A provision existed that allowed any Heathrow-authorised British carrier operating as the sole carrier between London Gatwick and any US city to switch that service to Heathrow as long as the incumbent airline had demonstrated its ability to carry a minimum of 300,000 non-stop passengers (both transfer and point-to-point) between that city and London, and vice versa, over a period of two consecutive calendar years. Using this method, BA was able to transfer Denver, San Diego and Phoenix services from Gatwick to Heathrow in 2002.