Another topic of intensive study has been the estimation,
through the use of queueing models, of the
delays caused by the lack of sufficient runway capacity.
This is a problem that poses a serious challenge to
operations researchers: The closed-form results developed
in the voluminous literature of classical steadystate
queueing theory are largely nonapplicable—at
least when it comes to the really interesting cases. The
reason is that airport queues are, in general, strongly
nonstationary. The demand rates and, in changing
weather conditions, the service rates at most major
airports vary strongly over the course of a typical
day. Moreover, the demand rates may exceed capacity
(>1), possibly for extended periods of time, most
often when weather conditions are less than optimal.