The Need for Speed: Obtaining Speed From Video

James R. Schmidt, Jr., BSME, Collision Reconstruction Engineering Analyst

You guessed it! If Jim Schmidt is writing something, it must be regarding speed from video! That’s because I’m passionate about what I do! They don’t call me “Captain Video” for nothing.

A new year. A new dash cam. A routine drive home in the evening. One thing leads to another, pushpins were plotted, speed was calculated, and wouldn’t you know it … the same results! Perfection!

Pushpins were plotted for 59 seconds and 0.6 miles of travel. A speed versus time profile was generated from this plotting, and a comparison was made with the GPS speeds shown on the dash cam display. Assuming for this example the GPS speed display to be the actual speed of the vehicle (which is a reasonable assumption for normal driving with gradual speed variations), the independently calculated speeds were within 2 mph of actual over this entire time and distance. Additional pushpins could be plotted to refine this even further.

A few stills from the dash cam video:

Dash Cam Images

GPS speed on the dash cam display was plotted versus time:
GPS Speed

Pushpins were plotted in Google Earth on an aerial photograph with the same lane striping as that shown in the video:
Google Earth Image

Select close-up views of the pushpin plotting:
Google Images

Speeds were calculated from the pushpin plotting and a speed versus time profile was generated:

Speed vs. Time

Independently calculated speeds were compared with GPS speeds (using a 1 second shift of the GPS data to account for the GPS delay/refresh rate):

Speed Comparison

An excellent match was obtained, within 2 mph of actual speed (as previously noted)! Typically, nighttime videos can be harder to work with, but in this instance given the high quality of the video, similar results to that of an analysis from a daytime video were obtained.

James R. Schmidt, Jr., BSME, Collision Reconstruction Engineering Analyst with DJS Associates, Inc., can be reached via email at experts@forensicDJS.com or via phone at 215-659-2010.

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