8 6.1 Defining Cities and Urban Centers
Cities and Metropolises
Must of us are “city people,” whether we like it or not. Many people say they do not like the city, with its noise, pollution, crowds, and crime, but living outside the city has its challenges as well. Living outside a city is inconvenient because rural areas lack access to the numerous amenities found in cities. The clustering of activities within a small space is called agglomeration, and it reduces the friction of distance for thousands of daily activities. Cities are convenient places for people to live, work, and play. Convenience has economic consequences, as well. Reduced costs associated with transportation, and the ability to share expenses for infrastructure creates what is known as economies of agglomeration, which is the fundamental reason for cities. The convenience and economic benefits of city life have led nearly 8 in 10 Americans to live in urban areas. In California, America’s most urban state, almost 95% of its people live in a city. This chapter explores the evolution of cities, why cities are where they are, and how the geography of cities affects the way urbanites live.
Though it seems simple enough, distinguishing cities from rural areas is not always that easy. Countries around the world have generated a plethora of definitions based on a variety of urban characteristics. Part of the reason stems from the fact that defining what constitutes urban is somewhat arbitrary. Cities are also hard to identify because they look and function quite differently in different parts of the world. Complicating matters are the great variety of terms we use to label a group of people living together. Hamlets are tiny, rural communities. Villages are slightly larger. Towns are larger than villages. Cities are larger than towns. Then there are words like metropolis and even megalopolis to denote huge cities. Some states in the United States have legal definitions for these terms, but most do not. The United States Census Bureau creates the only consistent definition of “city,” and it uses the terms “rural” and “urban” to distinguish cities from non-city regions. This definition has been updated several times since the 1800s, most radically in recent years as the power of GIS has allowed the geographers are working for the U.S. Census Bureau to consider multiple factors simultaneously. It can get involved.
For decades, the U.S. Census Bureau recognized an area as “urban” if it had incorporated itself as a city or a town. Incorporation indicates that a group of residents successfully filed a town charter with their local state government, giving them the right to govern themselves within a specific space within the state. Until recently, the U.S. Census Bureau classified almost any incorporated area with at least 2,500 people as “urban.”
There were problems though with that simple definition. Some areas which had quite large populations but were unincorporated, failed to meet the old definition or urban. For example, Honolulu, Hawaii, and Arlington, Virginia, are not incorporated, therefore they were technically labeled “census-designated places,” rather than cities. Conversely, some incorporated areas may have very few people. This can happen when a city loses population, or when the boundaries of a city extend far beyond the populated core of the city. Jacksonville, Florida, is the classic example of this problem. Jacksonville annexed so much territory that its city limits extend far into the adjacent countryside, making it the largest city in land area in the United States (874.3 square miles!).
Therefore, the Census Bureau created a complex set of criteria capable of evaluating a variety of conditions that define any location as urban or rural. Among the criteria now used by the Census is a minimum population density of 1,000 people per square mile, regardless of whether the location is incorporated or not. Additionally, a territory that includes non-residential but still urban land uses is included. Therefore, areas with factories, businesses, or a large airport, that contains few residences still counted as part of a city. The Census uses a measure of surface imperviousness to help make such a decision. This means that even a parking lot may be a factor in classifying a place as urban. Finally, the Census classifies locations that are reasonably close to an urban region if it has a population density of at least500 persons per square mile. That way, small breaks in the continuity of built-up areas do not result in the creation of multiple urban areas, but instead form a single, contiguous urban region. Therefore, people in the suburbs within five miles of the border of a larger city, are counted by the Census as residents of the urban region, associated with a central city.
City Push and Pull Factors
Cities began to form many thousands of years ago, but there is little agreement regarding why cities form. The chances are that many different factors are responsible for the rise of cities, with some cities owing to their existence to multiple factors and cities that arose as a result of more specific conditions.
Two underlying causal forces contribute to the rise of cities. Site location factors are those elements that favor the growth of a city that is found at that location. Site factors include things like the availability of water, food, good soils, a quality harbor, and characteristics that make a location easy to defend from attack. Situation factors are external elements that favor the growth of a city, such as distance to other cities, or a central location. For example, the exceptional distance invading armies have had to travel to reach Moscow, Russia has helped the city survive many wars. Most large cities have good site and situation factors.
Indeed, the earliest incarnation of cities offered residents a measure of protection against violence from outside groups for thousands of years. Living in a rural area, farming or ranching, made any family living in such isolation vulnerable to attack. Small villages could offer limited protection, but larger cities, especially those with moats, high walls, professional soldiers, and advanced weaponry, were safer.
The safest places were cities with quality defensible site locations. Many of Europe’s oldest cities were founded on defensible sites. The European feudal system was built upon an arrangement whereby the local lord/duke/king supplied protection to local rural peasants in exchange for food and taxes. For example, Paris and Montreal were founded on defensible island sites. Athens was built upon a defensible hillside, called an acropolis. The Athenian Acropolis is so famous that it is called merely, The Acropolis. On the other hand, Moscow, Russia, takes advantage of its remote situation. Both Napoleon and Hitler found out the hard way the challenges associated with attacking Moscow.
In the United States, the Atlantic and Pacific Oceans have primarily functioned as America’s defensive barriers, and therefore few cities are located on defensive sites. Washington, D.C. has no natural defense-related site or situation advantages. On the only occasion the U.S. was invaded, the city was overrun by the British in the War of 1812. The White House and the Capitol were burned to the ground. The poor defensibility of the American capital led to numerous calls for its relocation to a more defensible site during the 1800s. This is partly the reason, so many state-capitol buildings in the Midwest closely resemble the U.S. Capitol building in Washington, D.C.; many states were trying to lure the seat of the Federal government to their state capital.
San Francisco is the best example of a large American city founded upon the basis of its defensibility. Located on a peninsula between the Pacific Ocean and a large bay, San Francisco was established where it is because of the military advantage provided by that site. San Francisco boasts two kinds of defensible site advantages. It is both a peninsula site and a sheltered harbor site. Cannons positioned on either side of the Golden Gate could fire upon any enemy ships trying to pass into the San Francisco Bay. Armies coming northward up the peninsula would be forced into a handful of narrow passes where the Spanish Army could focus their defenses. These site advantages led the Spanish to establish the fort, El Presidio Real de San Francisco, there in 1776. The U.S. Army took control of the fort in 1846, and it remained a military base until 1994.
People who possess a specific skill set to become a site factor that can significantly affect the location and growth of a city. One specialized skill set was confined to the priestly class, and proximity to religious leaders is another probable reason for the formation of cities. Priests and shamans would have likely gathered the faithful near to them, so that, as the armies of the lordly class, they could offer protection and guidance in return for food, shelter, and compensation (like tithes). The priestly class was also the primary vessels of knowledge – and the tools of knowledge like writing and science (astronomy, planting calendars, medicine, e.g.), so a cadre of assistants in those affairs would have been necessary. Mecca and Jerusalem are probably the best examples of holy cities, but others dot the landscape of the world. Rome existed before the Catholic faith, but it assuredly grew and prospered as a result of becoming the headquarters of Christianity for hundreds of years.
Cities may have evolved as small trading posts where local farmers and wandering nomads exchanged agricultural and craft goods. The surplus wealth generated through trade required protection and fortifications, so cities with walls may have been built to protect marketplaces and vendors. Some trace the birth of London to an ancestral trading spot called Kingston upon the Thames, a market town founded by the Saxons southwest of London’s present core. The place-names of many ancient towns in England reveal their original function – Market Drayton, Market Harborough, Market Deeping, Market Weighton, Norton Chipping, Chipping Ongar, and Chipping Sodbury. “Chipping” is a derivation of a Saxon word meaning “to buy.”
Throughout history, cities, big and small, have served market functions for those who live in adjacent hinterlands. Some market cities grow much more substantial than others because they are more centrally located. Central location relative to other competing marketplaces is another example of an ideal situation factor. Large cities have excellent site and situation characteristics. Every major US city, including New York, Chicago, Los Angeles, Atlanta, and Houston is located ideally for commerce and industry.
Some cities grow large because of specific site location advantages that favor trade or industry. All cities compete against one another to attract industry, but only those with quality site factors, like excellent port facilities and varied transportation options, grow large. Cities ideally located between significant markets for exports and imports have unique situation factor advantages versus other competing cities and will grow most.
Most large cities in the United States emerged where two or more modes of transportation intersect, forming what geographers call a break of bulk point. Breaking bulk happens whenever cargo is unloaded from a ship, truck, barge, or train. Until the 1970s, unloading (and reloading) freight required a vast number of laborers, and therefore any city that had a busy dock or port or station attracted workers. Los Angeles, Chicago, New Orleans, and Houston all grew very large because multiple transportation modes well served each.
New York City is the largest city in the United States, but it was not always that way. It outgrew competitors on the East Coast because of the specific advantages of transportation. Early on, Boston and Philadelphia were more significant, but New York City’s break of bulk advantages helped it immensely. Key among the factors helping New York out-compete rivals were its additional transportation options. First, it had a port on the Atlantic Ocean. Second, it had the navigable Hudson River, which served inland cities far from the ocean via riverboat and barge. Then, in 1825, the Erie Canal opened, effectively connecting the Atlantic Ocean with Lake Erie and all the markets of the Great Lakes Region via New York City. The canal was a massive advantage. With the opening of the canal, agricultural products coming from the Midwest could be transported across the Great Lakes and Erie Canal to New York City, where it was off-loaded from riverboats to ocean-going ships headed for Europe. Simultaneously, goods coming from Europe and destined for any location in the Midwest had to be unloaded at the port in New York City. The additional jobs working at docks and warehouses attracted other industries, and a snowball effect was achieved by the mid- 1850s that made New York City, for a time, the largest city in the world.
With all of this in mind, it is possible to develop a view of cities that is based on innovations and diffusions of technology. This is what was done by the geography of John R. Borchert during the 1960s. Borchert developed a view of the urbanization of the United States that is based on the epochs of technology. As the components of technology wax and wane, the urban landscape undergoes dramatic changes.
- Stage 1: Sail-Wagon Epoch (1790–1830); the only means of international trade was sailing ships. Once goods were on land, they were hauled by wagon to their final destination.
- Stage 2: Iron Horse Epoch (1830–70); characterized by the impact of steam engine technology, and development of steamboats and regional railroad networks.
- Stage 3: Steel Rail Epoch (1870–1920); dominated by the development of long-haul railroads and a national railroad network.
- Stage 4: Auto-Air-Amenity Epoch (1920–70); with growth in the gasoline combustion engine.
- Stage 5: Satellite-Electronic-Jet Propulsion (1970–?), also called the High-Technology Epoch. This stage has continued to the present day as both transportation and technology improves.
Rivers have also played an essential role in the establishment of cities. Most cities are established along rivers of some sort. Rivers provide fresh water for drinking (and irrigation), but the effect navigable rivers have had on urban growth is hard to overstate. Before the age of trains and highways, rivers were by far the most efficient way to transport heavy cargo, especially over long distances. Interestingly, the interruptions to river navigation were most often responsible for creating conditions that attracted settlement and favored growth. Waterfalls were for many years a complete nuisance to river traffic, but they also are responsible for several cities. Not only do waterfalls provide a source of power for industry (see fall line cities below), but they also create a special kind of break of a bulk point called a head of navigation. At a waterfall, people had to stop, get out of their boats and carry the boat, and their cargo. Louisville, Kentucky, is an excellent example of a head of the navigation site because it arose next to the Falls of Ohio. In this place, the Ohio River tumbled over a waterfall forcing all boats to stop and breakbulk, again providing jobs at the boat dock, in warehouses, and encouraging manufacturing.
Those process of carrying boats and/cargo between two navigable stretches of the river (or to another river) is called Portage. Towns evolve where critical portage zones arose. Indiana, New York, Ohio, Wisconsin, Michigan, and Maine all have municipalities named “Portage,” but the essential portage zone in the United States appeared in Chicago, Illinois. Just southwest of what is now downtown Chicago, near Midway Airport was a portage zone where the Chicago River, which flows north into the Lake Michigan nearly intersected the Des Plaines River, which flows southward into the Mississippi River system. Around 1850, the people of Chicago built a canal connecting America’s two most significant navigable water systems, and by doing so, gave Chicago an enormous transportation advantage over other locations in the Midwest.
Business people value break of bulk because they offer opportunities for warehousing and manufacturing. Those industries not only attract migrants seeking work, but also additional transportation modes, which in turn create even more jobs. For example, the completion of the Illinois-Michigan canal in 1848 made Chicago an especially attractive terminus for multiple railroad companies that sprang up in the 1850s. It took Chicago just over 30 years to grow from the 100th most populous American city to the number two spot. Later still, interstate highways and airline routes also converged in Chicago.
Rivers also create chokepoints for the movement of goods and people traveling by land. Rivers are often difficult to cross in many locations because the water either the water is too deep or the river too wide. In such places, before bridges were standard, those trying to cross a river would seek out a ford, which is a shallow place to cross the river without a boat. City names like Stratford, Oxford, and Frankfurt all contain clues that they were once good places to cross a river. These fording sites often were simultaneously ideal locations for bridge construction because engineering a bridge across a shallow part of a wide river is simpler at a ford. Bridges funnel overland traffic to specific points, and provide another break of bulk opportunity, especially if the river is navigable.
Sometimes two rivers merge into a single, more massive river at a confluence site, creating yet another unique opportunity to gain an advantage over competitors. Pittsburgh, Pennsylvania, lies at America’s best-known confluence site. The steel industry thrived in Pittsburgh for over 100 years thanks in large part to the industrial advantages created by its location.
Los Angeles (L.A.) is the great metropolis on the west coast of the United States. The Spanish chose a location near what is now downtown L.A. for a pueblo (town) because they found fertile soil and a consistent source of water there alongside a large population of Indians that they hoped would form the core of a vibrant Spanish colony. As the years went by, Los Angeles’ only significant advantage over potential competitors in Southern California was its river. Spanish water law declared all the water in the L.A. River belonged to the people of Los Angeles. This law prevented other towns from forming either upstream or downstream from the original pueblo. People living along the L.A. River and hoping to use its precious waters were forced by Los Angelenos to become part of L.A.
Los Angeles remained a small town until the Santa Fe/Southern Pacific Railroad opened a second transcontinental railroad terminus in L.A. in 1881. Not long afterward, the local port facilities at San Pedro were upgraded, and L.A. began competing with San Francisco for business. With the invention of refrigerated boxcars and the discovery of oil in the region, L.A. proliferated. Good weather helped encourage migrants to journey westward to take jobs in the petroleum and citrus industries. The same great weather helped attract the movie and aeronautical industries decades later. Water resources, though, have remained a problem. The Los Angeles River was never sufficient to serve the needs of a large city, so a series of canals and pipelines have been constructed over the years to bring fresh water from vast distances into the Los Angeles region.
Sanctuary Cities
Sanctuary cities are common in many large cities across the United States. They are jurisdictions that have local policies that prevent or limit cooperation between local law enforcement and federal immigration enforcement officials. Based on the Center for Immigration Studies, there were roughly 300 sanctuary cities across the nation. The National Conference of State Legislatures currently states that there are 12 states and the District of Columbia that have laws allowing undocumented immigrants to obtain a driver’s license.
There is not a singular definition of what a sanctuary city is. They are also very controversial from a liberal and conservative perspective. Those who want sanctuary cities believe undocumented immigrants offer a workforce to fill employment gaps that major cities need. Others believe the immigration system is broken, so sanctuary cities provide places of refuge for law-abiding undocumented immigrants. Whereas, those against sanctuary cities think they’re locations violating federal laws by intentionally employing undocumented immigrants.
Because of the separation of power between local, state, and federal governments, as noted in the 10th Amendment, neither Congress nor the President can stop sanctuary cities. But Congress and the President can block federal funding to these cities.
Understanding Distribution and City Size
Under very unusual circumstances, one might find that among a group of cities, no single city has unique site location advantages over others. This might happen out on a vast plain, like in Kansas, where there are no navigable rivers, waterfalls, or ports. In instances like this, situation advantages come to the fore, and a regular, geometric pattern of cities may emerge. This process was more pronounced when transportation was primitive, and the friction of distance was considerable, but it can still be witnessed by picking up a map of almost any flat region of the earth. Geographer Walter Christaller noticed the pattern and developed the Central Place Theory to explain the pattern and the logic driving it forward.
According to Christaller, if a group of people (like farmers) diffuse evenly across a plain (as they were when Kansas opened for homesteaders), a predictable hierarchy of villages, towns, and cities will emerge. The driving force behind this pattern is the basic need everyone has to go shopping for goods and services. Naturally, people prefer to travel less to acquire what they need. The maximum distance people will travel for a good or service is called the range of that good or service. Goods like a hammer have a short-range because people will not travel far to buy a hammer. A tractor, because it is an expensive item, has a much higher range. The cost of getting to a tractor dealership is small about the value of the tractor itself, so farmers will travel long distances to buy the one they want. Hospital services have even higher ranges. People might travel to the moon if a cure for a deadly disease was available there.
Each merchant and service provider also requires a minimum number of regular customers to stay in business. Christaller called this number the threshold population. A major-league sports franchise has a threshold population of probably around a million people, most of whom must live in that team’s range. There are only 30 Major League Baseball teams in the United States, and the team with the smallest market (Milwaukee Brewers) has a threshold population of 2 million people. An ordinary Wal-Mart store probably has a threshold of about 20,000 people, so they are far more numerous. Starbuck’s Coffee shops probably have a threshold of about 5,000 people or less, because there are so many locations.
When customers and merchants living and working on featureless plain interact over time, some villages will attract more merchants (and customers) and grow into towns or even cities. Some communities will not be able to attract or retain merchants, and they will not grow. Competition between towns on this plain prevents nearby locations from increasing simultaneously. As a result, centrally located villages tend to grow into towns at the expense of their neighbors. A network of centrally located towns will emerge, and among these towns, only a few will grow into cities. One very centrally located city may evolve into a much larger city.
The largest cities will have businesses and functions that require significant thresholds (like major league sports teams or highly specialized boutiques). People from villages and small towns can access only the most essential goods and services (like gas stations or convenience stores) and are forced to travel to larger cities to buy higher-order products and services. Those goods and services not available to the nearest large city (regional service center) require customers to travel further. Some goods and services are only available at the top of the urban hierarchy, the mega-cities. In the United States, a handful of cities (New York, Los Angeles, Chicago, and Dallas) may offer exceptionally high order goods, unavailable in other large cities like Cleveland, Seattle, or Atlanta.
Geographer Mark Jefferson developed the law of the primate city to explain the phenomenon of huge cities that capture such a large proportion of a country’s population as well as its economic activity. These primate cities are often, but not always, the capital cities of a country. An excellent example of a primate city is Paris, which truly represents and serves as the focus of France. They dominate the country in influence and are the national focal point. Their sheer size and activity become a strong pull factor, bringing additional residents to the city and causing the primate city to grow even larger and more disproportional to smaller cities in the country. However, not every country has a primate city.
Some scholars define a primate city as one that is larger than the combined populations of the second and third-ranked cities in a country. This definition does not represent real primacy; however, as the size of the first ranked city is not disproportionate to the second.
The law can be applied to smaller regions, as well. For example, California’s primate city is Los Angeles, with a metropolitan area population of 16 million, which is more than double the San Francisco metro area of 7 million. Even counties can be examined about the Law of the Primate City.
Examples of Countries with Primate Cities
- Paris (9.6 million) is the focus of France, while Marseilles has a population of 1.3 million.
- Similarly, the United Kingdom has London as its primate city (7 million), while the second-largest city, Birmingham, is home to a mere one million people.
- Mexico City, Mexico (8.6 million) outshines Guadalajara (1.6 million).
- A considerable dichotomy exists between Bangkok (7.5 million) and Thailand’s second city, Nonthaburi (481,000).
Examples of Countries that Lack Primate Cities
- India’s most populous city is Mumbai (formerly Bombay) with 16 million; second is Kolkata (formerly Calcutta) with more than 13 million, and third is less than 13 million.
- China, Canada, Australia, and Brazil are additional examples of non-primate- city countries.
- Utilizing the metropolitan area population of urban areas in the United States, we find that the U.S. lacks an actual primate city. With the New York City metropolitan area population at approximately 21 million, second-ranked Los Angeles at 16 million, and even third-ranked Chicago at 9 million, America lacks a primate city.
In 1949, George Zipf devised his theory of rank-size rule to explain the size cities in a country. He explained that the second and subsequently, smaller cities should represent a proportion of the largest city. For example, if the largest city in a country contained one million citizens, Zipf stated that the second city would contain one-half as many as the first, or 500,000. The third would contain one-third or 333,333, the fourth would be home to one-quarter or 250,000, and so on, with the rank of the city representing the denominator in the fraction.
While some countries’ urban hierarchy somewhat fits into Zipf’s scheme, later geographers argued that his model should be seen as a probability model and that deviations are to be expected.
Understanding Internal City Structure and Urban Development
Most urban centers begin in the downtown region called the central business district (CBD). The CBD tends to be the node or of transportation networks along with commercial property, banking, journalism, and judicial departments like City Hall, courts, and libraries. Because of high competition and limited space, property values for commercial and private ownership tend to be at a premium. CBDs also tend to use land above and below ground in the form of subways, underground malls, and high- rises. Sports facilities and convention centers also tend to be dominating forces in CBDs.
Urban planning is a sub-field of geography and until recently was part of geography departments in academia. An urban planner is someone trained in multiple theories of urban development along with developing ways to minimize traffic, decrease environmental pollution, and build sustainable cities. Urban planners, sociologists, along with geographers, have come up with three models to demonstrate and explain how cities grow.
The first model is called the concentric zone model, which states that cities can develop in five concentric rings. The inner zone of the cities tends to be the CBD, followed by a second ring that tends to the zone of transition between the first and third rings. In this transition zone, the land tends to be used by industry or low-quality housing. The third ring is called the zone of independent workers and tends to be occupied by working-class households. The fourth ring is called the zone of better residences and is dominated by middle-class families. Finally, ring five is called the commuter’s zone, where most people living there have to commute to work every day.
The second model for city development and growth is called the sector model. This model states that cities tend to grow in sectors rather than concentric rings. The idea behind this model is that “like groups” tend to grow in clusters and expand as a cluster. The center of this model is still the CBD. The next sector is called the transportation and industry sector. The third sector is called the low-class residential sector, where lower-income households tend to group. The fourth sector is called the middle-class sector, and the fifth is the high-class sector.
The third and final urban design is called the multiple nuclei model. In this model, the city is more complex and has more than one CBD. A node could exist for the downtown region, another where a university is situated, and maybe another where an international airport is located. Some clustering does exist in this model because some sectors tend to stay away from other sectors. For example, the industry does not tend to develop next to high-income housing.
The multiple nuclei model also features zones common to the other models. Industrial districts in these new cities, unfettered by the need to access rail or water corridors, rely instead on truck freight to receive supplies and to ship products, allowing them to occur anywhere zoning laws permitted. In western cities, zoning laws are often far less rigid than in the East, so the pattern of industrialization in these cities is sometimes random. Residential neighborhoods of varying status also emerged in a nearly random fashion as well, creating “pockets” of housing for both the rich and poor, alongside large zones of lower-middle-class housing. The reasons for neighborhoods to develop where they do are similar as they are in the sector model. Amenities may attract wealthier people, transport advantages attract industry and commerce, and disamenity zones are all that poor folks can afford. There is a sort of randomness to multiple nuclei cities, making the landscape less legible for those not familiar with the city, unlike concentric ring cities that are easy to read by outsiders who have been to other similar cities.
Another model is referred to as “Keno Capitalism.” In this model, based in Los Angeles, different districts are laid out in a mostly random grid, similar to a board used in the gambling game keno. The premise of this model is that the internet and modern transportation systems have made location and distance mostly irrelevant to the location of different sorts of activities within a city.
Geographers Ernest Griffin and Larry Ford recognized that the popular urban models did not fit well in many cities in the developing world. In response, they created one of the more compelling descriptions of cities formerly colonized by Spain – the Latin American Model. The Spanish designed Latin American cities according to rules contained in the Spanish Empire’s Law of the Indies. According to these rules, each significant city was to have at its center a large plaza or town typical for ceremonial purposes. A grand boulevard along which housing for the city’s elite was built stretched away from the central plaza and served as both a parade route and an opulent promenade. For several blocks outward from this elite spine was built the housing for the wealthy and powerful.
The rest of the city was initially left for the poor because there was almost no middle class. The poorly built houses close the central plaza where jobs and conveniences existed. Over time, the houses built by the poor, perhaps little more than shacks, were improved and enlarged. Ford and Griffin called this process in situ accretion. As the city’s population grew, young families and in-migrants built still more shacks, adding rings of housing that is always being upgraded. At the edges of the city are always the newest residents, often squatting on land they do not own.
Sociologists, geographers, and urban planners know that no city exactly follows one of the urban models of growth. However, the models help us understand the broader reason why people live where they do. Higher-income households tend to live away from lower-income households. Renters and house owners also tend to segregate from each other. Renters tend to live closer to the CBD, whereas homeowners tend to live in the outer regions of the city. It should be noted that the three models were developed shortly after World War II and based on U.S. cities; many critics now state that they do not truly represent modern cities.