D.C. Mun. Regs. tit. 18, r. 18-2106
The meanings of lane use control signals and the requirements for obedience to such signals are set forth in this section.
A steady DOWNWARD GREEN ARROW means that a driver is permitted to drive in the lane over which the arrow signal is located.
A steady YELLOW X means that a driver should prepare to vacate, in a safe manner, the lane over which the signal is located because a lane control change is being made, and to avoid occupying that lane when a steady RED X is displayed.
A flashing YELLOW X means that a driver is permitted to use a lane over which the signal is located for a left turn, using proper caution.
A steady RED X means that a driver shall not drive in the lane over which the signal is located. This indication shall modify the meaning of all other traffic controls present accordingly.
When lane-use control signals are in use, drivers shall obey all other traffic controls and follow normal safe driving practices.
D.C. Mun. Regs. tit. 18, r. 18-2106