Final answer:
Mass production techniques, particularly Ford's implementation of the assembly line, made cars affordable for the average American, leading to widespread car ownership and making it the main mode of transportation.
Step-by-step explanation:
The quality that made mass-produced cars into America's most popular mode of transportation was affordability caused by the innovation of mass production techniques. The introduction of the assembly line by Henry Ford, in particular, dramatically reduced the cost of automobiles, making car ownership possible for the average American. Coupled with the rise of related industries and infrastructure, the automobile became a symbol of American prosperity and freedom of movement.
Ford's Model T is a notable example, where the price significantly dropped from $850 in 1908 to $300 in 1924. Thus, not only the wealthy, but also a larger percentage of the population, could afford a car. This caused a major shift in the economic landscape and everyday lifestyles of Americans as suburban living and cross-country travel became accessible to many.