With all this discussion, I went out just to test it out for you guys.
I engaged TACC after the car is moving past 5 mph. I had the car following another car at 6 distance unit. It stayed quite far away at high speed. When we get to a red light, the car started slowing down as the car in front slowing down. Eventually, my car continue on slowly and get to less than 1 car length away from the car in front. And then it stopped. When the light turned green, my car started by itself. As the speed increase, the distance between my car the the car in front also increase. All worked very smoothly!
Another person had doubt about the car distance even set to max distance. This is a great example. Since the car in front was slowing to almost stop, and eventually stopped. The 6 distance unit is actually less than 1 car away due to the slow moving speed. In real world application, this is perfect.
Before I did the test, I was also thinking I would have to keep adjusting the car distance in stop and go traffic. No way I could keep so far apart from the car in front in NYC. LOL