The US embarked on a huge effort in the early airmail days to build enough airports that aircraft could follow federal airways without ever being out of glide r

The US embarked on a huge effort in the early airmail days to build enough airports that aircraft could follow federal airways without ever being out of glide range of one of them - these were the CAA intermediate fields, many of them not much more than a grass strip with a beacon, but many others were built up over time and remain municipal airports today. Later, WWII lead to the construction of even more airfields for pilot training. So many pilots were being trained that the training bases didn't have the capacity for the pattern practice, so many training airfields had multiple "outlying fields" that were just extra sets of runways a few miles away. Some of these turned into municipal airports later, but many more were abandoned for lack of need. By the time the freeway system was built, the US was positively lousy with airports, much denser than seen in most other countries.