Greenbelt–BWI Airport Line

The Greenbelt–BWI Thurgood Marshall Airport Express Line (commonly shortened to the Greenbelt–BWI Airport Line), designated Route B30, is a bus route operated by the Washington Metropolitan Area Transit Authority between Baltimore-Washington International Thurgood Marshall Airport and the Greenbelt Metro Station of the Green Line of the Washington Metro. The line operates every 40 minutes seven days a week along the Baltimore-Washington Parkway between these two locations with no intermediate stops, with the exception of the BWI Business District Light Rail Stop, and the last bus leaving BWI at 10:44pm. The trip is approximately 28 minutes long.

The line is unique for being the only WMATA bus line that has a connection with any Maryland Transit Administration non-commuter services, and is responsible for providing a regular link between the two services. The original $3 one-way fare was seen as a bargain compared with other transportation modes in the area, including Greyhound and MARC Train Service. The one-way fare was increased to $6 at the end of June 2010, which matches the fare for the MARC train service between BWI's Amtrak/MARC station and Washington's Union Station, as well as the Metrobus service between L'Enfant Plaza and Washington Dulles Airport. As of June 2014, the fare increased to $7.

The B30 line is also partly responsible for BWI Airport ranking as one of the nation's top 10 airports in ease of access. WMATA also provides higher levels of service on the line in special times of need, especially on certain holidays.

The state of Maryland, which provides funds for operating the service, has been criticized for doing little to promote its use.