adaptive cruise control shutting off

I was traveling at night once again on the freeway with the adaptive cruise control on. It shut off 4 times on the trip back. The first time I was almost rear ended by a semi truck, flashing his lights and honking his horn.

Very unnerving as this is a new vehicle and not completely used to the car. The second and third time there was not any cars close to me, so not as scary. The fourth time, a vehicle came up on me very quickly honking and flashing lights, I think he was just annoyed that I had slowed down on the freeway. I think this is huge safety problem. Now that I know it may happen, and what it is that is happening, I can react faster. This issue was not disclosed to me in any manner. It is one of the main factors for purchasing this package on the car. I can't tell you how disappointed and alarmed I am over this situation. The vehicle has gone back to bmw and have not gotten a satisfactory response yet.