Safety testing of advanced driver assistance systems (ADAS) and advanced driving functions (ADF) is a challenging task due to the impossibility of performing a sufficient road testing. In order to overcome this limitation, simulations are usually included in testing. In a previous work, the safety of and autonomous overtaking function - proposed as a part of ADAS - has been evaluated with respect to collision rate performance for a particular scenario. Notwithstanding the advantages of that approach, there are also limits, in particular when a reaction from other traffic participants can significantly alter the collision risk, e.g. when an overtaking autonomous vehicle is reached by another vehicle on the overtaking lane during the maneuver. According to when and how this vehicle will brake, the collision risk will strongly change independently from the ADAS reaction. Against this background, instead of modeling the human driver's response to a cut-in maneuver, we suggest using three key variables, namely Time-To-Collision, Time-Headway and inter-vehicle distance, which in some way capture the instantaneous behavior of the vehicle coming from the rear. These variables are then used as an alternative performance assessment metrics for the autonomous overtaking function.