[Vienna, 06.02.2025]—"In this study, we use the spatial social connections of people within the 50 largest cities in the US to test whether the built environment—in this case, urban highways— is indeed a barrier to social ties, as has long been assumed in urban studies. For the first time, we are also finding quantitatively that this is the case," explains co-author Sándor Juhász. During his postdoctoral fellowship at the Complexity Science Hub (CSH), Juhász participated in the study.
This barrier effect by urban highways is particularly strong for short distances of less than 5 km (or ca. 3 miles) in all the US cities studied. It only reverses at around 20 km (ca. 12.4 miles) —above this distance, urban highways tend to help connect places within a city, according to the study. "In concrete terms, this means that if two people live on different sides of an urban highway, it is less likely that there will be social contact between them," says Juhász, who's now a research fellow at the Corvinus University of Budapest.
"Urban highways, especially those built in the US in the 1950s and 1960s, were entirely geared towards car traffic," explains co-author Anastassia Vybornova from the IT University of Copenhagen. They were intended to shorten the commute to work and make traveling within the city easier. "But this comes at a price, especially over short distances. If someone wants to cross a multi-lane highway, it takes a lot of effort. So highways connect over long distances, but divide over short ones," adds Juhász.
Effects Quantified for the First Time
A special feature of this study is that, for the first time, the researchers not only described these barrier effects qualitatively, but also quantified them. To do this, the team, led by the IT University of Copenhagen, combined geographical data with location information from online social networks.
The researchers added a second layer to a city map with streets, rivers and, other geographical features: a social network of 1 million X users (formerly Twitter) from 2013. The nodes of this network represent people's estimated places of residence, while the connections represent their friendships on X within the city.
"The fact that the mutual followerships on X reflect real friendships is of course a highly generalized assumption," says Juhász. "It is extremely difficult to map social connections at this level of detail. There are currently no comprehensive datasets for such analysis—most, such as public Facebook data, are only aggregated at the zip code level. But to examine the influence of highways, much finer data is needed. It's interesting to see that the barrier effect of urban highways is already detectable in this dataset."
Cleveland Has the Largest Barrier Effect
The average barrier effect of urban highways is highest in Cleveland, a city with particularly high segregation, according to the results of the study.
"In the US, there is a long tradition of highway construction through densely populated areas, which has led to segregation and a decline in local opportunities," explains Juhász. "Urban planners often used to decide to build highways in such a way that they cut through large, homogeneous neighborhoods or physically separate different population groups—mostly Black and White communities".
The best-known example is probably Detroit with the 8 Mile Road. "Our measurements clearly show that there is still less social contact there today than would be the case without the highway," adds Juhász.
In the US, there were already major programs such as the "Reconnecting Communities" initiative, which invested millions of dollars to make urban infrastructure more socially just. "So the problem was recognized, and there were active policies with increasing budgets. But more importantly, this program—like many others—was stopped by the new US government a few weeks ago," says Juhász. This makes studies like this one, which show that urban highways have a significant influence on social connectivity, all the more important, according to Juhász.
About the Study
The study "Urban highways are barriers to social ties" by L.M. Aiello, A. Vybornova S. Juhász, M. Szell, and E. Bokányi was published in PNAS (doi: 10.1073/pnas.2408937122 ).
About CSH
The Complexity Science Hub (CSH) is Europe's research center for the study of complex systems. We derive meaning from data from a range of disciplines—economics, medicine, ecology, and the social sciences—as a basis for actionable solutions for a better world. Established in 2015, we have grown to over 70 researchers, driven by the increasing demand to gain a genuine understanding of the networks that underlie society, from healthcare to supply chains. Through our complexity science approaches linking physics, mathematics, and computational modeling with data and network science, we develop the capacity to address today's and tomorrow's challenges.
