Yet the limitations that structure air transportation are mainly human creations.
First,in the interest of air safety, air traffic is channeled along specific corridors so that only a relatively
small portion of sky is in use.
Jetway 554, for example, which passes from high over the Michigan-Indiana state line towards Jamestown, New York via Southern Ontario,accommodates flights from many different cities in the West and Midwest bound for the Northeast,with
nonstop city-pairs such as San Diego-Boston, Chicago-Albany, Phoenix-Providence, and Lose Angeles-Hartford.
China is facing significant air capacity constraints not because its airports are congested,but mostly because a large segment of the airspace is regulated by the military.
Strategic and political factors have also influenced route choice.
For example,the flights of South African Airways were not allowed to over-fly many African nations during the apartheid period, and Cubana Airlines has been routinely prohibited from over-flying the US.
Even more significant was the opening up of Siberian airspace to Western airlines after the Cold War.
The new freedom permitted more direct routes not only between cites like London and Tokyo or New York and Hong Kong but also between transpacific city pairs like Vancouver-Beijing.
Few large areas of airspace forbidden to carriers on political grounds remain.
However, the intervention of the state in airline networks remains pervasive.