During the late 1960's, air congestion
often involving long delays or "stacks" was
common at major airports. The right to land
and take off was allocated on a first-come,
first-served basis with little coordination
among scheduled carriers. Since 1968, the
four major airports in the United States,
La Guardia, Washington National, John
F. Kennedy International, and O'Hare International,
have been operating under a
Federal Aviation Administration (FAA)
high-density ruling which limits the number
of slots (takeoffs and landings per hour) at
each of these airports
