In Which I Ponder the Enigma of Signs

Through Tanzania, some, though not all, probably not even most, stops for dala dalas and coasters are marked with a blue sign with a symbol of a bus on it.  Thusly:

Some, though not all, of the signs are further marked with an R.  This begs a deep question.  What in the name of Cranky Stallman's Beard can the R possibly stand for??  The only vehicle that I can think of, in either English or Kiswahili, that starts with an r is rickshaw.  I feel safe in stating that rickshaws are very unlikely to be found in Tanzania.  Rest seems unlikely as well, as the sign is marking a bus stop.  Gari, the generic Kiswahili word for vehicle, has an r in it, so maybe the sign makers are just being whimsically unhelpful by not using a G.  The R could maybe indicate rasmi, which means official, but that's a slight stretch.  Still, what else is there?  Rabelaisian?  Rhizome?  Rhinoceros?  Riddle?  Rheingold?

Thoughts, darlings?