“calcualting velocity of many objects in a video sequence.

Hi, I want to compute the velocity of moving objects from a video sequence.I have detected the objects using vision.ForegroundDetector and and I have found the centroid of each object in each frame. Further I have calculated the distance traveled and the speed of the car. But I could calculate the speed of only one car in the frame.How do I calculate speed of two or more cars that are in the same frame