East of Tucson, I‑10 parallels and in some cases overlies old US 80 to Benson, and was originally co-signed as US 80 and SR 86. The section of I-10 from Valencia Road to Rita Road was the first construction project in the state of Arizona funded by the Federal Aid Highway Act of 1956. Construction began in 1957 and was completed in 1960. From Benson, the Interstate follows the Southern Pacific mainline east through Willcox and Bowie to New Mexico, rather than bend south to the Mexican border along old US 80 (signed as SR 80 after 1989), through Douglas. The road from Benson east through Willcox was designated SR 86 in about 1935, that route number was subsequently shifted west and exists now between Why and Tucson. The bypass around Benson was opened about 1979, and other than the Phoenix gap was the last section of I‑10 to be opened.