Get busses by route or by vehicle ID.
        
        Arguments: either
            `vid`: "Set of one or more vehicle IDs whose location should be returned." 
                   Maximum of 10 `vid`s, either in a comma-separated list or an iterable.

            `rt`: "Set of one or more route designators for which matching vehicles should be returned."
                  Maximum of 10 routes, either in a comma-separated list or an iterable.
                  
        Response:
            `vehicle`: (vehicle container) contains list of
                `vid`: bus #
                `tmstmp`: local date/time of vehicle update
                `lat`, `lon`: position
                `hdg`: vehicle heading (e.g., 0: north, 180: south)
                `pid`: pattern ID of current trip (see `self.geopatterns`)
                `pdist`: distance into trip
                `rt`: route (e.g, 88)
                `des`: bus destinations (e.g., "Penn to Bakery Square")
                `dly` (optional): True if bus is delayed
                `spd`: speed in mph
                `zone`: current zone (usually `None` here)
                `tablockid`, `tatripid`: unsure, seems internal?
                
        http://realtime.portauthority.org/bustime/apidoc/v1/main.jsp?section=vehicles.jsp

def vehicles(self, vid=None, rt=None):
        """
        Get busses by route or by vehicle ID.
        
        Arguments: either
            `vid`: "Set of one or more vehicle IDs whose location should be returned." 
                   Maximum of 10 `vid`s, either in a comma-separated list or an iterable.

            `rt`: "Set of one or more route designators for which matching vehicles should be returned."
                  Maximum of 10 routes, either in a comma-separated list or an iterable.
                  
        Response:
            `vehicle`: (vehicle container) contains list of
                `vid`: bus #
                `tmstmp`: local date/time of vehicle update
                `lat`, `lon`: position
                `hdg`: vehicle heading (e.g., 0: north, 180: south)
                `pid`: pattern ID of current trip (see `self.geopatterns`)
                `pdist`: distance into trip
                `rt`: route (e.g, 88)
                `des`: bus destinations (e.g., "Penn to Bakery Square")
                `dly` (optional): True if bus is delayed
                `spd`: speed in mph
                `zone`: current zone (usually `None` here)
                `tablockid`, `tatripid`: unsure, seems internal?
                
        http://realtime.portauthority.org/bustime/apidoc/v1/main.jsp?section=vehicles.jsp
        """
        
        if vid and rt:
            raise ValueError("The `vid` and `route` parameters cannot be specified simultaneously.")
        if not (vid or rt):
            raise ValueError("You must specify either the `vid` or `rt` parameter.")

        # Turn list into comma separated string
        if listlike(rt): rt = ",".join( map(str, rt) )
        if listlike(vid): vid = ",".join( map(str, vid) )

        url = self.endpoint('VEHICLES', dict(vid=vid, rt=rt))        
        return self.response(url)