Traffic accidents have been serious problems. Therefore, the necessity for driving safety support systems are high. In order to verify the validity of these systems, it is necessary to evaluate risk values of each vehicle. Conventional risk indices as TTC(Time-To-Collision) are not appropriate to be applied to more complex situation like the case that a high speed vehicle is surrounded by multiple moving vehicles. In this research, we proposed a criteria which evaluates the risk of each vehicles over moving obstacles as high-speed vehicles on an expressway. In this criteria, a potential risk of surrounding vehicles is calculated by an elliptic potential function, whose major axis is along the velocity of moving vehicle and center is the future position of the vehicle. The proposed criteria is applied to a driving safety support system and verified by a realistic traffic simulation including the driver models calibrated by real traffic data in the situation with multiple surrounding vehicles.