The VW I was driving had the lane assist. I tested it out a few times by taking my hands off the wheel. The car would usually approach to within about 6" if the right line, then push the car back a few inches into the lane. After about 8 seconds of this, it would sense I wasn't holding the wheel and flash some warnings. I didn't push it any further. I should have to see if it would have braked the car.
One of the nights during the trip when we were at home, I was reading a story about Cadillac's super cruise technology that uses cameras to make sure the driver is attentive. Other companies use torque sensing technology to sense if the driver's hands are on the wheel. It senses that the driver isn't responding to the force applied to the wheel by the computer and electric motor. I'm guessing the VW uses that, but haven't looked into it.
Like it or not, it sure is interesting how they approach this.