Which U.S. route became popularized as the "Mother Road" of America?

U.S. Route 66 is one of the original highways in the United States Numbered Highway System. It was established on November 11, 1926, with road signs erected the following year. The highway, which became one of the most famous roadways in the United States, ran from Chicago, Illinois, through Missouri, Kansas, Oklahoma, Texas, New Mexico, and Arizona before terminating in Santa Monica in Los Angeles County, California, covering a total of 2,448 miles (3,940 km). In John Steinbeck's novel The Grapes of Wrath (1939), the highway symbolizes escape, loss, and the hope of a new beginning; Steinbeck dubbed it the Mother Road.

Source: Wikipedia