1. They were the key to Rome’s military might.

The first major Roman road—the famed Appian Way, or “queen of the roads”—was constructed in 312 B.C. to serve as a supply route between republican Rome and its allies in Capua during the Second Samnite War. From then on, road systems often sprang from Roman conquest. As the legions blazed a trail through Europe, the Romans built new highways to link captured cities with Rome and establish them as colonies. These routes ensured that the Roman military could out-pace and out-maneuver its enemies, but they also aided in the everyday maintenance of the Empire. Reduced travel time and marching fatigue allowed the fleet-footed legions to move as quickly as 20 miles a day to respond to outside threats and internal uprisings. Even the most isolated parts of the Roman world could expect to be swiftly supplied or reinforced in the event of an emergency, lessening the need for large and costly garrison units at frontier outposts.

2. They were incredibly efficient.

Since Roman roads were designed with speed of travel in mind, they often followed a remarkably straight trail across the countryside. Land surveyors, or “gromatici,” began the building process by using sighting poles to painstakingly chart the most direct route from one destination to another. The resulting roads often shot straight up steep hills, and small bridges and tunnels were built to ensure the path could traverse rivers or pass right through mountains. Even in instances where the road was forced to divert from its course, the Romans typically opted for sharp turns and switchbacks over sweeping curves to preserve their arrow-straight design. Britain’s Fosse Way, for example, only veered a few miles off course over its entire 180-mile distance.

3. They were expertly engineered.

Roman builders used whatever materials were at hand to construct their roads, but their design always employed multiple layers for durability and flatness. Crews began by digging shallow, three-foot trenches and erecting small retaining walls along either side of the proposed route. The bottom section of the road was usually made of leveled earth and mortar or sand topped with small stones. This was followed by foundation layers of crushed rocks or gravel cemented with lime mortar. Finally, the surface layer was constructed using neatly arranged blocks made from gravel, pebbles, iron ore or hardened volcanic lava. Roads were built with a crown and adjacent ditches to ensure easy water drainage, and in some rainy regions they were even nestled on raised berms known as “aggers” to prevent flooding.

4. They were easy to navigate.

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As the made their way down one of Rome’s many roadways, weary travelers could guide themselves by a detailed collection of mile markers. Much like the road signs on modern interstates and freeways, these stone pillars gave the distance to the nearest town in Roman miles and instructed the traveler on the best places to stop. They also provided information on when the road was built, who constructed it and who last repaired it. To embody the idea that “all roads lead to Rome,” the Emperor Augustus even saw that a so-called “golden milestone” was placed in the Roman Forum. Cast from gilded bronze, this monument listed the distance to all the city’s gates and was considered the convergence point of the Empire’s road system.

5. They included a sophisticated network of post houses and roadside inns.

Along with road signs and mile markers, Roman roads were also lined with state-run hotels and way stations. The most common of these ancient rest stops were the horse changing stations, or “mutationes,” which were located every ten miles along most routes. These simple posthouses consisted of stables where government travelers could trade their winded horse or donkey for a fresh mount. Switching horses was especially important for imperial couriers, who were tasked with carrying communications and tax revenues around the Empire at breakneck speed. By stopping off at multiple posthouses, couriers could move as far as 60 miles in a single day. Along with the more common “mutationes,” travelers could also expect to encounter roadside hotels, or “mansiones,” roughly every 20 miles. Each “mansio” offered basic lodgings for people and their animals as well as a place to eat, bathe, repair wagons or even hire a prostitute.

6. They were well-protected and patrolled.

To combat the activities of thieves and highwaymen, most Roman roads were patrolled by special detachments of imperial army troops known as “stationarii” and “beneficiarii.” These soldiers manned police posts and watchtowers in both high traffic and remote areas to help guide vulnerable travelers, relay messages and keep an eye out for runaway slaves. They also doubled as toll collectors. Like modern highways, Roman roads were not always free of charge, and troops were often waiting to levy fees or taxes on goods whenever the route reached a bridge, mountain pass or provincial border.

7. They allowed the Romans to fully map their growing empire.

Much of what historians know about Rome’s road system comes courtesy of a single artifact. Named for its medieval owner, Konrad Peutinger, the Peutinger Table is a 13th century copy of an actual Roman map created sometime around the 4th century A.D. This eye-catching atlas was drawn on a 22-foot-long collection of parchment and shows the entire Roman world in full color along with several thousand place names. Cities are illustrated with sketches of small houses or medallions, but the map also includes the locations of lighthouses, bridges, inns, tunnels, and—most importantly—the Roman highway system. All the major Roman roads are listed, and the map even gives the distances between various cities and landmarks. The Peutinger map has proven indispensable to scholars studying the Roman transit system, yet historians still debate its original purpose. Some have claimed it was a field guide for government figures traveling on official business, while others contend it was displayed in an imperial palace.

8. They were built to last.

Thanks to their ingenious design and careful construction, Roman roads remained technologically unequaled until as recently as the 19th century. But while modern asphalt highways might offer a smoother ride than the Via Domitiana or the Appian Way, Rome’s 2,000-year-old roadways take the prize for durability. Many Roman roads were used as major thoroughfares until only recently, and some—including the Via Flaminia and Britain’s Fosse Way—still carry car, bike and foot traffic or serve as the guiding route for highways. Rome’s enduring engineering legacy can also be seen in the dozens of ancient bridges, tunnels and aqueducts still in use today.