It will nudge you back into your lane if the car doesn't interpret your crossing the line as being under your controlled power. If you're like me and make very shallow moves changing lanes it will almost feel like it's fighting you to make the lane change. If you make an obvious strong turn of the wheel it will not try to correct you. Makes sense so it doesn't fight you in a panic lane change.
If the turn signal is on in the direction you're changing lanes, it will not activate.
It's nothing like an autopilot... It isn't designed to keep you driving in the middle of your lane, just prevent you from drifting into the other lane unintentionally. It usually doesn't activate until you're a bit over the lane already.
I tried just letting go of the wheel many times to see what would happen after slightly nudging off center of the lane. The car will correct itself, and in fact over-correct enough to put you on course to cross the lane on the other side of you let it. It will then start bouncing back and forth within the lane with sloppier and sloppier control and going further and further over the line on each oscillation. You'd surely be pulled over if a cop saw you doing this, and I felt like after a few bounces the car would probably just go off course completely.
It does work for safety for me. It's nudged me back on more than once when the radio controls, or yes, a text message distracted me. I'm glad my car has the option.
The system uses a camera to read the lane lines, so if your road doesn't have good lines, it won't work at all.
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