A Traffic Monitoring System (TMS) is the most important element in the Intelligent Transportation Systems (ITS), especially in a developing country like Vietnam with a mixed traffic flow of vehicles including motocycles and cars. A traffic flow varies in different daytime, therefore the adoption of the TMS for an adaptive control of the traffic light timing is very essential. The proposed TMS system uses one CCD camera mounted to view from the rear of moving vehicles appearing in a distance from a viewed span of the camera to the stopline of the cross-roads, which is denoted by L and statistically determined. The timing of the traffic light can be adaptively adjusted according to the estimated traffic flow which is based on the video sequence captured by the camera in its span on the road, where the gaps between the vehicles are used to determine the density of vehicles appearing in the camera span. This method enables to implement a simple but efficient traffic flow control especially in crossroad context compared to other convenient methods.