Amongst automotive functionalities which generate brakings like AEBS, it is necessary to assess the consequences of an inopportune braking in the presence of a following vehicle. In order to meet this requirement, a specific methodology was developed. This methodology uses data recorded on open roads and the ISO 26262 fundamentals. The main risk for ADAS function in particular for automatic braking, is to trigger a braking which can be perceived as inopportune, whereas the vehicle is followed by another vehicle which does not react quickly enough and collides. In order to define the failure rate criterion, we use the relation between the “pre-existing risk” and the product Exposure (E) by Controllability (C) for each Severity (S). By using data from accidentology, it is make it possible to evaluate the pre-existing risk and using a recorded database, compute the product [E × C]. Finally, the methodology allows to define the failure rate requirement necessary to drive without safety-related incident in order to validate the function.