11. Cities and Urbanization

Understanding Cities

Cities are densely settled places where people, economic activities, transportation systems, institutions, and cultural practices are concentrated. Urban geography studies how cities grow, how land is organized within them, and how urban areas influence people and physical environments.

A city can be understood in several ways. A legally defined municipality may be part of a larger urban agglomeration, which includes the continuously built-up city and surrounding settlements. A metropolitan area includes the city and surrounding communities that are economically connected to it. Because countries define “urban” differently, international comparisons require consistent measures of population size, density, settlement continuity, and related characteristics.

Takeaway: Cities are both physical places and networks of social, economic, cultural, and environmental relationships.

Urbanization and Its Processes

Urbanization describes a change in the share of people living in urban places. Several processes can produce it:

• Rural-to-urban migration: People move to cities for employment, education, health care, security, or other opportunities.

• Natural increase: Births exceed deaths within the urban population.

• Urban-to-urban migration: People move from one city to another, often toward larger or faster-growing metropolitan areas.

• Reclassification: A settlement or rural area is administratively redefined as urban after changes in population, density, or economic functions.

• Annexation and administrative expansion: A city incorporates neighboring territory or settlements.

• In-situ urbanization: A rural place gains urban infrastructure, services, and nonagricultural employment while many residents remain in place.

Urbanization is uneven. Some countries are already highly urbanized, while others have lower urban shares but much faster urban growth. Global population growth through 2050 is expected to occur substantially in cities, especially in countries in Africa and Asia. In 2025, cities contained about 45 percent of the world's population, towns about 36 percent, and rural areas about 19 percent.

Takeaway: A rising urban population can result from migration, demographic change, administrative decisions, or the transformation of rural places; it does not necessarily mean that rural migrants are arriving.

Why Urban Areas Grow

Urban growth is the increase in the population, land area, economic activity, or built environment of a particular urban place. It differs from urbanization, which measures the changing proportion of a population living in urban areas. A country may experience urban growth without a large rise in its urbanization level if its rural and urban populations both increase.

Cities often grow through agglomeration, the concentration of related activities in one place. Businesses may benefit from proximity to suppliers, customers, skilled workers, transportation hubs, universities, and financial institutions. Workers may be attracted by employment diversity and access to services.

Physical geography also shapes urban growth. Ports, river crossings, fertile plains, mineral deposits, defensible sites, and favorable climates have historically encouraged settlement. Modern transportation, digital communication, government investment, and global economic connections can reduce some physical constraints, but they do not eliminate them.

Rapid growth can produce both benefits and costs:

• More jobs, markets, schools, hospitals, and cultural institutions

• Greater innovation and economic productivity

• Higher demand for housing, water, energy, transportation, and sanitation

• Traffic congestion and air pollution

• Expansion onto agricultural land, wetlands, forests, or floodplains

• Informal settlements when housing and services do not keep pace

• Greater exposure to heat, flooding, storms, drought, and other hazards

The consequences depend on planning, public investment, income distribution, and local governance.

Takeaway: Urban growth creates opportunities, but its effects depend on how land, infrastructure, and resources are managed.

Suburbs, Sprawl, and Smart Growth

Suburbanization moves people, housing, businesses, and institutions from a central city toward surrounding communities. It may be encouraged by highway construction, automobile ownership, lower land prices, larger residential lots, demand for detached homes, zoning, mortgage policies, public infrastructure, and concerns about congestion or housing costs.

Urban sprawl is a particularly dispersed form of outward expansion. It commonly includes low-density development, separated land uses, long travel distances, extensive roads, large parking areas, and discontinuous construction. Sprawl can increase land consumption, infrastructure costs, traffic, and environmental impacts.

Exurbs lie beyond the suburbs and generally contain lower-density housing. Their residents may commute long distances to metropolitan jobs, extending urban influence farther into rural areas.

Smart growth seeks compact, connected, and mixed-use communities. Strategies include:

• Reusing already developed land through infill development

• Combining housing, employment, retail, and services in walkable areas

• Supporting public transportation, cycling, and pedestrian travel

• Preserving farmland, forests, wetlands, and open space

• Providing a range of housing types and prices

• Coordinating decisions across municipal boundaries

Takeaway: Outward growth is not a single process; its form depends on density, transportation, land use, governance, and the relationship between central cities and surrounding communities.

Megacities and Metropolitan Networks

A megacity is an urban agglomeration with at least 10 million inhabitants. Megacities often serve as national or international centers of finance, government, manufacturing, transportation, communications, and culture. Their large labor markets and extensive infrastructure can create economic advantages, but their scale can also intensify housing shortages, congestion, pollution, unequal access to services, and vulnerability to hazards.

A megalopolis is a connected urban corridor or network made up of multiple metropolitan areas. Its cities are linked by transportation, trade, commuting, and shared infrastructure. This differs from a megacity, which is one very large urban agglomeration.

The largest cities do not represent all urban growth. Most cities have fewer than 1 million inhabitants, and many small and medium-sized cities are growing rapidly. These places may have fewer financial and administrative resources for managing expansion, making land-use planning particularly important.

Takeaway: Urban networks operate at several scales, from small and medium-sized cities to megacities and connected metropolitan corridors.

Models of Urban Structure

Urban models simplify the spatial arrangement of land uses and social groups. They help identify patterns, but no single model describes every city because actual urban form is shaped by history, culture, physical geography, government policy, transportation, inequality, and globalization.

The concentric zone model, associated with Ernest Burgess, represents a city as rings surrounding a central business district. A simplified sequence includes the central business district, a zone of transition, working-class residential areas, middle-class residential areas, and a commuter zone. The model emphasizes invasion and succession, in which land uses or populations replace one another over time.

The sector model, associated with Homer Hoyt, proposes wedge-shaped sectors extending outward from the center along railways, highways, rivers, coastlines, or other corridors. Industrial and high-income residential areas may each develop along particular routes and continue expanding in the same direction.

The multiple-nuclei model, associated with Chauncy Harris and Edward Ullman, describes cities with several specialized centers. These may include a downtown, airport, port, university or medical district, suburban office center, regional shopping center, industrial park, or entertainment district. Edge cities are suburban concentrations of offices, shopping, entertainment, and related activities that function as secondary business centers.

The peripheral or galactic model describes a central city surrounded by suburbs, satellite business districts, shopping centers, industrial areas, and beltways or ring roads. It reflects automobile-oriented metropolitan development and multiple suburban nodes.

Takeaway: The models emphasize different forces: distance from the center, transportation corridors, or several specialized centers. A single metropolitan area may contain elements of all three major models.

Land Use, Accessibility, and Inequality

Urban land use results from the interaction of accessibility, land value, zoning, transportation, historical development, environmental conditions, and social inequality.

The central business district is often highly accessible because many transportation routes converge there. High accessibility can produce high land values, tall buildings, intensive commercial activity, and limited residential space. In some cities, employment has decentralized to suburban office centers, while redevelopment has brought more residents back into the central area.

Residential patterns reflect income, housing costs, transportation, culture, family structure, zoning, and discrimination. Higher-income households may seek larger lots, environmental amenities, or exclusive neighborhoods. Lower-income households may be pushed toward less expensive areas with fewer services or greater exposure to hazards. Ethnic neighborhoods can provide cultural institutions, social networks, and businesses, while also reflecting historical discrimination or unequal housing access.

Industrial activity often locates near highways, railroads, ports, airports, rivers, or inexpensive land. Heavy industry may be separated from residential areas because of pollution, noise, and safety concerns. Retail and services may cluster in downtowns, neighborhood centers, shopping corridors, malls, or edge cities.

Transportation networks create nodes and corridors of development. Automobile-oriented systems generally encourage dispersed development, while rail and pedestrian-oriented systems can support higher-density, mixed-use development near stations.

Environmental justice examines whether environmental benefits and burdens are distributed fairly and whether affected communities can meaningfully participate in decisions. Low-income communities and marginalized groups may face greater exposure to air pollution, industrial hazards, extreme heat, flooding, inadequate housing, or limited access to parks and reliable transportation.

Takeaway: Urban form reflects physical accessibility and economic forces, but it also reflects policy, discrimination, and unequal power over land and infrastructure.

Cities and the Physical Environment

Cities replace vegetation and soil with buildings, roads, and other impervious surfaces. These changes can increase storm-water runoff and flood risk, reduce wildlife habitat and agricultural land, intensify demand for water and energy, and concentrate air pollution and solid waste.

The urban heat-island effect occurs when built-up areas are warmer than nearby rural areas. Pavement and buildings absorb and retain heat, while the reduction of vegetation limits cooling. Urban development can also alter local drainage, air circulation, and ecological processes.

Cities can also support environmental efficiency. Dense development may reduce per-person travel distances and infrastructure needs, especially when paired with public transportation and energy-efficient buildings. Urban parks, green roofs, wetlands, street trees, and restored waterways can reduce heat and flooding while improving quality of life.

Effective planning must balance population needs, economic activity, housing, mobility, public health, cultural identity, and environmental limits. Sustainable urbanization depends on land-use planning, resilient infrastructure, environmental protection, and equitable access to services.

Takeaway: Cities both create environmental pressures and offer opportunities for efficient, resilient, and healthier development.