Empirical Longitudinal Driving Behavior in Authority Transitions between Adaptive Cruise Control and Manual Driving

Automated vehicles are expected to have a substantial impact on traffic flow efficiency, safety levels, and levels of emissions. However, field operational tests suggest that drivers may prefer to disengage adaptive cruise control (ACC) and resume manual control in dense traffic conditions and for maneuvers such as changing lanes. These so-called authority transitions can have substantial effects on traffic flow. To gain insight into these effects, a better understanding is needed of the relationships between these transitions, longitudinal dynamics of vehicles, and behavioral adaptations of drivers. In this context, a driving simulator experiment was set up to gain insight into the effects of authority transitions between ACC and manual driving on longitudinal dynamics of vehicles. Participants were assigned randomly to one of three conditions. In the control condition, participants drove manually. In the first experimental condition, a sensor failure was simulated at a specific location where drivers were expected to resume manual control. In the second experimental condition, drivers switched ACC off and on by pressing a button whenever they desired. Statistical tests indicated that the distributions of speed, acceleration, and time headway differed significantly between the three conditions. In the first experimental condition, the speed dropped after the sensor failure, and the time headway increased after the discretionary reactivation of ACC. These results seem to be consistent with previous findings and suggest that authority transitions between ACC and manual driving may significantly influence the longitudinal dynamics of vehicles and potentially mitigate the expected benefits of ACC on traffic flow efficiency.