Controllability (C) is the parameter that determines the Automotive Safety Integrity Level (ASIL) of each hazardous event based on an international standard of electrical and/or electronic systems within road vehicles (ISO 26262). C is classified qualitatively in ISO 26262. However, no specific method for classifying C is described. It is useful for C classification to define a specific classification based on objective data.This study assumed that C was classified using the percentage of drivers who could reduce Severity (S) in one or more classes compared with the S class in which the driver did not react to a hazardous event.An experiment simulated a situation with increased risk of collision with a leading vehicle due to insufficient brake force because of brake-assist failure when the experiment vehicle decelerated from 50 km/h on a straight road. First, the relationship between the S class and the difference of speed at the moment of collision obtained in the experiment was classified according to ISO/DIS 26262 Part 3 Annex B. C was then classified as C3 base on the classification result and the method of classifying C Class. We then attempted to classify C using the experiment time sequence of brake-pedal effort when the characteristics of brake-assist differed from those of the experiment.The result indicated that C can be classified in the experiment and that it is possible to classify C when the characteristics of brake-assist differ from those of the experiment.