Complex sentence: San Diego International is the busiest single-runway commercial airport in the United States, and the second-busiest single-use runway in the world after Gatwick Airport Gatwick has two runways, but is said to use one at a time.
Split into simpler sentences: San Diego International is the busiest single-runway commercial airport in the United States. It is also the second-busiest single-use runway in the world after Gatwick Airport Gatwick has two runways, but is said to use one at a time.