The city's primary commercial airport is the San Diego International Airport (SAN), also known as Lindbergh Field. It is the busiest single-runway airport in the United States. It served over 17 million passengers in 2005, and is dealing with an increasingly larger number every year. It is located on San Diego Bay three miles (4.8 km) from downtown. San Diego International Airport maintains scheduled flights to the rest of the United States including Hawaii, as well as to Mexico, Canada, Japan, and the United Kingdom. It is operated by an independent agency, the San Diego Regional Airport Authority. In addition, the city itself operates two general-aviation airports, Montgomery Field (MYF) and Brown Field (SDM). By 2015, the Tijuana Cross-border Terminal in Otay Mesa will give direct access to Tijuana International Airport, with passengers walking across the U.S.–Mexico border on a footbridge to catch their flight on the Mexican side.
What isn't the purpose of the Tijuana Cross-border Terminal? (If the question is unanswerable, say "unanswerable")
unanswerable