[oldE]

"I typically know that a car ahead is about to change lane before the driver ahead has made his decision, simply because I can tell he's moving around while he looks, and based on his relative speed and position on the traffic around him."

Yeah, this is a habit I have developed too. I like reading traffic around me so there are no surprises.
The idea of being aware of what is going on at least 15 seconds ahead of you is a minimum, as noted. That's a quarter mile at most road speeds, more on a limited access. You would almost always be aware of what's going on beyond that parameter.

Just as I don't like surprises on the road, I don't like surprising other drivers. I have for a long time felt if I make someone apply their brakes in a rush, I have somehow messed up.

Our driver training instructor in high school worked us on Defensive Driving/Smith system as something to which we should aspire. Some of us (who had more behind the wheel time outside the course go it, others not so much.
I also remember researching the UK police driver training about forty years ago. I found it helps as well as The Pace for motorcyclists.
Drive to arrive.

Best
Les