The road layout in this screenshot shows what caused the problem.
The cars only choose the "turn left" or "turn right" lane, when they plan to make another left or right turn at the very next(!) junction. As you can see, the blue lane is only used, if the car plans to turn left at the very next junction. All other cars choose the middle lane (turn left and turn right) to avoid getting in the turn left or right lane in the next street. This is a game design issue in my opinion. To solve the problem, I changed all one-way roads to normal two-way roads, to make them accessible from both sides.