While driving down on CA-1 to Monterey yesterday 5PM, when left lane merged to right (reducing from 3 lane to 2 lanes), my car (autopilot engaged) couldn't follow the left yellow line (when the right side white lane divider disappeared for a while, the car has always followed the left yellow line to stay in the lane) and crossed it. I was watching closely (as everyone should) and gently nudged the steering wheel to get back into lane without disengaging AP. There were passengers taking nap and disengaging usually causes jerky movement that can wake them up. The car complied with the gentle steering pressure and didn't resist too much. I think if I had not taken action, I could have gone into the grassy left areas. What could be the reason for this AP1 failing to follow the well marked yellow lane marking in the absence of the other lane marking? Running the latest 8.1 firmware for AP1. the sun was on the right (ocean side) and I was driving south. Distance following was set to 5.