In order to understand how people drive a car in a real road environment, quantitative behavioral data of a hundred drivers were collected using equipped vehicles. The collected data were stored to establish a driving behavior database for convenient use in industry. We also have applied the database to develop a driving assistance system based on the driver’s behavior. Focusing on deceleration maneuver when approaching to an intersection with a stop sign, we identified range of the normal driving behavior. Based on a concept that the driving risk increases when a driving situation is out of the range of the normal driving, an on-board driving assistance system has been developed that gives a caution information to the driver when the driver behaves differently from the normal driving.