docs: def route_directions(self, rt):
          """
          Return a list of directions for a route.
          
          The directions seem to always be INBOUND and OUTBOUND for the busses
          currently, where INBOUND is towards downtown and OUTBOUND is away from
          downtown. (No idea if this is going to change.)
          
          Arguments:
              `rt`: route designator
          
          Response:
              list of `dir`: directions served (e.g., INBOUND, OUTBOUND)    
              
          http://realtime.portauthority.org/bustime/apidoc/v1/main.jsp?section=routeDirections.jsp    
          """
          url = self.endpoint('R_DIRECTIONS', dict(rt=rt))
          return self.response(url)