Are you looking for someone to write the code? Or are you just looking for some brainstorming help?
In terms of helping you brainstorm. You will first need to know the distance the camera is from the car and what the linear width of the viewable frame is at the street lane. In other words, how far is it from the left side of the viewable screen to the right side (Note, you'll want to involve trig in your code to get an accurate measurement of speed. If you are lucky enough to have a clear background you could easily do an image recognition on the frame to locate the bumper of the car and track it across the screen. More likely you'll have to track the tires. Since all cars have circular tires, it will not be subjective based upon car type. You will have to write the code to track the center of the tire (or any traceable point on the cars) across the screen. By logging its location (left to right) on the screen for one frame and comparing it to the location a frame or two later (and combined with knowing what the overall distance is in the viewable screen) you can figure out how far the car has travelled. Take that distance, divide by the time between frames. And viola, you have the speed of the car.
