About: Missouri Route M (Jefferson County)

Route M is a short arterial highway in Jefferson County, Missouri. 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. 61/67. 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.