public-private partnerships between the District of Columbia Department of Transportation, WMATA, and DC Surface Transit, Inc. (DCST). The bus system services each stop approximately every 10 minutes. Many other public bus systems operate in the various jurisdictions of the Washington region outside of the city in suburban Maryland and Virginia. Among these are the Fairfax Connector in Fairfax County, Virginia; DASH in Alexandria, Virginia; and TheBus in Prince George's County, Maryland. There are also numerous commuter buses that residents of the wider Washington region take to commute into the city for work or other events. Among these are the Loudoun County Transit Commuter Bus and the Maryland Transit Administration Commuter Bus. The city also has numerous buses used by tourists and others visiting the city. Among the most popular tourist buses are Big Bus Tours, Old Town Trolley Tours, and DC Trails. The city also sees many charter buses carrying young students and other tourists from across the country to the Washington region's historic sites. These buses are often found parked beside famous tourist locations, such as the National Mall. Air Three major airports serve the district, though none are within the city's borders. Two of these major airports are located in suburban Virginia and one in suburban Maryland. The closest is Ronald Reagan Washington National Airport, which is located in Arlington, Virginia just across the Potomac River about 5 miles from downtown DC. This airport is primarily reserved for domestic flights and has the lowest number of passengers of any of the three airports in the region. The busiest by international flights and the largest by land size and amount of facilities is Washington Dulles International Airport, located in Dulles, Virginia, about 24 miles west of the city. Dulles has the most international passenger traffic of any airport in the Mid-Atlantic outside the New York metropolitan area, including approximately 90% of the international passenger traffic in the Washington-Baltimore region. The busiest by number of total passenger boardings is Baltimore/Washington International Airport (BWI), located in Anne Arundel County, Maryland about 30 miles northeast of D.C. Each of these three airports also serves as a hub for a major American airline: Reagan is a hub for American Airlines, Dulles is a major hub for United Airlines and Star Alliance partners, and BWI is an operating base for Southwest Airlines. The President of the United States does not use these airports for