The Bridge Act of 1906, 33 U.S.C. § 494, requires, that if a bridge over a navigable stream 'shall be constructed with a draw, then the draw shall be opened promptly by the persons owning or operating such bridge upon reasonable signal for the passage of boats and other water craft.' This general language cannot reasonably be construed to require that all drawbridges over all navigable streams in all fifty states shall be tended at all times of day or night, summer or winter, despite the near certainty that no traffic will approach.
Alternatively it is arguable that a signal given when no traffic was to be expected would not be a 'reasonable signal' unless this gave the bridge owner reasonable time to get someone down to the bridge to open it. However, an older statute, 28 Stat. 362 (1894), as amended, 33 U.S.C. § 499, makes it 'the duty of all persons owning, operating, and tending the drawbridges across the navigable rivers and other waters of the United States, to open, or cause to be opened, the draws of such bridges under such rules and regulations as in the opinion of the Secretary of the Army ...