Final answer:
The 1920s saw increased interest and investment in passenger air travel due to advancements in aviation technology, Charles Lindbergh's historic flight, and the development of aviation infrastructure including airports and radio communications.
Step-by-step explanation:
The increase in interest and investment in passenger air travel in the 1920s can be attributed to several key factors. Major technological advancements in the aviation industry post-World War I, symbolized by the transition from producing a few hundred airplanes to around 5,000 per year by the end of the decade, played a pivotal role.
Additionally, the successful solo transatlantic flight of Charles Lindbergh in 1927 captured public imagination and demonstrated the potential of long-distance air travel, leading to a boom in the airline industry and the emergence of airlines such as Boeing and Ford focusing on passenger aircraft.
Meanwhile, radio technology also flourished, further boosting transportation and communication innovations. The late 1920s saw scheduled flights connecting major cities and the beginning of an infrastructure with airports and terminals that permitted efficient boarding and luggage handling, key components for enabling commercial passenger air traffic.