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.
History
thumb|Intersection of Route 21 and Route 110 right outside of De Soto
The section through northern Jefferson County was considered dangerous. As a result, the road was rerouted and built to freeway standards. Construction to reroute the highway to just south of Hillsboro was completed on December 15, 2008, and Route 21 is currently freeway standard from Route 141 to Highway B. Plans to extend the freeway south to De Soto have been approved, but funds are lacking to complete this part of the project.
Route description
Route 21 begins at the Arkansas state line, where it continues southward as Highway 115. As the highway enters Ripley County, it passes through the city of Doniphan, where it intersects US 160 and Route 142. Route 21 runs concurrently with US 160 for a short distance before separating east of Doniphan.
Continuing northward, Route 21 enters Carter County, where it meets US 60 near Van Buren. The highway briefly overlaps US 60 before diverging and heading northeast into Reynolds County. In Reynolds County, Route 21 passes through Ellington, where it intersects Route 72 and continues northward toward Iron County.
|location=Otto–Barnhart
Route M begins as a four-lane divided highway at a diamond interchange with Route 21 near the community of Otto. West of the interchange, the highway is called Route MM. The highway heads east for less than where it has a partial cloverleaf interchange with the former alignment of Route 21, appropriately named Old Route 21. It continues east where there are two turn-offs which connect to nearby grade-separated highways. The first highway is the former alignment of Route M (Old Route M), the second is Old Lemay Ferry Road near the community of Antonia. Only the eastbound lanes have direct access to Old Lemay Ferry Road. Access is provided to the westbound lanes via median u-turn crossovers on either side of the intersection.
Route M continues east, where it has two at-grade intersections before entering Barnhart; one at St. Luke's Church Road and the other at Moss Hollow Road. At Barnhart, the intersection with Marriott Parkway connects Interstate 55 traffic with gas stations and retail stores. , the I-55 interchange was being reconstructed to better handle access to southbound I-55.
Due to an increasing number of accidents at the intersection of Route M with Old Lemay Ferry Road, MoDOT rebuilt the intersection in 2007, with the eastbound lanes only having access to Old Lemay Ferry Road. Turnarounds built short distances east and west of the intersection allow access to and from the westbound lanes. As of 2011, the interchange is fully open. Southbound I-55 access is possible through a roundabout.
Connector route
A connector between Route M and Old Lemay Ferry Road exists in Jefferson County.