Others have answered the braking question. I like this feature, it's good knowing that on downhill stretches of motorway I'm not going to speed away.
In UK we can opt on the Driving Assist which will at least warn you of a collision but won't actually stop / slow you down:
"The Lane Departure Warning includes an integrated camera-based Collision Warning. As soon as the system detects that the vehicle in front is too close, it warns the driver with a display on the instrument panel as well as an acoustic signal.
Source:
BMW UK
But I had the same question thinking this was adaptive cruise control which would be awesome on lazy motorway drives.