Robert T. Lynch, P.E., Principal Collision Reconstruction Engineer

With the growing prevalence of dashcam and surveillance cameras, the cases we get with video of the collision have increased dramatically. In just about all of these cases, we are asked “How fast was the vehicle going?”

In order to calculate speed, one must know a distance and a time, as speed is simply distance divided by time (i.e.- miles per hour). The time component is determined from the frame rate of the provided video and by counting the frames it takes for the vehicle to travel a distance. The frame rate can be gathered from the metadata for the video, the imprinted timestamp on the video (if there is one), and/or under the general assumption that the provided video is being played back in real time. The distance component can be between two known objects that the vehicles passes by, or by using the dimensions of the vehicle itself (i.e.- wheelbase, overall length).

For example, if the frame rate of the video is 10 frames per second, and it takes 2 frames (0.2 seconds) for the vehicle to travel its wheelbase (10 feet) as it passes by a fixed object (i.e.- utility pole in the video), then the speed of the vehicle is:

10 feet / 0.2 seconds = 50 feet per second (or 34.1 miles per hour).

If the vehicle is traveling above the speed limit, an additional analysis can then occur to determine the contribution of the excessive speed to the occurrence and/or severity of the collision.

Robert T. Lynch, P.E., Principal Collision Reconstruction Engineer with DJS Associates, Inc., can be reached via email at experts@forensicDJS.com or via phone at 215-659-2010.

Categories: Accident Reconstruction | Collision Reconstruction | Speed Calculations | Video Analysis