Bus travel is the most economical and flexible form of transportation. Air travel saw revolutionary growth following World War II as airlines carried mail and more people gained familiarity with air travel. Advances like charter services further reduced airfares while improving quality. International air travel requires bilateral agreements between nations regarding rights like overflight, landing, and carrying traffic. The lodging industry has a long history and includes accommodations like hotels, motels, resorts, and campgrounds that developed in response to travelers' needs.