## 3 Experiment
### Results and Discussion
#### 3.2.2 Vehicle Detection and Tracking  
In this section, we compare the performance of an existing algorithm [7] (without occlusion handling) and the proposed algorithm (with occlusion handling) under two scenarios of two-lane and four-lane urban roads. To describe the detected vehicles in the images, the preceding vehicles are marked as white rectangles and the oncoming vehicles are marked as yellow rectangles. Table 2 shows the quantitative evaluation results for the test videos recorded on two-lane urban roads. It is clear that the system performance is improved significantly in both oncoming and preceding vehicle detection when the occlusion handling mechanism is applied. As presented in the vehicle tracking section, missed detections may occur because of the disappearance or the distorted appearance of one lamp in a light pair. In the two-lane scenario, for both preceding and oncoming vehicles, the typical situation that causes vehicle detection failure involves turning vehicles. When the front vehicle turns left or right, the appearance of a vehicle light is distorted, which breaks the pre-defined conditions for light pairing. Figure 14 illustrates the comparative results with and without occlusion handling in turning situations. Furthermore, for oncoming vehicles, the blooming effect of only one light in a light pair sometimes occurs, which also causes failed detection. Figure 15 shows the successful vehicle detection of the proposed method compared with an existing work when the blooming effect occurs. Figure 14: Results of vehicle detection in the turning situation: (**a,c**) without occlusion handling; (**b,d**) with occlusion handling. Figure 15: Results of vehicle detection in the blooming effect situation: (**a**) without occlusion handling; (**b**) with occlusion handling. Table 3 shows the quantitative evaluation results for the test videos recorded on four-lane urban roads. As with the results in the two-lane scenario, the vehicle detection performance is significantly enhanced when the proposed method is applied for detecting both oncoming and preceding vehicles. Compared to a two-lane road, the width and traffic volume of a four-lane road are both larger and thus there are various situations that may cause incorrect vehicle detection (missed detection or false positive). In addition to the situations mentioned for the two-lane scenario, partial vehicle occlusion occasionally happens owing to the viewpoint of the host vehicle's camera or the passing maneuver of vehicles in front. Figure 16 shows vehicle detection results for partial vehicle occlusion. As with the occlusion handling mechanism, the proposed method uses the information of the vehicle that has been tracked in the previous frame to update the new vehicle position, even if the light pair process for the occluded vehicle has failed. Occasionally, when one vehicle is moving very close to another vehicle, one of the two vehicles may be missed in the detection because two adjacent vehicle lights are detected as one vehicle light, as shown in Figure 17. However, the proposed algorithm is still able to obtain both vehicles, as shown in Figure 17.