This was the point I was trying to say, but I think I didn't communicate it well.

I would like to know just how fast these cars can react. I'm betting they could be actually slowing the car before a human is even aware something is wrong, but I would like to have some numbers to cite.

Also - keep in mind that this type of Driver Assist is (in the scheme of things) a very straightforward problem compared to others. It's pretty easy to calculate what type of braking is required to avoid running into something in front of you based on the relative velocities.

Not sure about the Model S, but Google's Prius and Lexus autonomous vehicles use a radar sensor on the front of the vehicle that can see past a semi tractor trailer in front of it, providing sensor input a human would never have.