The world's busiest airports by passenger traffic are measured by total passengers (data from Airports Council International), defined as passengers enplaned plus passengers deplaned plus direct-transit passengers. Hartsfield–Jackson Atlanta International Airport has been the world's busiest airport every year since 2000; with all airports combined London has the world's busiest city airport system by passenger count. As of 2016, seven countries have at least two airports in the top 50; the United States of America has 16, China has 8 (including Hong Kong), and the United Kingdom, Japan, Germany, India and Spain have two airports each.