Automated vehicles are expected to have a substantial impact on traffic flow efficiency, safety levels and levels of emissions. However, Fields Operational Tests suggest that drivers may prefer to disengage Adaptive Cruise Control (ACC) and resume manual control in dense traffic conditions and before performing manoeuvres such as lane changing. 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 behavioural 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 to one of three conditions randomly. In the control condition, participants drove manually. In the first experimental condition, a sensor failure was simulated at a specific location after which drivers were expected to resume manual control. In the second experimental condition, drivers switched ACC off and on pressing a button whenever they desired. Statistical tests indicate that the distributions of speed, acceleration and time headway significantly differ between the three conditions. In the first experimental condition, the speed drops after the sensor failure and the time headway increases after the discretionary re-activation of ACC. These results seem to be consistent with previous findings and suggest that authority transitions between ACC and manual driving influence significantly the longitudinal dynamics of vehicles, potentially mitigating the expected benefits of ACC on traffic flow efficiency.