I've flown into the top ten busiest airports (movements) in the world with 9 out 10 airports located in the U.S. See the table. Note the three New York airports (EWR, LGA, JFK) don't make the cut, but the airspace is littered with airplanes criss-crossing the skies.
I don't know how we safely flew without the system. A true marvel and one of the top aviation innovations as far as I'm concerned.
WORLD'S BUSIEST AIRPORTS
|1.||Hartsfield-Jackson Atlanta International Airport||Atlanta, Georgia, United States||ATL/KATL||978,824||▬||▼1.6%|
|2.||O'Hare International Airport||Chicago, Illinois, United States||ORD/KORD||881,566||▬||▼4.9%|
|3.||Dallas/Fort Worth International Airport||Dallas/Fort Worth, Texas, United States||DFW/KDFW||656,310||▬||▼4.3%|
|4.||Los Angeles International Airport||Los Angeles, California, United States||LAX/KLAX||622,506||▬||▼8.6%|
|5.||Denver International Airport||Denver, Colorado, United States||DEN/KDEN||619,503||▬||▲0.9%|
|6.||McCarran International Airport||Las Vegas, Nevada, United States||LAS/KLAS||578,949||▬||▼5.0%|
|7.||George Bush Intercontinental Airport||Houston, Texas, United States||IAH/KIAH||576,062||▬||▼4.6%|
|8.||Paris-Charles de Gaulle Airport||Seine-et-Marne/Seine-Saint-Denis/Val-d'Oise, Île-de-France,France||CDG/LFPG||559,816||▬||▲1.3%|
|9.||Charlotte/Douglas International Airport||Charlotte, North Carolina, United States||CLT/KCLT||536,253||▲1||▲2.6%|
|10.||Phoenix Sky Harbor International Airport||Phoenix, Arizona, United States||PHX/KPHX||502,499||▼1||▼6.8%|