From automated to manual: traffic effects of control transitions in level 3 conditional driving

Conditional automated driving at Level 3 marks a decisive step on the path from manual to fully automated driving. Under defined operating conditions, the driver may divert attention from the traffic situation and hand over the driving task entirely to vehicle automation. When the system reaches the limits of its operating range, it prompts the driver to retake control within a specified time window. This takeover, termed a transition of control (ToC), exposes a central automation problem. A driver who has been out of the control loop for an extended period must, with reduced situational awareness, rapidly re-establish full vehicle control, make decisions and, where necessary, respond under time pressure. Such takeovers are particularly demanding in dynamic traffic, since even well-prepared, non-emergency ToCs can temporarily impair vehicle behaviour and the quality of manual control. To account for this, the automation takes preparatory measures by moderately increasing the time gap to the preceding vehicle during ToCs in dense traffic. Even without any failed takeovers, a high frequency of such ToCs can disturb traffic flow. This thesis therefore examines the resulting effects and their implications using microscopic traffic simulations and shows how system-initiated ToCs at the limits of Level 3 automation aggregate from single events into noticeable traffic-level effects. Several studies consider efficiency, safety and capacity. The focus is on the mechanisms associated with the sequence, timing and location of takeover triggers, as well as automation behaviour during ToCs. The results show that ToCs occurring in clusters cause a disproportionate disturbance of traffic flow up to complete breakdown; they degrade safety metrics and reduce capacity. The thesis also investigates traffic-management and V2X-enabled measures that can partially mitigate the consequences by distributing ToCs in time and space, but cannot fully restore baseline conditions of undisturbed, takeover-free traffic.