Missouri Route DD

Missouri Supplemental Routes
Highway markers for Routes A and BB
System information
Formed:	1952
Highway names
State:	Route xx
System links
Missouri State Highway System Interstate US State Supplemental

A supplemental route is a state secondary road in the U.S. state of Missouri, designated with letters. Supplemental routes were various roads within the state which the Missouri Department of Transportation was given in 1952 to maintain in addition to the regular routes, though lettered routes had been in use from at least 1932. The goal of the secondary highway system was to place state-maintained roads within 2 miles (3 km) of more than 95% of all farm houses, schools, churches, cemeteries and stores. The four types of roads designated as Routes are:

Supplemental routes make up 19,064 miles (30,681 km) (59%) of the state highway system.

The more major supplemental routes of the system are ones assigned with single-letter designations (such as "K"). Minor branch routes and farm-to-market roads, which often end at county roads or are former alignments of the other highways, are typically assigned with two-letter designations consisting of two of the same letter (e.g. "KK"). Additionally, combinations of letters may be used, but always with A as the first letter (such as "AD"); the only exception of this is on Route BA in western St. Louis County. Combinations beginning with the letter R are also used for routes that connect with state parks or other recreational facilities, which is the only use of R on the system; Route AR, a non-recreational route south of Bakersfield, is the only exception to this.

The vast majority of the highways in the system are designated with 19 letters of the alphabet. The letters "G", "I", "L", "Q", and "S" are not used because of the potential confusion with other letters and numbers. The only current use of X is on Route AX in Macon County.

These routes rarely run for more than a few miles, although they may cross county lines. At a U.S. Route or Interstate highway, they change their letters; for example, in Greene County, southbound Route J crosses U.S. Route 60 and becomes Route NN, and Route M in Cole County becomes Route J after passing U.S. Route 50. But when Route MM (Greene County) crosses Route 360, it remains Route MM, only changing into Route B when crossing Interstate 44. Route J in Boone County is one of the few exceptions to this, as it continues past U.S. 40 for three miles before becoming Route O upon passing I-70. The names are also reused, but not near one another. Rarely is a letter re-used in a county; there are two Route Z's in Randolph County and two route K's in Carter County. Route D exists in the counties of Cole, Greene, Newton, St. Louis, and several others.