Sandstruck, I'll also add that it takes a lot of courage to not only share a story like this, but also to accept blame for it, so I commend you for that.
There are two possibilities that I can think of that may have been the cause for this.
1) In general, the radar filters out stationary objects--otherwise the radar would be going haywire passing signs, stopped cars on the side of the road, traffic cones, barriers, etc. In general I've found the radar to be good at recognizing stopped cars in traffic ahead and as I mentioned in an earlier post have never had a situation where TACC didn't see a stopped car ahead--but as a general rule it's a good idea to assume the radar won't see anything stationary. In other words, if TACC is engaged and you're coming up on a block of stopped traffic, get ready to brake. (Personally, TACC brakes a bit harder than I would, so I tend to disengage it when approaching stopped traffic to regen at my own rate).
It's possible in this case that with a car changing lanes ahead, the radar was tracking the car changing lanes but didn't have enough time to lock on a stationary car further ahead.
2) No offense, but it's also possible that you either accidentally disengaged TACC or misinterpreted something. After all, although you're not new to the S you are new to the autopilot features. The logs should help clear up any questions.
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But, I kept wondering how the car would have reacted if the car in front of me just slammed the brakes all of a sudden. I had no trust that the AP would've reacted better than I could (in my most alert state), and this is based on my observations of the AP behavior in 2 hr bumper-to-bumper traffic.
That happened to me. I was following a car with TACC engaged and the car in front slammed on his brakes. I wasn't in immediate danger of a collision because my following distance was somewhere in the 5-7 range at the time, but the TACC reacted instantly...much faster than the half second or so reaction time that I had, even though I was alert.
I think hands down TACC will beat any human reaction time. As I recall it's operating somewhere in the 60-120 Hz range...so can react on the order of about 10-20 milliseconds. Human reaction time is an order of magnitude larger than that.