Three performance areas can be singled out in the humanmachine system, consisting of the driver and the vehicle, as key factors for improving active safety. (1) The vehicle shouldp rovide good responsea nd controllability in relation to the driver's operational inputs. (2) Vehicle behavior should be highly stable and free ofany sudden changes. (3) There should be a sufficient flow of information from the vehicle to the driver to assure accurate recognition of present vehicle conditions and good predictability of subsequenvt ehicle behavior. Based on this framework, an analysis was made of the torque split between the front and rear wheels of a 4WD vehicle, and an optimum traction control method was devised for improving the performance areas noted above. That method was incorporated into an electronically controlled torque split 4WD system which employs a wet multiplate clutch in the 4WD transfer assembly to achieve optimum traction control. Vehicle tests conducted with the system confirmed that it significantly improves cornering propefties and ABS performance under all road surface conditions, and thereby contributes to improved active safety.