D.C. Code § 9-603
It shall be the duty of the Director of National Park Service within the first 8 hours of daylight after the ceasing to fall of any snow or sleet, or after the accumulation of ice upon the paved sidewalks within the fire limits of the District of Columbia, to remove or cause to be removed from such sidewalks as are in front of or adjacent to all buildings owned or leased by the United States, except the Capitol buildings and grounds and the Library of Congress building, and from all paved sidewalks or crosswalks used as public thoroughfares in front of, around, or through all public squares, reservations, or open spaces within the fire limits of the District of Columbia, owned or leased by the United States, such snow, sleet, and ice; but in the event of inability to remove such accumulation of snow, sleet, and ice, by reason of the hardening thereof, without injury to the sidewalk, it shall be his duty, within the first 8 hours of daylight after the hardening of such snow, sleet, and ice, to make reasonably safe for travel, or cause to be made reasonably safe for travel, by the sprinkling of sand or ashes thereon, such paved sidewalks and crosswalks, and shall, as soon thereafter as the weather shall permit, thoroughly clean said sidewalks and crosswalks.
D.C. Code § 9-603