The first airport in the area was Arlington's Hoover Field, which opened in 1926. Near the present site of the Pentagon, its single runway was crossed by a street; guards had to stop automobile traffic during takeoffs and landings. The following year, in 1927, Washington Airport, another privately operated field, began service next door. In 1930, the Great Depression led the two terminals to merge to form Washington-Hoover Airport. Bordered on the east by U.S. Route 1, with its accompanying high-tension electrical wires, and obstructed by a high smokestack on one approach and a dump nearby, the field was inadequate. The need for a better airport was acknowledged in 37 studies conducted between 1926 and 1938, but a statute prohibited federal development of airports. When Congress lifted the prohibition in 1938, President Franklin D. Roosevelt made a recess appropriation of $15 million to build National Airport by reallocating funds from other purposes. Construction of Washington National Airport began in 1940–1941 by a company led by John McShain. Congress challenged the legality of FDR's recess appropriation, but construction of the new airport continued. The airport is located southwest of Washington, D.C., in the Crystal City section of Arlington County, Virginia, adjacent to National Landing. The western part of the airport was once within a large Virginia plantation, a remnant of which is now inside a historic site near the airport's Metrorail station. The eastern part of the airport was built in the District of Columbia on and near mudflats in the tidal Potomac River near Gravelly Point, about 4 statute miles (6.4 km) from the United States Capitol, using landfill dredged from the Potomac River. The airport opened June 16, 1941, just before U.S. entry into World War II. The public was entertained by displays of wartime equipment including a captured Japanese Zero war prize flown in with U.S. Navy colors. In 1945 Congress passed a law that established the airport was legally within Virginia, mainly for liquor sales taxation purposes, but under the jurisdiction of the federal government. On July 1 of that year the airport's weather station became the official point for D.C. weather observations and records by the National Weather Service, in Washington, D.C. Until 1946, nonstop airline flights did not reach beyond New York City, Detroit, Cincinnati, Memphis, Atlanta, and Jacksonville. In 1946, Boston, Chicago, Dallas, and Miami were added; nonstops reached Denver in 1951 and Los Angeles in 1954. The April 1957 Official Airline Guide shows 316 weekday departures: 95 Eastern (plus six per week to/from South America), 77 American, 61 Capital, 23 National, 17 TWA, 10 United, 10 Delta, 6 Allegheny, 6 Braniff, 5 Piedmont, 3 Northeast and 3 Northwest. Jet flights began in April 1966 (727-200s were not allowed until 1970). In 1974 the airport's key carriers were Eastern (20 destinations), United (14 destinations after subsuming Capital) and Allegheny (11 destinations). The grooving of runway 18–36 to improve traction when wet, in March 1967, was the first at a civil airport in the United States. Service to the airport's Metro station began in 1977. The Washington National Airport Terminal and South Hangar Line were listed on the National Register of Historic Places in 1997. The runway layout has changed little since the 1956 closure of the east–west runway at the south end of the field. Changes to the terminal complex over the years include: Extension of the original Main Terminal (today's Terminal 1) to the south in 1950 The construction of a North Terminal supplemented the original terminal in 1958; construction connected the two terminals in 1961. A United Airlines holdroom complex was built in 1965, a facility for American Airlines was completed in 1968, and a facility for Northwest Airlines and TWA (still in use today as the Terminal A concourse), along with a commuter terminal in 1970.