Explain the missouri road system of using letters and double letters on highways?

Question

Explain the missouri road system of using letters and double letters on highways?

Answer 1

Those are county highways.

Letters are assigned by each county.

There could be identical lettered roads in adjoining counties.

Answer 2

These are Missouri supplemental highways. It is a misconception that they are county highways, they are not.  Even though the same designation might exist as a different road in a different county. (ex. theres a hwy BB in at least Crawford, Jefferson & Phelps counties).  They are maintained entirely by MoDOT (Missouri Department of Transportation).

Answer 3

Numbers confuse them.