The northern sections of the Parkway were mostly completed in the 1950s-1960s. The portion of the parkway from Glebe Road to I-495, was built primarily to provide access for workers at the new Central Intelligence Agency headquarters in Langley, Virginia, in 1959. The portion of the parkway just north of the Key Bridge was considered a model of modern highway design, and it was featured in many scholarly papers, engineering journals, and textbooks of the day. The Capper-Cramton Act received significant amendments in 1946, 1952, and 1958, both funding and terminating significant portions of the unbuilt parkway. The most significant changes came when Congress declined to fund construction of the segments from Fort Washington to the District of Columbia, from I-495 in Virginia to the Great Falls, and from MacArthur Boulevard/Carderock north to the Great Falls. Significant opposition to these segments emerged from the Izaak Walton League, the Wilderness Society, and other groups, which argued that the environmental damage caused by these segments would be too severe to justify their construction.