Until automated cars function perfectly, drivers will have to take over control when automation fails or reaches its functional limits. Two simulator experiments (N = 24 and 27) were conducted, each testing four automation levels ranging from manual control (MC) to highly automated driving. In both experiments, participants about once every 3 min experienced an event that required intervention. Participants performed a secondary divided attention task while driving. Automation generally resulted in improved secondary task performance and reduced self-reported physical demand and effort as compared to MC. However, automated speed control was experienced as more frustrating than MC. Participants responded quickly to the events when the stimulus was salient (i.e., stop sign, crossing pedestrian, and braking lead car), but often failed to react to an automation failure when their vehicle was driving slowly. In conclusion, driving with imperfect automation can be frustrating, even though mental and physical demands are reduced.@en