For five years, archaeologists meticulously examined historical records, old journals, and archaeological sites. They then used satellite imagery and aerial photography, including newly digitized World War II photographs, to find evidence.
Scientists analyzed the topography, where ancient sources had indicated lost routes, searching from above for hidden traces. This included subtle variations in vegetation, soil variations, and changes in elevation, along with Roman engineering features such as raised ridges or manicured hillsides, which ultimately revealed the forgotten paths.
"It's become a huge game of connecting the dots on a continental scale," said archaeologist and co-author of the study published in Scientific Data, Tom Brugmans.
The data and an interactive digital map are also available online for researchers, history teachers, or anyone interested in the history of ancient Rome.
Brugmans, a researcher at Aarhus University in Denmark, explained that previous research focused on the "Roman Empire's highways"—the main roads most frequently mentioned in familiar historical records. The updated map, however, fills in more obscure details about "secondary routes, such as rural roads connecting villas and farms," and other locations.
Researchers had previously estimated the length of Roman roads at approximately 117,163 miles (188,555 kilometers). The new work reveals nearly 186,000 miles (300,000 kilometers) of roads across the Roman Empire, enabling travel from Spain to Syria.
The study added much to the knowledge of archaeologists about ancient routes in North Africa, the plains of France, and the Peloponnese peninsula in Greece.
Archaeologist Benjamin Duque of the German Archaeological Institute in Berlin, who was not involved in the project, stated that "this work will be fundamental to many other research projects." However, he pointed out one caveat: it remains unclear whether all these routes are open and active simultaneously.
Brugmans asserted that being able to visualize the ancient routes taken by Roman farmers, soldiers, diplomats, and other travelers would provide a better understanding of the major historical trends that relied on the movement of people during the Roman era, including the rise of Christianity across the region and the spread of ancient epidemics.
"The Romans left a huge impact through this road network," said co-author and archaeologist Adam Bagot of the Autonomous University of Barcelona, "which laid the basic groundwork for many of the roads that are still in use today."
He added that Roman engineering achievements in road construction and maintenance – including arched stone bridges and tunnels through the hills – continue to shape the geography and economy of the Mediterranean region and beyond.