James R. Schmidt, Jr., BSME, Sr. Collision Reconstruction Engineer ::::
I’m a collision reconstruction engineer with over 23 years of experience in the field. I’m affectionately known in the office as Captain Video, given my love for the evaluation of vehicle speed and crash-related parameters from surveillance videos.
Basic evaluation from stationary camera:
A 2019 Toyota Sienna minivan passes in front of a stopped dash cam. Speed from video is evaluated therefrom. Speed is distance over time. The easiest way to perform the evaluation is to look at the timeframe required to travel the vehicle’s wheelbase (i.e. the distance from the front wheel or axle to the rear wheel or axle). So, in this example, the minivan travels its 119 inch wheelbase in 7 frames of a 30 frame-per-second video. Distance is 119 inches, or 9.92 feet. Time is 7 frames divided by 30 frames per second, or 0.233 seconds. Calculating speed … 9.92 feet divided by 0.233 seconds is 42.5 feet per second, or ~29 mph. FYI, this was a 35mph speed limit roadway.
Complex evaluation from moving camera:
That same Toyota minivan passes through a crosswalk up ahead, after the traffic signal turns green. First, let’s take a look at the travel of the wheelbase relative to the FIRST crosswalk line using footage from a moving dash cam. The vehicle travels its 119 inch (or 9.92 foot) wheelbase in 13 frames of a 30 frame per second video (or 0.433 seconds). Speed is calculated at 22.9 feet per second, or ~16 mph.
As a double-check, let’s now look at the travel of the minivan’s wheelbase relative to the SECOND crosswalk line. The vehicle travels its 119 inch (9.92 feet) wheelbase in 12 frames (0.400 seconds), which equates to 24.8 feet per second, or ~17 mph. This is an excellent match to the calculation above, and the result is expected given that it’s generally a fraction of a second later and the vehicle is accelerating!
These evaluations were completed to show an example of determining speed from video, my specialty. “Captain Video” is ready to calculate your speed from video next!
James R. Schmidt, Jr., BSME, Sr. Collision Reconstruction Engineer with DJS Associates and can be reached via email at experts@forensicDJS.com or via phone at 215-659-2010.
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