Demise and Resurgence of interest in Route 66
Excessive truck use during World War II and the comeback of the automobile industry immediately following the war brought great pressure to bear on America's highways. Automobile production jumped from just over 65,000 cars in 1945 to 3.9 million in 1948. Meanwhile, the deterioration of the national highway system was appalling. Virtually all roads, including Route 66, were functionally obsolete because of narrow pavements and antiquated structural features that reduced carrying capacity.

Emergency road building measures developed during wartime left bridges and culverts woefully inadequate for postwar needs. In the 1940s, most bridges in Illinois and Missouri used wood as a substitute for steel. Steel reinforcements were virtually nonexistent in concrete pavement, and sporadic maintenance left U.S. 66 and other highways riddled with potholes and gaping fissures.

The need for a modern system of national highways was painfully obvious. In 1941, Thomas MacDonald, director of the Public Roads Administration, told of the urgency for improved highways across the country in his report, "Highway for the National Defense." MacDonald estimated that 78,000 miles of roads and highways vital to the war effort needed improvements. The director estimated the cost for maintenance and repair to be $458 million. In anticipation of postwar traffic needs, MacDonald proposed a transcontinental expressway not to exceed 40,000 miles, designed to connect all of the major metropolitan centers in the United States. The Interregional Highway Committee, President Roosevelt's advisory group on national defense highways, adopted the so-called MacDonald Plan with the recommendation that $500 million be allocated over three years to implement the interstate highway system. National defense priorities during the war, however, tabled MacDonald's proposal until the surrender of Germany and Japan.

The Federal Aid Highway Act of 1944 incorporated both civilian and military highway needs into a single piece of legislation, the legal embodiment of the MacDonald Plan. The act incorporated the idea of a 40,000-mile national system of interstate highways, but Congress failed to appropriate funds for its construction. Not until the 1950s, and the War Department's prediction that the Korean Conflict was merely a prelude to a more widespread involvement in Asia, did the dream of an interstate system of expressways linking all regions of the United States become a reality.

Ironically, the public lobby for rapid mobility and improved highways that gained Route 66 its enormous popularity in earlier decades also signaled its demise beginning in the mid-1950s. Mass support for an interstate system of divided highways markedly increased during President Dwight D. Eisenhower's second term in the White House. General Eisenhower returned from Germany very impressed by the strategic value of Hitler's Autobahn. "During World War II," he recalled later, "I saw the superlative system of German national highways crossing that country and offering the possibility, often lacking in the United States, to drive with speed and safety at the same time." Heightened global tension hastened by the Cold War affirmed Eisenhower's resolve to improve the defense capabilities of the nation's highways.

Congress responded to the president's commitment by passing the Federal Aid Highway Act of 1956, which provided a comprehensive financial umbrella to underwrite the cost of the national interstate and defense highway system. In accord with the legislation, Interstate 40 west from Oklahoma City through the Texas Panhandle, New Mexico, northern Arizona, and finally ending in Barstow, California would replace the major segment of U.S. 66. By 1960, each of the States along the original U.S. 66 spent between $14 and $20 million to construct their portions of the interstate, designed to accommodate 1975 traffic projections. By 1970, two equally modern four-lane highways, Interstate 55 between Chicago and St. Louis and Interstate 44, which absorbed the old diagonal section from St. Louis to Oklahoma City, replaced the remaining segments of the original Route 66. On June 26, 1979, the American Association of State Highways and Transportation Officials (AASHTO) accepted the recommendation to eliminate the designation of Route 66. The committee noted that "U.S. 66 markings no longer served as a through-state guide to tourists, but in fact generated confusion because the route coincided with interstate designations over much of its length." Many of the States along this part of Route 66 pledged to preserve some symbol of the historic highway with signs reading "Old U.S. 66."

In many respects, the physical remains of Route 66 mirror the evolution of highway development in the United States from a rudimentary hodge-podge of State and county roads to a federally subsidized complex of uniform, well-designed interstate expressways. Various Route 66 alignments, many still detectable, illustrate the evolution of road engineering from coexistence with the surrounding landscape to domination of it. One outstanding example of the highway in its early form is the 3.5 mile section near Miami, Oklahoma, constructed between 1919 and 1924. Many of the original segments of Route 66 have been either abandoned or modified for secondary use.  Modern improvements such as widened shoulders, adequate swales, gentler curves, resurfaced pavement, and brightly painted safety stripes have not been able to keep the highway from becoming obsolete.

The last outdated, poorly maintained vestiges of U.S. Highway 66 succumbed to the interstate system in October 1984 when Interstate 40 at Williams, Arizona, replaced the final section of the original road. In 1985, the highway was officially decommissioned. Soon after, members of the public, private organizations, and local, State, and Federal agencies who understood the historic and social significance of the road began campaigns to preserve and commemorate the highway. As part of these efforts, many historic resources associated with Route 66 have been nominated and listed in the National Register of Historic Places. Numerous associations developed to promote travel and preservation of the road. State agencies worked to mark the road with signs so that the traveling public could remain aware of the route’s location. Some States designated Route 66 as a State and/or National Scenic Byway. Businesses along the road began catering to tourists who continued to seek out the alignments of the route.

In 1990, the United States Congress passed Public Law 101-400, the Route 66 Study Act of 1990, recognizing that Route 66 had “become a symbol of the American people's heritage of travel and their legacy of seeking a better life." In accord with the legislation, the National Park Service conducted the Route 66 Special Resource Study to evaluate the significance of Route 66 in American history and to identify options for its preservation, interpretation, and use. This study led to the enactment of Public Law 106-45 and the creation of the National Park Service Route 66 Corridor Preservation Program. This program provides financial and technical assistance to individuals; nonprofits; local, State, tribal and Federal agencies; and others to help preserve the most significant and representative historic resources along the route for people to learn from and enjoy.