2016 Buick Regal - Question About Cruise Control

With adaptive cruise control and AEB: do the brake lights go on when AEB is engaged while adaptive cruise control is on? Going down a steep mountain grade my car “brakes” for slower traffic while in adaptive cruise control. Is the traffic behind me alerted (via brake lights) of my AEB engaging?

Have some on follow you and communicate with them via your Bluetooth.

Yes, they do if they are actually applying the brake. The system can slow the car exactly as you might on an older car by keeping off the gas and downshifting to maintain speed coasting down a hill. That would not trigger a brake light just as you doing it wouldn’t.