In recent years, in addition to manual driving in which a vehicle travels based on user's driving maneuvers, a new drive mode has been proposed regarding autonomous driving assistance systems that assist the user in driving a vehicle by performing a part or all of user's driving maneuvers by the vehicle.  For example, the autonomous driving assistance system autonomously performs vehicle control such as control of steering, a driving source, and a brake so as to keep the vehicle approximately centered in the lane while maintaining a preset vehicle speed and a certain distance to a vehicle ahead.  The autonomous driving assistance system is advantageous in that it eases the driving burden on the user.  However, depending on the surrounding environment and the road conditions, situations (obstructive factors that obstruct continuation of autonomous driving assistance) may arise in which it is difficult to continue autonomous driving assistance.  Examples of such situations include the situation where it is necessary to perform complicated vehicle maneuvers when the user merges into another lane or when another vehicle merges into the user's lane, and the situation where lane markings cannot be detected.  If such obstructive factors occur, there is a possibility that autonomous driving assistance may be cancelled. 
As a solution to this problem, a technique is conventionally proposed in which autonomous driving assistance is continuously performed by avoiding such obstructive factors that obstruct continuation of autonomous driving assistance.  For example, Japanese Patent Application Publication No.  2012-51441 (JP 2012-51441 A) proposes a technique in which, if a sensor detects an obstacle (e.g., a parked vehicle) ahead on the driver's lane, a new route is created to avoid the obstacle, and the vehicle is moved along the route thus created, so that autonomous driving assistance is continued without being cancelled.