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 the 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 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.