29 C.F.R. § 1926.909 – Firing the blast.

(a) A code of blasting signals equivalent to Table U-1, shall be posted on one or more conspicuous places at the operation, and all employees shall be required to familiarize themselves with the code and conform to it. Danger signs shall be placed at suitable locations.
(b) Before a blast is fired, a loud warning signal shall be given by the blaster in charge, who has made certain that all surplus explosives are in a safe place and all employees, vehicles, and equipment are at a safe distance, or under sufficient cover.
(c) Flagmen shall be safely stationed on highways which pass through the danger zone so as to stop traffic during blasting operations.
(d) It shall be the duty of the blaster to fix the time of blasting.
(e) Before firing an underground blast, warning shall be given, and all possible entries into the blasting area, and any entrances to any working place where a drift, raise, or other opening is about to hole through, shall be carefully guarded. The blaster shall make sure that all employees are out of the blast area before firing a blast.
Table U-1 Warning Signal—A 1-minute series of long blasts 5 minutes prior to blast signal. Blast Signal—A series of short blasts 1 minute prior to the shot. All Clear Signal—A prolonged blast following the inspection of blast area.