A driving assistance method comprising: acquiring the position of a subject vehicle; extracting an interference traffic line interfering with a planned travel route of the subject vehicle, the interference traffic line being a route along which other vehicles can move on a road; analyzing a shape of the road by referring to map information and road information, the map information and road information being obtained from a database; determining whether the other vehicles traveling along the interference traffic line decelerate, on the basis of the analyzed shape of the road; determining a necessary length of the extracted interference traffic line, the necessary length being necessary for determining a driving action of the subject vehicle; when the other vehicles traveling along the interference traffic line do not decelerate, calculating a moving speed of the other vehicles as a constant speed, wherein the necessary length of the extracted interference traffic line is determined on a basis of the calculated constant moving speed; when the other vehicles traveling along the interference traffic line decelerate, calculating the moving speed of the other vehicles in accordance with the deceleration, wherein the necessary length of the extracted interference traffic line is determined on the basis of an assumption that the deceleration is a constant deceleration; detecting, by using an object detection device, whether other vehicles traveling along the determined necessary length of the interference traffic line exist around the subject vehicle; and determining the driving action of the subject vehicle to avoid contact with any of the detected other vehicles. acquiring the position of a subject vehicle; extracting an interference traffic line interfering with a planned travel route of the subject vehicle, the interference traffic line being a route along which other vehicles can move on a road; analyzing a shape of the road by referring to map information and road information, the map information and road information being obtained from a database; determining whether the other vehicles traveling along the interference traffic line decelerate, on the basis of the analyzed shape of the road; determining a necessary length of the extracted interference traffic line, the necessary length being necessary for determining a driving action of the subject vehicle; when the other vehicles traveling along the interference traffic line do not decelerate, calculating a moving speed of the other vehicles as a constant speed, wherein the necessary length of the extracted interference traffic line is determined on a basis of the calculated constant moving speed; when the other vehicles traveling along the interference traffic line decelerate, calculating the moving speed of the other vehicles in accordance with the deceleration, wherein the necessary length of the extracted interference traffic line is determined on the basis of an assumption that the deceleration is a constant deceleration; detecting, by using an object detection device, whether other vehicles traveling along the determined necessary length of the interference traffic line exist around the subject vehicle; and determining the driving action of the subject vehicle to avoid contact with any of the detected other vehicles.