Detailed Instructions: Answer the question from the given passage. Your answer should be directly extracted from the passage, and it should be a single entity, name, or number, not a sentence.
Problem:Passage: Southern California is home to Los Angeles International Airport, the second-busiest airport in the United States by passenger volume (see World's busiest airports by passenger traffic) and the third by international passenger volume (see Busiest airports in the United States by international passenger traffic); San Diego International Airport the busiest single runway airport in the world; Van Nuys Airport, the world's busiest general aviation airport; major commercial airports at Orange County, Bakersfield, Ontario, Burbank and Long Beach; and numerous smaller commercial and general aviation airports. Question: What is the metric they use to determine how busy airports are?
Solution:
passenger volume