I was in a new Prius the other day that had the high tech option. I have to tell you it was creepy.
When activated, it sees the car in front of you AND the lane lines on each side of you (by some sort of radar). If the car sways out of the lane, the steering wheel gently nudges you back in your lane and if the car you're following slows down, the Prius slows down too.
Was a weird feeling to have the car control those functions.