It's usually very good at staying perfectly centered between painted lane lines, even when following curves. This is fine usually, except when a lane on the right merges into your lane. In the United States, interstate highways don't always have any sort of markers establishing the non-merging lane. Because of that, about halfway into where the lanes merge, Autopilot considers that portion just one very wide lane and aggressively tries to center itself, rather than just keeping a relative distance from the lane markings on the left.