The maps below show the development of the US air travel network from 1990 to 2020. The red circles represent the airports in the contiguous US in the given year, with the radius of each being proportional to the number of departing flights in that year. The purple lines represent all routes with at least one endpoint in the contiguous US and with at least 1,000 passengers in the given year.
A version of the 2015 map, with domestic routes within the contiguous US only and displaying the metropolitan areas, appears in Sheard, N., 2021, “The Network of US Airports and its Effects on Employment”, Journal of Regional Science 61(3), 623–648. That paper analyses the effects of the air network on the distribution of the US labour force and employment.