At Level 2 airports, the principles governing slot allocation are less stringent; airlines periodically submit proposed schedules to the administrating authority, rather than historic performance.
[4] To avoid pollution and financial losses caused by an excessive number of empty flights, these rules have occasionally been waived during periods of temporary but widespread travel disruption, including after the September 11, 2001 attacks and during the SARS epidemic, the Great Recession, and the COVID-19 pandemic.
[5] Maintenance of the rules forces airlines to schedule extra unnecessary flights to keep their slots, wasting fuel; a 2021 expiration of a waiver in the United States was projected to cause 20,000 tons of carbon dioxide emissions per day.
In 2008 Continental Airlines paid US$209 million for four pairs of landing slots from GB Airways at London Heathrow Airport, $52.3m each.
[9] In the United Kingdom, the Department for Transport has stated the slot system "is not designed to stimulate a competitive market environment and has no means of taking into account broader objectives".