D.C. Code § 10-123

When, in the judgment of the Mayor of the District of Columbia, the public necessity or convenience requires them to enter upon any of the spaces or reservations under the jurisdiction of the Director of the National Park Service for the purpose of widening the roadway of any street or avenue adjacent thereto or to establish sidewalks along the same, the Director of the National Park Service is authorized to grant the necessary permission upon the application of the Mayor.