I just realized after 1400 miles that this feature was not turned on from day one from the Dealer. Was this off by default on your guys Cars? Is it a PIA when turned on? I guess I'll find out tomorrow on my 100 mile trip down to San Diego.
See I thought it would measure input and response but I think it does a calculation. I think it calculates your responsiveness plus drivetime.
I took a long road trip with it on and after three hours I was still very awake and attentive but it pinged me to take a break. So it seemed it was really going off distance/time. It probably takes movement into consideration but I'd say it has programming that tells it "hey, they should be taking a break now".
This kind of technology will fall by the wayside as more autonomous systems work their way into new models. Everyone should make it a habit to pull over from time to time on longer road trips. My guess is that its based on uninterrupted mileage.