Last year I drove my parent’s car which is equipped with one of these cameras that determine if the driver is distracted or dozing. And I can say for certain that it works.

I rented two different modern (2015-2016) Mercedes SUVs. They both had systems that detected tired/inattentive driving. I was neither but after several hours on the road both vehicles would alert that it was time to take a break with a nice little coffee icon. I was conversing with a passenger, driving fine, not wandering between lanes/etc.. The first time I kind of doubted myself but subsequent notifications both the passenger and myself were agreeing that we had no idea what it was upset about.

The newer car had another sensor that would get upset if your grip on the steering wheel got too light. That was kind of neat to see how much leeway it'd give you before it got antsy.