Objective:With the overall goal to harmonize prospective effectiveness assessment of activesafety systems, the specific objective of this study is to identify and evaluate sources of variationin virtual precrash simulations and to suggest topics for harmonization resulting in increasedcomparability and thus trustworthiness of virtual simulation-based prospective effective-ness assessment.Methods:A round-robin assessment of the effectiveness of advanced driver assistance systemswas performed using an array of state-of-the-art virtual simulation tools on a set of standard testcases. The results were analyzed to examine reasons for deviations in order to identify and assessaspects that need to be harmonized and standardized. Deviations between results calculated byindependent engineering teams using their own tools should be minimized if the research ques-tion is precisely formulated regarding input data, models, and postprocessing steps.Results:Two groups of sources of variations were identified; one group (mostly related to theimplementation of the system under test) can be eliminated by using a more accurately formu-lated research question, whereas the other group highlights further harmonization needs becauseit addresses specific differences in simulation tool setups. Time-to-collision calculations, vehicledynamics, especially braking behavior, and hit-point position specification were found to be themain sources of variation.Conclusions:The study identified variations that can arise from the use of different simulation set-ups in assessment of the effectiveness of active safety systems. The research presented is a first ofits kind and provides significant input to the overall goal of harmonization by identifying specificitems for standardization. Future activities aim at further specification of methods for prospectiveassessments of the effectiveness of active safety, which will enhance comparability and trust-worthiness in this kind of studies and thus contribute to increased traffic safety.