1. A system for assisting a driver operating a vehicle traveling on a road, the system comprising: an engine controller regulating operation of an engine of the vehicle to provide a desired engine output characteristic; control logic configured to modify the engine output characteristic in response to at least one of a detected state of an environment around the vehicle and a determined risk; and an accelerator having different positions, wherein a reaction force applied to the driver via the accelerator varies with different positions of the accelerator, according to a preset characteristic specifying how the reaction force varies with the positions of the accelerator; wherein the control logic includes: a first control section that determines the risk based on the detected state of the environment around the vehicle, and a second control section configured to modify, according to the determined risk, the preset characteristic specifying how the reaction force applied to the driver via the accelerator varies with different positions of the accelerator; and wherein: the control logic controls the operation of the engine to switch between a first operation mode and a second operation mode according to the determined risk; under the first operation mode, the engine output varies with positions of the accelerator, according to a predefined normal engine output characteristic,; under the second operation mode, the engine output varies with positions of the accelerator, according to a modified engine output characteristic selected from multiple predefined modified engine output characteristics according to the determined risk; each of the modified engine output characteristics defines a range of accelerator positions, and the engine output corresponding to a position within the range is lower than the engine output corresponding to the same position according to the normal engine output characteristic; when the determined risk is lower than a preset threshold, the control logic controls the vehicle to operate under the first operation mode; and when the determined risk is higher than the preset threshold, the control logic controls the vehicle to operate under the second operation mode.