A method for determining an adaptive reaction time of a driver of a motor vehicle, wherein the adaptive reaction time is determined from the activity of the driver by determining the driving behavior of a vehicle driving ahead by a surroundings sensor system, determining activities of the driver of the motor vehicle, determining chronological correlations between the driving behavior of the vehicle driving ahead and the activities of the driver, and determining the driver's activity from the chronological correlations. The method is applied in assistance systems with observation of the surroundings in front of the motor vehicle, in which systems the driver's reaction time is used to determine a warning time.