Final answer:
After 1920, streetcars began to disappear as middle-class Americans increasingly acquired automobiles, leading to suburban development and the decentralization of cities. Electrification and subways also changed public transport, leading to the consolidation of city bus services by mid-20th century.
Step-by-step explanation:
By World War I, most cities in North America had streetcar systems. However, after 1920, streetcars began to disappear due to significant changes in transportation and urban planning. Large numbers of middle-class Americans began acquiring automobiles after 1920, which led to a shift away from the use of streetcars and the development of automobile suburbs. This also contributed to the decentralization of cities, as people were now able to live farther from their workplaces and not rely solely on streetcar lines or railway stations. The introduction of streetcars had previously allowed cities to grow and create neighborhoods along these lines, making daily commutes more accessible to workers.