Since the tragic accident occurred in South Korea on April 16 2014, the safety has become more and more important issue in the country. The ferry named Sewol had sunken in Yellow sea. More than three-hundred people were missing or lost their life by a one single accident. One of the causes of this accident is lack of a safety device for the passengers. It is no good fixing problem after it happens. Therefore, it is important to prevent accident before it happens. Besides of water transportation, in South Korea, there are an increasing number of accidents in the city bus. When taking bus on and off, passengers are very vulnerable with vehicle driving through the side of the bus. A bus stopping, and bus driver opens the rear door of bus, the driver is hardly taking a concern of incoming vehicle. Though, there is a caution sign for passenger to make them have in mind of incoming transportation, it is not enough to prevent the accident. This paper suggests a method to prevent this accident by controlling the door electrically and mechanically with sensing an incoming vehicle. The system consists of three parts. First part is mainly about sensing the incoming vehicle and the distance between bus and car with ultrasonic sensor. In addition, the signal is sent to the microprocessor called Arduino. After the distance signal is compared in operational circuit with reference data, it makes the motor of door move or not move. These three parts are main components of this system, and the system is demonstrated in a miniature bus designed for this system.