The introduction of the Model-T automobile by Henry Ford in 1908 spurred the third stage of suburbanization. The rapid adoption of the mass-produced automobile by Americans led to the creation of the automobile-oriented suburb [†] of single-family houses on spacious lots that has become the quintessential American landscape of the twentieth century.
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Between 1910, when Ford began producing the Model-T on a massive scale, and 1930, automobile registrations in the United States increased from 458,000 to nearly 22 million. Automobile sales grew astronomically: 2,274,000 cars in 1922, more than 3,000,000 annually from 1923 to 1926, and nearly four and a half million in 1929 before the stock market crashed. According to Federal Highway Administration statistics, 8,000 automobiles were in operation in 1900, one-half a million in 1910, nine-and-a-quarter million in 1920, and nearly 27 million in 1930. 
The rise of private automobile ownership stimulated an intense period of suburban expansion between 1918 and the onset of the Great Depression in 1929. As a result of the increased mobility offered by the automobile, suburban development began to fill in the star-shaped city created by the radial streetcar lines. Development on the periphery became more dispersed as workers were able to commute longer distances to work, as businesses moved away from the center city, and as factories, warehouses, and distribution centers were able to locate outside the railroad corridors due to the increased use of rubber-tired trucks. 
The popularity of the automobile brought with it the need for a new transportation infrastructure that included the construction and improvement of roads and highways, development of traffic controls, building of bridges and tunnels, and widening and reconstruction of downtown streets. One of the most unheralded structures that facilitated the growth of the suburbs was the perfected mechanical road. Automobiles required smooth, hard surfaces, and before 1900, even in cities, most roads were unpaved. Asphalt, introduced in the 1890s, became the common road surface by 1916. 
Beginning in the 1890s, the City Beautiful movement spurred advances in city planning and urban design. Transportation planning, as well as the improvement of streets, was recognized as central to the coordinated growth of urban areas. In cities such as Kansas City, Denver, and Memphis, the collaboration of planners, landscape architects, architects, and local political leaders, forged a rich legacy of parkways and boulevards that linked new residential suburbs with the center city. Highly influential were the writings of Charles Mulford Robinson, a journalist and advocate for Denver's park and parkway system. These included Improvement of Towns and Cities (1901), Width and Arrangement of Streets (1911), and City Planning, with Special Reference to the Planning of Streets and Lots (1916).
Proposed in 1906 and built between 1916 and 1924, the Bronx River Parkway was one of the first modern parkways designed for automobiles. Sixteen miles in length, the parkway connected suburban communities in Westchester County with downtown New York. The parkway followed the Bronx River through a reservation initially established to reclaim what had become a polluted and unsightly watershed. Featuring a right-of-way ranging from 300 to 1,800 feet, the parkway was extensively planted with trees and shrubs, provided scenic river views, and achieved the illusion of being totally separated from adjoining development. The alignment featured graceful curves and gently followed the undulating topography to give motorists, many of whom were daily commuters, a pleasurable driving experience. 
Metropolitan areas expanded as streets, parkways, and boulevards extended outward, opening up new land for subdivision. As new radial arterials were built, suburban development became decentralized, creating fringes of increasingly low densities. With commuters no longer needing to live within walking distance of the streetcar line, residential suburbs could be built at lower densities to form self-contained neighborhoods that afforded more privacy, larger yards, and a parklike setting. Neighborhood improvements typically included paved roads, curbs and gutters, sidewalks, and driveways, as well as connections to municipal water systems and other public utilities. 
Concerns over pedestrian safety emerged as automobile use increased, and by the late 1920s, subdivision designers and housing reformers alike were examining ways to separate neighborhood traffic from arterial traffic and to design neighborhoods that remained safe, quiet, and free of speeding traffic. The "Radburn Idea," first introduced by Clarence Stein and Henry Wright in their 1928 design for a "Town for the Motor Age," called for separate circulation systems to serve pedestrians and automobiles. Published a year later in the regional plan for metropolitan New York City, Clarence Perry's Neighborhood Unit Formula called for a hierarchy of streets of varying widths to control automobile traffic.
In 1916 the United States Congress passed the Federal Aid Highway Act, authorizing expenditure of Federal funds for up to 50 percent of the cost of State road projects within the Federal aid network. During the 1920s, most States established highway departments, and the total miles of surfaced highway in the Nation doubled. 
During the "golden age of highway building" from 1921 to 1936, more than 420,000 miles of roads were built in the United States. The increase in intercity highways and roads connecting farms with markets made new land available for suburbanization. Advances in highway engineering, including the development of divided highways, bridges and tunnels, and cloverleaves, made automobile travel faster and safer. 
Suburban areas continued to grow faster than central cities, and the planning of metropolitan highway systems gained increasing attention. High speed roads extending outward from central cities appeared in major metropolitan areas: Lakeshore Drive to Chicago's northern suburbs opened in 1933; and, in 1936, the Grand Central Parkway was added to the already extensive system of roads on Long Island built under Robert Moses's direction. In 1940, the opening of the Arroyo Seco Freeway in Los Angeles heralded a new age of freeway construction connecting city and suburb. 
The Futurama exhibit sponsored by General Motors Corporation at the 1939 New York World's Fair presented one of the most influential and memorable visions for the future of highway engineering, and with it suburban life. Designed by Norman Bel Geddes, the exhibit featured a huge diorama of the American landscape overlaid with an intricate network of high-speed, multi-lane, limited-access highways joining country and city. Called "magic motorways," the highways featured total separation of grades and graduated speeds. A ring highway surrounded the city interconnecting with radial freeways that guided suburban commuters to the center city where exit ramps eventually led to underground garages.
In its 1938 report, Toll Roads and Free Roads, the Bureau of Public Roads called for a master plan for highway development, a series of upgraded interregional roads, and the construction of express highways into and through cities to relieve urban traffic congestion. The report also outlined the routes for six transcontinental highways and debated the feasibility of using tolls to support highway construction. 
The emergency of World War II intervened, and Federal highway spending was limited to the improvement of roads directly serving military installations or defense industries. In 1941 President Franklin D. Roosevelt appointed a seven-member Interregional Highway Committee to work with Public Roads administrator Thomas H. MacDonald on recommendations for national highway planning following the war. The committee's recommendations for an extensive 32,000-mile national network of expressways resulted in the Federal Aid Highway Act of 1944. The act authorized a National System of Interstate Highways, which included metropolitan expressways designed to relieve traffic congestion and serve as a framework for urban redevelopment. 
Since Congress did not appropriate additional funds for the system's construction until the mid-1950s, State highway departments were forced to rely on other sources, including public bonds, toll revenues, and the usual matching Federal funds earmarked for the improvement of the Federal aid highway network. 
From the end of World War I until 1945, increasing automobile ownership accelerated suburbanization and significantly expanded the amount of land available for residential development. This trend further stimulated the design and construction of a new infrastructure of roads, highways, bridges, and tunnels, laying the groundwork for highway systems that would transform metropolitan areas after World War II.
† Adapted from: Linda Flint McClelland, Historian and Sarah Dillard Pope, Historian, National Park Service; David L. Ames, University of Delaware, Historic Residential Suburbs in the United States, 1830-1960, nomination document, 2002, National Park Service, National Register of Historic Places, Washington, D.C.