In recent years, a specific type of navigation system is used to navigate a vehicle when the vehicle is merging from an onramp into a through traffic on a main lane of an expressway or the like. For example, Japanese patent document JP-A-H11-86192 discloses a technique that facilitates a smooth merging of a subject vehicle by reporting to a driver of the subject vehicle a position and appearance of an approaching vehicle in the through traffic on a main lane. In addition, the navigation system in the above disclosure provides for the driver of the subject vehicle a predicted position of the approaching vehicle at a certain period of time after an initial report of the position and appearance of the approaching vehicle.
However, the driver of the subject vehicle has to determine a merge speed by him/herself based on the information provided by the navigation system. That is, the driver has to decide the merge speed by him/herself based on the predicted position of the approaching vehicle provided by the navigation system. In addition, the driver has to find an appropriate merge timing and/or a merge position at the same time for merging. Therefore, the driver of the subject vehicle has to process a large amount of information for merging operation provided by the navigation system at his/her own cost. In other words, the navigation system leaves a huge load on the driver's disposal even when it provides navigation information for merging into the traffic on the main lane.