I've always wondered if there isn't an intermediate step, a sort of 80/20 rule to gain most of the benefits for 20% of the work, without your car actually taking you for a drive while you sleep in the back seat.

For example, lets assume highway traffic is caused primarily from exceeding road capacity, and road capacity is a function primarily of the size of the gap that we are taught to leave between cars. Why are we taught to leave a 2 second gap between cars? In theory, that's how much time/space you need if the car in front of you decides to go full-tilt on the breaks with no warning.

I've read about autonomous caravans (the article calls them platoons) where cars line up behind a follow vehicle. The follow-cars in that scenario typically also take over steering. Like in the 2014 Mercedes S-Class from the article;

A jovial safety engineer drove me around a test track, showing how the car can lock
onto a vehicle in front and follow it along the road at a safe distance. To follow
at a constant distance, the car’s computers take over not only braking and
accelerating, as with conventional adaptive cruise control, but steering too.

But why does it have to steer? If the car takes over JUST breaking and acceleration the software and sensors required are vastly simplified. Given a target maximum speed, set by the user (so it can be set at 85mph and not 65mph), the car drives at the designated speed, or else maintains a close follow (250ms is ~30ft at 85mph). Why can't the driver still be responsible for steering while this is happening?

I think the key is giving the driver very high confidence that "no, there is no way that my car will let me rear-end the guy in front of me" even if you're just 30ft back at 85mph. That's not "scary close" but at that distance, you are trusting the car in front not to apply maximum breaks given human reaction times. Computers could apply sufficient stopping force in time, although the responsiveness required of the algorithm might make regular driving a bit "twitchy" depending on the human driver you're following.

I've never even driven a vehicle with adaptive cruise control, so I have no idea how "aggressive" the system is, or how it feels as the driver. I don't think any of the adaptive cruise systems out there will take you down to 0mph and then also start moving again, which seems like a must-have. But I bet if Tesla added "maintain Xmph or close follow" to their Model S, owners would trust it, use it, and look quite badass in the process.

You might benefit from some obvious (but not distracting) signal to other cars when this mode is active, and spend a boat-load of money on awareness, to try to avoid the inevitable "oh this asshole is tail-gating me, I better slow down."

Another caveat is that it's easier to steer smoothly at high speeds when you look far down the road ahead of you, which is kind of hard to do when you're breathing down the neck of the car in front of you. Obviously if drivers start losing the ability to stay in lane when following that closely, the idea falls apart.

Ultimately I think "cruise control" is something every driver understands and trusts. Make cruise control better. Call it "super cruise" and put it in the Tesla Model S. Try to educate other drivers about "super cruising" so that they know you're not actually driving like an ass. More brands will follow.

If most highway traffic is caused by exceeding road capacity by just 1 or 2% (personally hard to believe, but that's what experts say) then in theory if "super cruise" reduced inter-car gap by 50% for 5% of cars on the highway, then POOF no more traffic jams. Of course as adoption increased much past 5%, only then you would need to add software to support zipper merging ;-)