def slowDown(self, vehID, speed, duration):
        """slowDown(string, double, int) -> None

        Changes the speed smoothly to the given value over the given amount
        of time in ms (can also be used to increase speed).
        """
        self._connection._beginMessage(
            tc.CMD_SET_VEHICLE_VARIABLE, tc.CMD_SLOWDOWN, vehID, 1 + 4 + 1 + 8 + 1 + 4)
        self._connection._string += struct.pack(
            "!BiBdBi", tc.TYPE_COMPOUND, 2, tc.TYPE_DOUBLE, speed, tc.TYPE_INTEGER, duration)
        self._connection._sendExact()
slowDown(string, double, int) -> None

        Changes the speed smoothly to the given value over the given amount
        of time in ms (can also be used to increase speed).