Section 401.51 - Signaling approach to a bridge(a) Unless a vessel's approach has been recognized by a flashing signal, the master shall signal the vessel's presence to the bridge operator by VHF radio when it comes abreast of any of the bridge whistle signs.(b) The signs referred to in paragraph (a) of this section are placed at distances varying between 550 m and 2990 m upstream and downstream from moveable bridges at sites other than lock sites.48 FR 20691, May 9, 1983, as amended at 49 FR 30936, Aug. 2, 1984; 73 FR 9954, Feb. 25, 2008; 76 FR 13089, Mar. 10, 2011 68 Stat. 93-96, 33 U.S.C. 981-990 , as amended and secs. 4 , 5 , 6 , 7 , 8 , 12 and 13 of sec. 2 of Pub. L. 95-474 , 92 Stat. 1471