Freeway driving is a bit like playing asteroids, especially when there are four or more lanes to each side. Fortunately, despite the fact that it sometimes seems otherwise, cars only go in a few specific directions rather than every which way.
Unfortunately, unlike asteroids that only move based on inertia, they can slow down, speed up, and change direction, sometimes for no apparent reason.
Fortunately, this doesn't generally happen without warning. Even when they aren't using turn signals, cars have body language. By watching how they slow down, speed up, tailgate, drift, or "lean" to one side of the lane or the other, it's possible to know what they want to do (or are about to do). And by watching all of the other cars around them, it's also possible to know why.
Unfortunately, not everyone knows how to read car body language. Most people probably aren't even aware of it - it's not exactly required knowledge for obtaining a basic drivers license. Even people who do know aren't necessarily always paying attention at all times. It's therefore good to assume that everyone else on the road is oblivious until they prove otherwise (or are a professional trucker).