The Apostle Paul travelled widely across the Roman Empire to spread the Christian faith. One key segment of his second missionary journey, which began in 49 CE in Jerusalem, involved an extensive overland trek across modern-day Turkey to Alexandria Troas, a city south of ancient Troy.

This route, if completed entirely by land, covered about 2012 kilometres and would have taken some 524 hours to walk, or 466 hours if traveling by donkey.

We know this because a study published last month in the Nature journal Scientific Data significantly updated the estimated size of the Roman Empire’s road system, increasing its total length to 3,01,688 kilometres from about 1,93,121 kilometres. Rome probably achieved peak road sometime around 150 CE, when the empire was at its most prosperous and extensive. But the database tallies all the roads presumed to have existed during Rome’s life span, from roughly 312 BCE to 400 CE.

Previous estimates focussed on the main thoroughfares radiating from Rome across Italy and the empire, including the Via Appia (Appian Way) to the south, the Via Flaminia to the north, and major arteries like the Via Egnatia across the Balkans, and the Via Domitia across southern Gaul. The Barrington Atlas of the Greek and Roman World, published 25 years ago, served as the most comprehensive resource.

The paper’s revised mileage figure incorporates major highways, strategic routes and local roads often omitted from previous analyses. The accompanying digital atlas Itiner-e offers a sophisticated tool for scholars by leveraging historical records alongside satellite and topographic data to model travel times accurately.

Tom Brughmans, an archaeologist at Aarhus University in Denmark who collaborated on the paper, and his colleagues defined Roman roads more broadly to include any walkable path and used a practical, terrain-following mapping technique, rather than imposing unrealistic straight lines. The change substantially increased mapped networks in North Africa, Greece and the Iberian Peninsula.

Because many Roman roads are now gone or paved over by modern highways, their exact positions were hard to determine; researchers used aerial analysis of the terrain, looking for faint signs in the plant life or subtle changes in height, to find these lost byways.

According to Brughmans, determining much of the Roman road network is a “game of connecting the dots”. The precise course of most of the roads is unknown, with less than 3 per cent of their locations confirmed. The paths are generally inferred from sparse evidence like scattered milestones, leaving the exact route between known places A and B a mystery.

One of the longest documented Roman trails is detailed in the Itinerarium Burdigalense, a travelogue from 333-334 CE, a time when the empire struggled with internal and external pressures, and the road network began to suffer as regions became less connected. This journal by an anonymous pilgrim from Bordeaux, France, recorded his 5,069-kilometre journey to Jerusalem and back, providing a textual map of the route’s stops and distances.

Mapping ancient transport paths provides researchers with a 2,000-year-old case study on the centuries-long societal impact of pandemics, Brughmans said. This strategy aids in comprehending events such as the Antonine Plague of 165 CE, estimated to have killed one-quarter of the Roman Empire’s populace, by connecting its spread to the Roman road system.

Daisy Dunn, a classicist and author of The Missing Thread: A Women’s History of the Ancient World, shared her enthusiasm for the project, calling it a “nerdy delight”. She pointed out that despite the Romans’ reputation as master road builders, their routes are often unmarked and easily missed today.

NYTNS