That's already been release since the previous version via the traffic aware cruise control and emergency braking.

Specifically, the Model S locks onto the car in front of you and matches its speed, maintaining a certain number of car lengths behind it (which you can set). When the car changes lanes, your Model S speeds up a bit to fill the gap, and when a car cuts into the lane, the Model S slows down (although this one is a bit scary). If the car in front comes to a complete stop, the Model S also smoothly slows down into a complete stop.