1. A system for assisting a driver in operating a vehicle having an accelerator pedal comprising: a controller configured to: calculate a repulsive force between the vehicle and an obstacle in front to give a calculated value of the repulsive force; calculate risk potential associated with the vehicle; determine which one of a plurality of predetermined risk regions the calculated risk potential falls in; limit the calculated value of the repulsive force by an upper bound depending on which one of the plurality of predetermined risk regions the calculated risk potential falls in; and change the upper bound from a predetermined value of the upper bound preset by one of the plurality of predetermined risk regions toward a different predetermined value of the upper bound preset by the next one of the plurality of predetermined risk regions at a predetermined rate with respect to time in response to determination that the calculated risk potential falls in the next one of the plurality of predetermined risk regions after leaving the one of the plurality of predetermined risk regions; and a device configured to modify, according to the limited calculated value of the repulsive force, at least one of a driving force applied to the vehicle, a braking force applied to the vehicle and a reaction force applied to the driver via the accelerator pedal.