I'm betting there's somebody watching a camera in a control room somewhere that pushed the down button on those barricades. Reminds me of the siren at the South Tower on Onslow Beach. I have occasionally had to call Blackburn (Fire Control) when I spotted people who had wandered too far into the impact area to hear me. They immediately turn on the tower siren which is a classic long wail air raid siren that goes up and down in intensity. One day I was walking toward the off-limits area at that end of the beach and noticed three beautiful women in bikinis had paused near the tower to dip their toes in the surf. Looked like a mother and two daughters in their early 20's. The Marine in the tower noticed too, and hit the siren on/off switch with such precision that it let out three notes of ear splitting wolf whistle. It startled the ladies, and they asked me what the noise was. I explained that it was just a young Marine on duty showing his appreciation to three beautiful ladies for visiting our beach.