Most likely, being in heavy traffic, the Sign Reading failed to pickup a change of speed limits along the way. The OP probably didn't recognize this, as the car was being paced by the surrounding speed of traffic, not the speed limit. Once traffic cleared, TACC resumed to the last known Speed Limit. I happen to live in an area where the opposite is true. I leave in a 35MPH signed area. I then turn onto a highway that is 55 MPH, but there is no posted speed limit sign for about 1/2 mile. I can't engage AP before I get to the speed limit sign, because it says it is limited to the 35MPH posted speed limit, and of course I am driving faster than 35MPH. Once I get to the 55MPH sign, all is well. JMHO