To mesure vehicle speed within a few percent accuracy, we recommend a fast webcam with at least 30, better 60 or 90 fps / frames per second. But also marking the road with chalk at least once during setup is important to get the exact measurement distance. You simply have to draw a line 90 degrees from the sidewalk into the road. Then move 15 to 60 feet and make a second linke 90 degrees to the road. The lines should be parallel within a few inches. Then you can measure the exact distance between the lines. In the software, you simply have to place the start(blue) and end(red) sensors over both lines. Place the middle sensor(green) somewhere in in between. Now you can measure within 3% accuracy.