Modern cities are often seen as ever-changing landscapes of glass and concrete, but beneath the surface of asphalt lies a persistent memory of the past. In a 2020 study published in the Journal of Physics: Complexity, researchers Michelle Cirunay, Maricor Soriano, and Rene Batac explored how road networks evolve over time. By treating cities as living, growing organisms, the team sought to understand whether the original elements of a settlement’s layout remains visible even after decades of urban expansion and densification.

The researchers focused on Manila, Philippines, an urban metropolis that has seen various colonial influences, endured the destruction of the war, and underwent rapid urbanization over the last few decades. The team analyzed and digitized historical maps of Manila to track the birth and transformation of road segments. Additionally, they used the contemporary maps from online sources to compare the historical data with the modern road network layout.

They discovered that while cities grow in size and complexity, they often exhibit the preservation of certain features of the road network layout. In particular, the statistical distributions of spatial and topological (network) properties are found to be robust and nearly universal.

Additionally, using complex network analysis, the study identified the spatial orientation of the most "central" locations in the road network. In particular, the nodes (intersections) with the highest closeness centrality values actually form the historical backbone of the city. On the other hand, the nodes with the highest betweenness centrality trace the skeletons of the most traffic-prone locations in the road network. 

These preserved statistical and spatial features are explained as a result of the delicate interplay between "exploration" (building into new territory) and "densification" (filling in gaps within the existing grid) mechanisms, which are also observed for other cities. 

Understanding these preserved features offers valuable insights for future urban design. The research highlights that once a road is laid down, it becomes a permanent fixture of the urban landscape, creating a "frozen" history that modern planners must navigate. By recognizing the persistent patterns in how roads develop, planners can better predict how a city might respond to new infrastructure or population shifts. The work of Cirunay, Soriano, and Batac reminds us that cities are not just collections of buildings, but historical records etched onto the earth—proving that the ghost of an old wagon trail can still be found in the rhythm of a modern highway. ◼