Human factors have been found to be the major cause of vehicle accidents. We studied the driver-vehicle closedloop system. Using the motion-type driving simulator the active-safety performance of 100 ordinary drivers was examined for two conditions :​ obstacle avoidance and slippery curve control. From the resulting data a computer model was developed. After confirmingthe validity of the computer model, the active-safety performance of drivers under the two conditions was better understood and in the case of the slippery curve condition, driver assistant devices are feasible.