holley, I.B. The Highway Revolution, 1895-1925: How the United States Got Out of the Mud. Durham: Carolina Academic Press, 2008. Book.

Kaszynski, William. The American Highway. Jefferson: McFarland & Company, 2000. Book.

Staccioli, Romolo Augusto. The Roads of the Romans. Los Angeles: Getty Publications, 2003. Book.