Air transport enjoys greater freedom of route choice than most other modes. while it is true that the mode is less restricted than land transport to ific rights of speci way, it is nevertheless much more constrained than what might be supposed. Earty i the such as the RockyMountainsand the great gap of the history of aviation, physical obstacles North Atlantic limited the articulation of air transport networks. while those limitations have fallen, physical geography still affects the geography of intercity air transportation. Aircraft seek, for instance, to exploit(or avoid upper atmospheric winds, in particular the jet stream, to enhance speed and reduce fuel consumption. volcanic eruptions may also impede air travel by releasing ash in the atmosphere, which can damage and even shot down turbofan engines. Such occurrences are however rare and punctual, with the exception of April 2010 when a volcanic eruption in Iceland forced the closing down of in most of Europe as well as several North Atlantic routes. is represented the largest natural disruption of air travel in history. Yet the limitations that structure air transportation are mainly human creations. First, in the interest ofair safety, air traffic is channeled along specific corridors so that only a relatively the sky isin use. Jetway 554, for example, which passes from high over the small portion o 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 such as San Diego Boston, Chicago-Albany, Phoenix- Northeast, with nonstop city-pairs Providence, and Los 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 cities like London and Tokyo or New York and Hong Kongbutalso between transpacificcity 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.