The Sector Model is an urban model which succeeded and modified the Concentric zone model. The Sector Model was the first urban model to include the aspect of transportation in urban expansion and the layout of social classes. The model hypothesized that zones of expansion develop in sections instead of rings like the Concentric zone model. These sections were centered around major railroads and highways, and social classes were also effected by these transportation networks.
Sections were also chosen for the model because they were more feasible of growth than rings. If a ring which was surrounded by other rings were to grow, it's expansion would cross over into the surrounding rings causing the surrounding rings to loose land area. While if a section were to grow then it would not interfere with any other section. Since no section was completely surrounded by other sections, a section could simply grow outward without crossing over into another section's area. The Sector Model was eventually succeeded by the Multiple nuclei model in 1945.