D.C. Mun. Regs. tit. 18 § 703
Each vehicle operated on a highway at any time from one half (½) hour after sunset to one-half (½) hour before sunrise, and at any other time when, due to insufficient light or unfavorable atmospheric conditions, persons and vehicles on the highway are not clearly discernible at a distance of five hundred feet (500 ft.) ahead shall display lighted lamps and illuminating devices as required in this chapter for different classes of vehicles, subject to exceptions with respect to parked vehicles.
The provisions of this chapter which set forth specific distances from which certain lamps and devices shall render objects visible, or distances within which such lamps or devices shall be visible, shall apply during the times stated in this section with respect to a vehicle without load when upon a straight, level, unlighted highway under normal atmospheric conditions, unless a different time or condition is expressly stated.
The provisions of this chapter which require a specific mounted height of lamps or devices shall mean a distance measured from the center of the lamp or device to the level ground upon which the vehicle stands when the vehicle is without a load.
The sections of this chapter which relate to clearance and marker lamps, reflectors, and stop lights shall apply as stated in those sections to vehicles of the type enumerated in those sections; namely, passenger buses, trucks, truck trailers, and certain trailers, semi-trailers, and pole trailers, respectively, when operated upon any street or highway.
The vehicles cited in § 703.4 shall be equipped as required and all lamp equipment required shall be lighted at the times mentioned in this section, Provided, that clearance and side marker lamps need not be lighted on any such vehicle when operated where there is sufficient light to render clearly discernible persons and vehicles on the highway at a distance of five hundred feet (500 ft.).
Whenever motor vehicles and other vehicles are operated in combination during the time when lights are required, any lamp (except tail lamps) need not be lighted which, by reason of its location on a vehicle of the combination, would be obscured by another vehicle of the combination; Provided, that this shall not affect the requirement that lighted clearance lamps be displayed on the front of the foremost vehicle required to have clearance lamps, nor that all lights required on the rear of the rear most vehicle of any combination shall be lighted.
Whenever the load upon any vehicle extends to the rear four feet (4 ft.) or more beyond the bed or body of the loaded vehicle, there shall be displayed at the extreme rear end of the load, at the times specified in this section, a red light or lantern plainly visible from distance of at least five hundred feet (500 ft.) to the sides and rear.
The red light or lantern required in this section shall be in addition to the red rear light required upon every vehicle.
At any other time when lights are not required, there shall be displayed at the extreme rear end of the extended load a red flag or cloth not less than twelve inches square (12 in.) which is so hung that the entire area is visible to the driver of a vehicle approaching from the rear.
In addition to the requirements of subsection 703.1, whenever a motor vehicle's windshield wipers are operated for a continuous period of time because of impaired visibility resulting from unfavorable atmospheric conditions, the vehicle headlamps shall also be lighted. Provided, however:
D.C. Mun. Regs. Tit. 18, § 703