Missouri Routes are Letters

This is something that I've never seen before.  Apparently in Missouri most (all?) of their state routes are actually letters instead of numbers.  Either a single letter or a double letter but I didn't decode the meaning.  I'm not sure what the rationale is, I would think numbers would be easier especially since there might be illiterate people out there driving around.  I just thought it was strange.  [caption align=left caption="Missouri Letter Routes"]Missouri Letter Routes[/caption]