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)