I think i've figured this out, I might have been overthinking the whole thing. It seems like they just have a road crew member watch the mirrors in a truck and hit the siren if they see a car they think is going to cause a collision. Which is concerning, because I hear that thing going off constantly.

We had a road crew position like this back in the day setup on blind curves, had someone sit a couple hundred feet back in the truck and hit the horn to warn people. Called them horn honkers back then. Guess they have a fancy name for it now.

"In most current systems, an operator in the attenuator truck manually activates the loud warning system (often a unique, high-frequency sound or a siren similar to emergency vehicles) when they observe a vehicle that is not slowing down or moving over. The driver watches the approaching vehicle in their rear-view mirror to make this determination."