Ancient Roman roads (light yellow) superimposed on 2010 satellite imagery of nighttime lighting in Europe. (Washington Post illustration using data from NOAA Earth Observatory, Natural Earth and Digital Atlas of Roman and Medieval Civilization)

Prosperity begets prosperity: On a global level, economists and historians have shown that places that prospered 100, 500, even 1,000 years ago tend to be more economically developed today.

But how? We’re less clear on the exact channels by which economic activity sustains itself over the millennia. Could dynastic wealth play a role? How about the concentration and transmission of knowledge via institutions such as schools and libraries? How does military might factor in?

Now, a team of Danish economists has put forth a forceful case for one largely overlooked driver of economic development in Europe: roadways built by the Roman empire nearly two thousand years ago. They demonstrate that the density of ancient Roman roads at a given point in Europe strongly correlates with present-day prosperity, as measured by modern-day road density, population density and even satellite imagery of nighttime lighting.

Their data shows that infrastructure investments are — if you’ll pardon an unpardonable pun — a pathway to long-term prosperity.

To arrive at this conclusion, Carl-Johan Dalgaard of the University of Copenhagen and his colleagues first obtained a geographic database of the major roads of the Roman era that had been compiled by Harvard University’s Digital Atlas of Roman and Medieval Civilizations.

Roman roadways were massive infrastructure projects even by modern standards. They consisted of several base layers, including stone, gravel and sand, over which large stone slabs were laid. At the empire’s peak in 117 A.D., scholars estimate, the Romans had built more than 80,000 kilometers of roadway across Europe, the Middle East and North Africa. Many of them have lasted well into the present day.

Dalgaard and his colleagues took a map of the major ancient Roman roads and superimposed it over satellite imagery showing the level of nighttime illumination in 2010. Economists often use nighttime lighting as a proxy for economic activity: more lights, more development. The image below shows the resultant map. Ancient Roman roadways are in light yellow, while the boundaries of the Roman Empire as of 117 A.D. are outlined in red. The background layer shows modern nighttime illumination.


(Washington Post illustration using data from NOAA Earth Observatory, Natural Earth and Digital Atlas of Roman and Medieval Civilization)

The visual relationship is particularly striking in France. There, you can clearly see the paths of ancient roadways connecting not just major modern cities, like Paris and Lyon, but also many minor ones, too. Across inland France, nearly every junction of ancient roads is marked by a splash of light in the modern era.


(Washington Post illustration uses data from NOAA Earth Observatory, Natural Earth and Digital Atlas of Roman and Medieval Civilization)

While just eyeballing it like this is certainly suggestive, it’s not good enough for social science research. So Dalgaard and his colleagues took it several steps further: They divided the entire ancient Roman empire into a grid of one degree latitude by one degree longitude squares and measured the density of Roman roads within each. For each square, they also measured modern-day population, the density of current roadways and economic activity as indicated by the satellite imagery.

They then ran a battery of statistical tests to determine how the presence of ancient roadways was related to the modern-day variables they measured. The answer: quite a bit. Places with more Roman roads in antiquity tended to have more roads today, as well as more people and greater levels of economic development.

Now, there’s a big question of causality looming over all this: Can we really say that ancient roads caused greater economic development down the line? Or is it more accurate to say that more prosperous areas in the ancient world simply had more of a tendency to build roads to other places as a natural result of their prosperity?

Dalgaard and his colleagues marshal convincing pieces of evidence to argue in favor of a causal link that runs from ancient roadbuilding to modern-day prosperity. For starters, Roman roads weren’t typically built with trade in mind: their primary purpose was to move troops and supplies to locations of military interest. Trade was an afterthought.

“Roman roads were often constructed in newly conquered areas without any extensive, or at least not comparable, existing network of cities and infrastructure,” Dalgaard and his colleagues write. In many instances, the roads came first. Settlements and cities came later.

Then there’s the fascinating question of what happened to Roman roads built in North Africa. At some point between 500 and 1,000 A.D., wheeled transport was essentially abandoned in the region. Goods were ferried around on the backs of camels, rather than in carts pulled by oxen. The exact reasons for this are up for debate and probably involved costs, advances in saddle technology and the increasing military and political might of groups that had traditionally relied on camels for transport, Dalgaard and his colleagues explain.

If you’re not pulling carts around, you have less of a need for paved roadways. As a result, the Roman roads in the Middle East and North Africa (MENA) weren’t maintained the same way they were in Europe, where cart-based transit remained dominant. “The implication of these developments is that since ancient roads fall into disrepair in the MENA region, to a much greater extent than in Europe, one should expect to see much less persistence in infrastructure density.”

Indeed, that’s exactly what Dalgaard and his colleagues found. The correlation between ancient roadways and modern-day development so prevalent in Europe is much smaller and less significant for the Middle East and North Africa. “As ancient roads are left to decay they ultimately become a less reliable predictor of modern road location in the MENA,” they found. “Roman road density does not predict current day economic activity within the MENA region.”

In sum, Dalgaard’s research adds historical heft to the idea that infrastructure investments can be a driver of economic growth. While most research into that question has focused on short-term results, Dalgaard’s paper suggests that infrastructure investments today could continue to bear fruit for thousands of years to come.