A conventional vehicle-mounted navigation device displays the current position of a vehicle on a digital map (referred to as a “map” from here on) displayed on a display unit while superimposing the current position of the vehicle on the map.  Furthermore, the conventional vehicle-mounted navigation device searches for a route leading from the current position of the vehicle to a destination, and displays the recommended route on the map.  In addition, the conventional vehicle-mounted navigation device carries out route guidance on the recommended route on the basis of map information read from a recording medium. 
By the way, a car pool lane (Car Pool Lane) or the like which is used in a road system mainly seen in, for example, a North American big city is known as a road on which restrictions showing which vehicles can enter the road are imposed.  A car pool lane is also called an HOV lane and is a lane along which only vehicles with two or more people are allowed to travel.  A special lane disposed along with lanes on a freeway, a lane which is a shortcut of an interchange, etc.  are known as car pool lanes.  A road system which adopts such a car pool lane provides users with a preferential treatment that makes it possible to arrive at the destination in a shorter time as long as they travel along a car pool lane so as to prod users to share a vehicle with one or more people, thereby reducing the traffic as a whole and therefore reducing traffic congestions. 
As a technology related to such a car pool lane, patent reference 1 discloses a navigation device that enables a vehicle to efficiently travel along a lane on which restrictions showing which vehicles can enter the lane from another lane and exit the lane to enter another lane are imposed.  This navigation device reads map data including road information about entering and exiting points in each highway equipped with a car pool lane from a DVD, and stores the map data in a data buffer.  A route search processing unit performs a route searching process in consideration of whether or not car pool lanes can be used by using the map data stored in the data buffer.  When performing a route guidance using car pool lanes, a car pool lane guidance unit performs a predetermined lane change guidance using an image and voice at a time when each of the entering and exiting points at which the vehicle should change its travelling direction falls within an area at a predetermined distance or less from the position of the vehicle. 
Patent reference 2 discloses a route searching device that can perform a search for a route in consideration of the conditions of a vehicle through an easy operation.  This route searching device includes a map data acquiring unit for acquiring map data defined by nodes and road links, a route searching unit for searching for a route leading to a destination in consideration of road links included in the map data acquired by this map data acquiring unit and each corresponding to a particular road (e.g., an HOV lane) before the conditions of the vehicle are set up, whether or not the vehicle can travel the particular road being determined according to the conditions of the vehicle, and an output unit for outputting information about the route which is searched for by this route searching unit.