Determining Closing Speed from Dashcam Video

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In a recent case, the client retained a collision reconstruction engineer to determine how quickly a dump truck had approached a stopped tractor-trailer, using only dashcam video.  By analyzing the footage, the engineer was able to provide a clear answer, based on a simple and reliable method.

As a vehicle moves closer, objects ahead appear larger in the camera’s view.  When the dump truck collided with the tractor-trailer, the trailer’s width on the screen was measured and compared to an earlier frame where it appeared half as wide.  Since the actual width of the trailer was known, that change in apparent size was directly related to how far away the dump truck was at each point.  When the trailer looked half as wide, the dump truck was about twice as far back.

The time between those two frames was then measured.  Since the distance from the camera to the front of the dump truck was known, the exact amount of ground that was covered in that time was able to be calculated.  The dump truck’s closing speed was determined by dividing distance by time.

This approach turned ordinary dashcam footage into valuable evidence.  Using video alone, the analysis demonstrated—in clear, measurable terms—how fast the dump truck was moving moments before the collision.