The paper describes a method and a device for a current permanent observation of driver's response in the road traffic was described. The task of presented system is not only the "on-line" analysis of the human behavior but primarily the determination of the physiological changes and their fuzzy classification in order to determine the states considered sub-critical. The article is based on authors research and results of experiments, related to the international scientific state of art, in terms of excessive fatigue and aggression as essential biocybernetic factors increasing the road accident risk. A variant of the warning system for notification of driver's psychophysical condition changes was presented. The system is based on a revised analysis of the human metabolism under a monotonous long-term load. The results of simulations and real measurements are prerequisites for the development of the presented system structure.