> use more subtle and inadvertent signals like the slight movement to the line telling me they're likely about to switch lanes
For what it's worth, this was explicitly taught in Northern European driving classes, especially in the context of turns. It would literally translate as "grouping" or "making a formation". You "group right" to make a right turn (think aligning text left/center/right). You can look at a line of cars and immediately know which ones are preparing to turn, even from an angle where turn signals aren't noticeable.
For what it's worth, this was explicitly taught in Northern European driving classes, especially in the context of turns. It would literally translate as "grouping" or "making a formation". You "group right" to make a right turn (think aligning text left/center/right). You can look at a line of cars and immediately know which ones are preparing to turn, even from an angle where turn signals aren't noticeable.