Final answer:
Increased air traffic is caused by factors such as deregulation and competition, resulting in lower fares and increased demand for air travel. Airlines have also implemented more efficient systems, such as hub-and-spoke, to provide greater service to more cities.
Step-by-step explanation:
The cause of increased air traffic can be attributed to several factors. One major factor is the greater competition and deregulation in the airline industry, which led to reduced airfares and increased demand for air travel. With lower fares, more people were able to afford flights, resulting in a doubling of the number of air passengers from the late 1970s to the start of the 2000s. Additionally, airlines implemented more efficient systems, such as hub-and-spoke systems, which allowed for greater service to more cities.