Researchers are reshaping what we know about Persia's Royal Highway, the ancient backbone of the Achaemenid Empire. A new study from archaeologist Davide Salaris uses digital modelling to trace the routes once used by the Persian kings to move their courts across vast distances.
The research, published in Antiquity, reveals that the road between Susa and Persepolis wasn't just a simple path. It was a purpose-built corridor, engineered to handle the massive scale of royal movement.
The study challenges long-held ideas that the Royal Road was just another communication route. Instead, Salaris argues that these roads were designed for one key purpose: moving the empire's royal machinery -- kings, soldiers, officials, wagons, and animals -- smoothly and reliably across rugged terrain.
Historical texts describe royal convoys numbering in the tens of thousands. Supporting this scale of movement required more than dirt tracks. It demanded water access, wide and level terrain, and gentle slopes to support wheeled vehicles.
Salaris used GIS-based modelling to find the most likely travel corridors between the two ancient capitals. This approach goes beyond guessing from historical maps or modern roads.
By analyzing elevation data and terrain features, the study identified realistic pathways where roads could support royal-scale logistics. The result is a map of broad zones -- not just single lines -- where movement was most feasible.
The model applied a critical slope threshold of eight percent, which is the steepest grade that wagons pulled by animals could manage. Steeper paths added extra "cost" to the model, reducing their likelihood of being part of a royal road. The study also excluded modern features like dams or altered landscapes to reflect ancient conditions more accurately.
What makes a road "royal" in this context is not just its direction or use. According to the study, it is its ability to support massive logistical operations. That included space for thousands of travelers, animals, and support staff, along with facilities for food, water and rest. These weren't simply courier stations.
Some were large, temporary settlements that acted like mobile capitals. Ancient records from Persepolis even describe detailed preparations for royal arrivals, including supplies delivered in advance to elite stations along the route.
The road's width also mattered. A narrow path could stretch a convoy out over 40 kilometers (25 miles), disrupting coordination and slowing progress. A road at least 5 to 7 meters (16 to 23 feet) wide, the study notes, would allow multiple columns of movement. This would match Roman road standards found elsewhere in West Asia.
One possible royal station, Qaleh-ye Kali in western Iran, fits these findings. Its location on flat, fertile land with access to water aligns with the model. Archaeological evidence -- such as elite buildings and storage areas -- points to its role in royal logistics.
The project, known as PersianTRAIL, offers a new way to identify ancient royal infrastructure using modern tools. It provides a framework for testing how roads were built to meet the specific needs of governance and power projection.
Future work will extend this analysis west toward Sardis, the empire's former capital in what is now Turkey. Researchers will also include more environmental and political data to refine the model.
By linking terrain with ancient texts and archaeological findings, Salaris' work shows that these roads were not just symbolic. They were part of a carefully managed system that kept a massive empire moving.