In 1939, Homer Hoyt expanded on the concentric zone model to create the sector model, which is also called the Hoyt model after him. The sector model proposes that zones in a city extend outward by transportation, such as railroads and highways.
While accepting the existence of a central business district, Homer Hoyt suggested in 1939 that zones expand outward from the city center along railroads, highways, and other transportation arteries. Using Chicago as an example, an upper class residential sector evolved outward along the desirable Lake Michigan shoreline north of the central business district, while industry extended southward in sectors that followed railroad lines.
In developing this model Hoyt observed that it was common for low-income households to be near railroad lines, and commercial establishments to be along business thoroughfares. Recognizing that the various transportation routes into an urban area, including railroads, sea ports, and tram lines, represented greater access, Hoyt theorized that cities tended to grow in wedge-shaped patterns -- or sectors -- emanating from the central business district and centered on major transportation routes. Higher levels of access meant higher land values, thus, many commercial functions would remain in the CBD but manufacturing functions would develop in a wedge surrounding transportation routes. Residential functions would grow in wedge-shaped patterns with a sector of low-income housing bordering manufacturing/industrial sectors (traffic, noise, and pollution makes these areas the least desirable) while sectors of middle- and high-income households were located furthest away from these functions. Hoyt's model attempts to state a broad principle of urban organization.