Here's what happens when a Tesla Model S is cruising down the highway on adaptive cruise control — not Autopilot — and the car its following splits the lane markings to avoid a truck stopped on the side of the road:

There are a couple of things to take away from this:

  1. This was using the adaptive cruise control system, not Autopilot. The "Traffic Aware Cruise Control" system uses the forward-looking radar in the car's nose to slow the cruising car down when following behind another slower vehicle. Autopilot, a $3,000 software upgrade for cars equipped with the hardware, adds in a camera for lane keeping and steering, plus an array of sensors for 360-degree awareness of the car's immediate surroundings. Adaptive cruise control is speed only.
  2. The TACC radar locked onto the car that was in front of the Tesla and maintained speed with that car even as it moved to the side to avoid the parked truck. — it's notable that the car the Tesla was following did not slow down as it split the lane, in fact it may have accelerated (the driver notes that his Model S started to go faster).
  3. As soon as the car the Tesla was following had passed the truck, the radar locked onto the stationary truck and sounded the forward collision warning.
  4. While this apparently was not an Autopilot accident, it's not clear if Autopilot would have helped in this situation. It uses the radar to detect other vehicles and the camera to pick up lane markings — while it would have seen that the first car was moving over, it may not have picked up on the threat posed by the parked truck until it was too late.
  5. The driver noted that his car has slowed down "correctly 1,000 times", this is clearly an edge case compared to expected driver behavior (i.e. don't drive like an asshole like the front car did — merge into the next lane in advance to give those behind you time to react and follow as well as give the stopped vehicles a wide berth).
  6. TACC requires even more input from the driver than Autopilot. Where Autopilot will follow lane markings and change lanes when directed, TACC is purely braking and acceleration, with steering left in the driver's hands. Even with Autopilot the driver must be aware and paying attention, as if the car encounters a situation it is not programmed to handle (perhaps like this one), it will demand the driver take control.

The car was left with damage to the front end, though thankfully nobody was injured in the wreck. It's clear at this point that adaptive cruise control — a so-called "Level 1" out of 4 autonomy system where the car takes over just one aspect of the drive (speed) — is nowhere near full driver disengagement. Heck, Tesla advises drivers using Autopilot — A Level 2 (or perhaps 2.5) autonomy system — to pay attention. Autopilot has demonstrated smarts enough to avoid collision with merging vehicles, but it's still nowhere near ready for the driver to be asleep at the wheel.

It's your car. Pay attention and be proactive.