The subject of the diptych US 66 (States)/US 66 (Cities) is Route 66, one of the original highways in the U.S. Highway System. Indiana first referenced the highway, along with highways 40, 37, and 29, all routes on which he had traveled, in his first and third American Dream paintings.
Numbers had specific personal meanings for Indiana, often linked to events and people in his life. Six was the number he associated with his father, who was born in June, the sixth month, worked for the gasoline company Phillips 66, and disappeared down Route 66 when he left his family. The route, which ran from Chicago, Illinois through Missouri, Kansas, Oklahoma, Texas, New Mexico, and Arizona before terminating in Los Angeles, California, covered a total of 2,448 miles. The names of the eight states through which the highway ran are incorporated into US 66 (States), and the name of a city in each of these states is incorporated into US 66 (Cities).
The colors of the paintings also recall signs that Indiana had seen along highways as a child: the red, green, and blue of US 66 (States) was inspired by the Philiips 66 sign, “standing high in a blue sky, red and green as the company colors were at the time,” and the black and yellow of US 66 (Cities) by advertising for 666 Cold Preparation, “small metal plates affixed to farmers’ fences—black and yellow—dotted the pastures and fields like black-eyed susans in perennial bloom.” 
 Excerpts from the artist statement for USA 666 (The Sixth American Dream), published in William C. Seitz and Lloyd Goodrich, São Paulo 9—United States of America: Edward Hopper [and] Environment U.S.A.: 1957–1967 (Washington, D.C.: Smithsonian Institution Press, 1967).