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Towns, cities, and rural landscapes never exist in true isolation. Human beings traverse these distinct locations daily, monthly, and annually, creating a complex web of interaction. Consider an individual who lives in a small town but commutes to an intermediate city for employment, while occasionally traveling to a major metropolitan hub to utilize an international airport or attend a global festival. These consistent travel patterns bind disparate locations together into what geographers and planners now term "functional city-regions." This conceptual framework fundamentally shifts the analytical focus from static political boundaries to the dynamic reality of how people move and access essential resources across a landscape.
Understanding the intricate connections between urban centers and their surrounding territories is critical for effective planning, resource management, and achieving sustainable development. This concept possesses deep historical roots, notably in Walter Christaller's Central Place Theory, which was introduced in 1933. For decades, cartographic analyses relied heavily on political borders or simplistic classifications of urban versus rural zones. While recent initiatives, such as the Degree of Urbanization (DEGURBA), have added necessary granularity to these classifications, the global interconnections between urban cores and their peripheries remained largely unstudied.
A pivotal paper by Cattaneo and colleagues, published in 2024, addresses this substantial gap in academic research. It provides the first systematic, worldwide delineation of multi-tier city-regions. This research, along with its accompanying dataset, offers a transformative methodology for understanding urban connections. The authors demonstrate how populations globally can realistically access different tiers of urban centers—ranging from small towns to massive metropolises—within practical travel times. This approach transcends static political lines to map the fluid reality of human movement and service accessibility.
A primary finding of this research is striking: in 2020, 41% of the global population, amounting to 3.2 billion people, could access multiple tiers of urban services within just one hour. This statistic reveals a remarkably high degree of interconnectedness in the modern world. Another critical insight underscores the vital role of intermediate cities, defined as settlements with populations between 250,000 and one million. The data indicates that individuals residing in or near towns and small cities were twice as likely to easily reach an intermediate city as they were to reach a large one. This suggests that for the majority of the global population, the most accessible and critical urban hubs are not the massive metropolises, but the intermediate centers that act as essential bridges between small towns and large cities.
Building upon this groundbreaking research, the FAO City-Region Explorer data platform provides an interactive map designed for a broad audience. This tool facilitates the exploration of the roles played by over 30,000 urban centers within their specific regions. Users can filter data by country and select travel-time thresholds of one, two, or three hours. Furthermore, they can explore "activity tiers," which describe the specific range of services a city-region provides. These tiers are categorized based on the complexity and specialization of services available.
Tier 1 (T1) covers everyday essentials such as grocery stores and primary schools. Tier 2 (T2) includes more specialized services like general hospitals. Tier 3 (T3) involves highly specialized facilities such as universities or major sports complexes. Tier 4 (T4) represents major hubs, including sizable international airports or specialized medical centers. These activity tiers are closely linked to urban tiers, which are categories determined by population size. Larger centers naturally offer a wider spectrum of activities. Towns, with populations between 20,000 and 50,000, primarily offer T1 services. Small cities, ranging from 50,000 to 250,000 people, provide T1 and T2 services. Intermediate cities, with populations between 250,000 and one million, offer T1, T2, and T3. Large cities, exceeding one million residents, serve as comprehensive hubs, providing all tiers from T1 through T4. This hierarchical structure allows researchers to precisely determine which level of service is accessible to any given population.
Possible combinations of multi-tier catchments and resulting city-region patches. Each box’s four-digit number indicates the patch type and urban tier for four activity levels at the starred location.Source: Cattaneo, A. et al. Worldwide delineation of multi-tier city–regions. Nature Cities 1, 469–479 (2024).
The United States employs a unique method to understand city connections. For decades, the Office of Management and Budget has defined Metropolitan and Micropolitan Statistical Areas, collectively known as Core Based Statistical Units or CBSAs. These maps track the economic and social integration between urban cores and nearby counties. This method aligns economic data with administrative boundaries and tracks commuting patterns effectively. However, the new city-region delineation, based on travel time rather than county lines, offers distinct advantages. While CBSAs depict established economic areas, they are constrained by rigid county boundaries and focus on shorter distances. In contrast, the city-region method reveals how people access services up to three hours away. This makes it especially useful for cross-country analysis, as it is not tied to specific national administrative units.
City-regions are constructed from "patches." A patch is a geographic area defined by a unique mix of urban catchments across activity tiers for a given travel time. Each patch is assigned a specific type based on its urban tier combination. A city-region is a collection of patches that share the same urban center across one or more tiers at a specific travel time. The interactive platform's City-Region Explorer Panel provides detailed insights into these patches.
The patch-level detail enables precise analysis of overlapping city-regions. For example, the map below shows how parts of the Milwaukee city-region—specifically patch types 1334, 2234, and 1234—are functionally connected to Chicago. The accompanying data table highlights urban centers like Racine and Kenosha, which fall within Milwaukee’s region for tier 3 services but access Chicago’s higher-tier services.
Because city-regions can overlap, it is crucial to distinguish between primary and secondary city-regions. In a primary city-region, all patches share the same highest-tier urban center. Large cities always form primary city-regions. Secondary city-regions include patches that can access a higher-tiered center in another city-region. For instance, parts of the Milwaukee city-region fall within Chicago's region. This means residents may rely on Milwaukee for everyday needs but turn to Chicago for specialized services, such as additional flight options from O'Hare International Airport.
Zooming in on Atlanta, it is clear how intermediate cities can serve as regional powerhouses. Beyond its urban core of over 430,000 residents, Atlanta’s influence extends widely – reaching over 6.6 million people within one hour, 8.5 million in two, and more than 10 million within three hours. These impressive numbers are dynamically calculated in the City-Region Explorer Panel.
In the United States, large cities are pivotal for regional connectivity. They provide access to 60% of the population within three hours. However, intermediate cities are also highly significant. They serve as the highest-tier urban center within three hours for nearly 29% of the population, or approximately 97 million people. Another 9% of people can only reach a small city within that timeframe. While large cities are vital, intermediate and small cities also play crucial roles. Their accessibility and services shape the daily lives of millions, contributing to a more balanced and resilient national network.
Examining Atlanta illustrates how an intermediate city can act as a regional powerhouse. Beyond its urban core of over 430,000 residents, Atlanta's influence is vast. It reaches over 6.6 million people within one hour, 8.5 million in two hours, and more than 10 million within three hours. These numbers are dynamically calculated in the City-Region Explorer Panel. This demonstrates that intermediate cities often provide a denser and more immediate network of services than large cities, which might be more distant for many residents.
Whether analyzing a single location, a specific country, or making international comparisons, the City-Region Explorer is a valuable and free resource. The full dataset is available on Zenodo. Tutorial videos are also available to assist users in getting started. By utilizing this data, planners, researchers, and citizens can gain a clearer understanding of how urban systems function, ensuring that development strategies align with the actual movement and needs of the population rather than arbitrary political lines.
This new framework for understanding city-regions provides a powerful tool for addressing the challenges of modern urbanization. As populations grow and climate change alters transportation networks, the ability to map functional connections becomes increasingly essential. The shift from static administrative maps to dynamic travel-time models represents a significant step forward in geography and urban planning. It allows for a more nuanced view of how communities interact, how economies develop, and how resources are distributed. The insights provided by Cattaneo and colleagues will likely influence policy decisions for years to come, ensuring that the needs of those living in the spaces between large cities are not overlooked. The data reveals a world that is far more connected than previously imagined, where the distance to a hospital or a university is often a matter of hours, not miles.
The implications extend beyond simple geography. Economic planners can use this data to identify where infrastructure investments are most needed. Social scientists can study how access to different service tiers affects community well-being. Environmentalists can analyze how travel patterns impact carbon emissions. The City-Region Explorer is more than a map; it is a lens through which we can view the complex interplay of human activity on a global scale. By understanding the true shape of our functional city-regions, we can build systems that are more efficient, equitable, and responsive to the needs of people everywhere.
The integration of these findings into standard planning practices will require collaboration between governments, academic institutions, and international organizations. The data must be made accessible to those who make decisions, not just those who study them. The tutorial videos and open-source dataset mentioned earlier are critical steps in this direction. They lower the barrier to entry for new users and encourage widespread adoption of this new methodology. As more people engage with this data, the collective understanding of urban connectivity will deepen, leading to better-informed policies and more sustainable development outcomes.
Ultimately, the story of the modern world is one of connection. People are not confined to their immediate surroundings; they move, work, and access services across vast distances. The concept of the functional city-region captures this reality. It acknowledges that a town is not just a town, but a node in a larger network. It recognizes that an intermediate city is not just a stepping stone, but a vital center of its own. By embracing this complexity, we can create a more comprehensive and accurate picture of our world. The research by Cattaneo and colleagues provides the foundation for this new perspective, offering a roadmap for understanding the intricate web of urban life that defines our era.