On a highway, you see a flashing yellow "X" signal above a lane. What does it mean?

Lane use control signals are special overhead signals indicating which lanes of a highway may be used at certain times. A flashing yellow "X" signal above a lane means that the lane can only be used for left turns. [Lane Use Control Lights, Rules of the Road, Part IV: Operating a Motor Vehicle, State of Maine Motorist Handbook and Study Guide]