 At exit 170 south of Festus, the Great River Road joins I-55, following that highway into St. Louis and using I-44 and I-70 to exit 220 on the St. Peters-O'Fallon border.From there the National Route follows the entire length of Route 79 to I-72 in Hannibal, where it crosses the Mark Twain Memorial Bridge back to Illinois.A second state alternate begins in Hannibal by running west on I-72 to its end at US 61, then turning north on US 61.