Final answer:
Transportation innovations were the most influential factor in the growth of cities and their expansion into suburbs, particularly with the advent of affordable automobiles and the interstate highway system.
Step-by-step explanation:
Transportation innovations had the greatest effect on the growth of cities and the expansion of cities to suburbs. The late nineteenth and early twentieth centuries witnessed a series of technological advancements that significantly shaped urbanization. Four major inventions – electric lighting, the telephone, skyscrapers, and especially innovations in intra-city transportation – played significant roles in this process.
The expansion of the railroad system, the creation of electric streetcars, and later the widespread adoption of automobiles made living in suburbs while working in city centers feasible. Henry Ford's affordable automobiles and the development of the interstate highway system, along with supportive federal legislation providing low-interest home construction loans, were crucial in facilitating suburban living. This enabled a broader segment of the population to embrace the suburban lifestyle, thus contributing to the growth and spatial expansion of cities.
In summary, while political changes, actions by the federal government, and leadership from mayors all played roles, it was transportation innovations that most dramatically influenced the way cities and suburbs developed during this era.