Missouri Route 21

Route 21 is a highway in eastern Missouri. Its northern terminus is at Route 30 in Affton. Its southern terminus is at the Arkansas–Missouri state line (where it continues as Highway 115). In the St. Louis area, it is known as Tesson Ferry Road, which was named after the 19th century proprietor of the ferry across the Meramec River. Route 21 from the Meramec River to Route B, along with Route M, make up the Jefferson County Scenic Byway.

Route M' is a short arterial highway. It is a major east–west route which connects Route 21 to Interstate 55. For the majority of its length, Route M is a four-lane divided highway with limited access. At its junction with Interstate 55, Route M becomes an undivided two lane road until its eastern terminus at U.S. 61/US 67. Route M was rerouted to its present location in the late 1990s after traffic became too great for the original road to handle. The original route is now known as Old Route M.