Sample survey: Example Court

How these speeds were measured

Two-line timing. Two reference points across the drive lane were measured on the ground with a tape. The distance between them is the baseline (45 ft in this report). Each car was filmed from the side in slow motion as it passed both points.

Marking the video. The video was stepped one frame at a time. One frame was marked when the front bumper reached the first point, and another when it reached the second point. The time between those two frames is the elapsed time. Speed is baseline ÷ elapsed time. Feet per second × 0.6818 gives miles per hour.

Frame rate. The clips were filmed at 240 / 120 frames per second, so each frame is about 4.2 thousandths of a second. Elapsed time comes from the timestamp the phone recorded on each frame.

Uncertainty. Each mark could be off by one frame, early or late. Across both marks, the elapsed time could be up to two frames longer or shorter, and every speed shows the range that produces. Example: the middle measurement in this report is Baseline 45 ft · 240 fps · 517 frames · 2.154 s. Two frames at 240 fps is 0.008 s, so the speed lies between 14.2 and 14.3 mph (shown as 14.2 ± 0.1).

These are estimates from video, not radar or lidar readings. Baseline accuracy also matters: a tape off by 6 inches on a 45 ft baseline changes a speed by about 1%. Formula: mph = (distance in ft ÷ seconds) × 0.6818.