Life’s Big Decisions: What do you do when the driver in front of you does something stupid

I was driving home the other day, cruising along the highway. The road eventually goes from four lanes down to two. The transition has proper signage, with one at 300 metres and another at 50. The lines get fatter and closer together as is standard.

 

As I don’t keep typical office hours, traffic during my commute tends to be very light. On this day there is only one other car going my direction, about 50 metres in front of me in the left hand lane. I’m traveling slightly faster than they are, but I likely will not have to adjust my speed before the road narrows.

Suddenly, to my surprise, after passing the ‘lane ending’ sign at 300 m, the car in front of me signals and changes into the right lane. There’s no one else on the road, and I’m certainly not close enough that they should feel like I’m tailgating. How did they not see the sign we just past or the next one fast approaching? What about the lines on the road? Can they simply not see far enough ahead to notice that that lane ends very soon?

What should I do? Do I speed up to pass them? Do I slow down and try and keep my distance? Or do I maintain my current speed and risk the person being surprised that the lane ends and either cut me off, or worse, turn into the side of my car?