From Wikipedia: "Reagan National is a hub for US Airways, Reagan National's largest carrier. The US Airways Shuttle offers near-hourly air shuttle service to LaGuardia Airport in New York City and Logan International Airport in Boston, Massachusetts. Delta Air Lines' Delta Shuttle also offers near-hourly air shuttle service to LaGuardia.
"Other than 40 slot exemptions, flights into and out of DCA are not allowed to exceed 1,250 statute miles in any direction nonstop, in an effort to send the major air traffic volume to the larger but more distant Dulles International Airport. In 2010, the airport served about 18.1 million passengers.
"Reagan National only provides US immigration and customs facilities for corporate jet traffic; the only international flights allowed to land at DCA are those from airports with U.S. Customs and Border Protection preclearance facilities. For all other international passenger flights, those in the Washington Metropolitan Area can use Dulles International Airport west of the city and Baltimore-Washington International Airport northeast of the city."