I have it set on medium (I think that's the default).
The brake sign will flash, but not always act. I think the system kicks in at the last possible moment to either avert a collision, or to reduce the severity of one.
This is exactly what I did and have not had an issue since. I would highly recommend turning it to "medium" or "near". You can leave the alert setting on high so it beeps and shows brake but it won't kick in unless you are truly going to hit someone.
