sentences:  Air travel first entered as a viable alternative to transcontinental railroads, and to the then-primitive (or non-existent) road networks that crossed the United States and Canada in the early 1930s, but truly increased in popularity after the Second World War.Most of the continent's busiest airports are located in the United States.In fact the U.S. has 9 of North America's 10 busiest airports, including the world's busiest, Hartsfield-Jackson Atlanta International Airport in Atlanta.