For example, in tollgates of expressways, passage of a vehicle is generally detected by a pole sensor (a vehicle detection apparatus) of a transmission type and a reflection type using infrared laser.
However, there are various types of vehicles having different shapes, and the length from the distal end portion of the vehicle detected by the pole sensor to a specific region (for example, a part around a windshield on which an on-board device is installed) differs for each type of vehicle. Thus, it is difficult to detect a specific region by the pole sensor.
In addition, pole sensors require excavation in installment, and require separate equipment to adjust positions of the left and right sensors. Thus, pole sensors have a disadvantage of requiring high cost for construction and adjustment. Thus, pole sensors may be desirably replaced with vehicle detecting apparatuses of other means.
On the other hand, vehicle detection apparatuses using cameras relatively easily achieve conditions for making the vehicle fall within a range of angle of view, by using existing pole sensors. The price of cameras has fallen in recent years, and thus cameras are available at a relatively low price. It is thus preferable to achieve a vehicle detection apparatus using a camera.
A vehicle detection apparatus of a stereoscopic type using a plurality of cameras will now be discussed. Generally, the feature points of a vehicle differ according to the shape of the vehicle and how the vehicle looks. Thus, in a stereoscopic system, the feature points that are correlated with one another between a plurality of images obtained by a plurality of cameras change each time, and the criteria for alignment are indistinct.
As described above, vehicle detection apparatuses adopting a stereoscopic system have a disadvantage that feature points that are correlated with one another between a plurality of images change each time, and the criteria for positioning are indistinct.