A driving assistance device for a vehicle includes an acquisition module configured to acquire information comprising at least one of vehicle state information and environment information surrounding the vehicle; a decision module configured to determine a driving behavior for the vehicle in an autonomous driving mode based on the acquired information; a pre-processing module configured to process the acquired information to identify a respective information category among a plurality of predefined information categories the acquired information belongs to; and a determining module configured to retrieve one or more traffic rules related to the information category the acquired information belongs to from a traffic rule database pre-stored in the driving assistance device; determine whether the driving behavior violates any of the retrieved traffic rules; and determine risks of said driving behavior if it is determined the driving behavior violates at least one of the retrieved traffic rules, said risks comprising one or more of the legal penalty, property damages and personal injury caused by the driving behavior.