In 1957, a new freeway, which is today's I-55, was opened as US 66 between Gardner and Welco Corners, bypassing both Braidwood and Plainfield. Most portions of the 1940 western alignment that were not incorporated into the new freeway reverted to their previous state routes, except for the section from Gardner through Braidwood, which became IL 129. This freeway was originally designated only as US 66 and was then also designated as I-55 in 1960, becoming the first complete section of I-55 in Illinois. It served as mainline US 66 for 19 years, from 1957 to 1976, longer than either of the two previous alignments.