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Tuesday, April 8, 2014

Can you outrun speed cameras?

File:New Zealand PW Speed Camera Area.svg

A group of physics students from the University of Leicester calculated whether someone could defy speed cameras by outrunning them. The good news is that it is definitely possible. The bad news is that he’d have to be traveling at 119 million miles per hour.

The students based their calculations on what it would take to create a sufficient Doppler effect to blur the images taken by a camera. The Doppler effect is the change in frequency of waves as the object emitting those waves first approaches and then passes the observer. You can notice this yourself in the changing sound of a siren as an emergency vehicle catches up with you and then passes you. The sound waves between you and the vehicle ‘bunch up’ as the distance closes, resulting in a higher pitched siren. As the vehicle moves away from you, the sound waves stretch out.

In light, objects moving away from an observer are shifted toward the red end of the spectrum. The faster the object, the bigger the shift. Because speed cameras take pictures of fleeing cars, the students worked out how fast a car would need to travel to create a redshift great enough to make the license plate undetectable. The answer: about a sixth of the speed of light.

I feel a couple of caveats are in order. If you’re a fan of the show Mythbusters, then you know that Jamie Hyneman and Adam Savage also tried to outrun speed cameras and were able to do so by going about 200 miles per hour. This is obviously considerably less than a hundred million miles per hour. One possible reason for the discrepancy is that the University of Leicester students ignored camera speed. If a camera has a shutter speed of 1/1000 of a second, a car traveling 200 miles per hour would have moved about three feet during the time it took the camera to snap the image. I’m not sure what the shutter speed or focal range is for traffic cameras, but it’s not hard to see how the world’s fastest cars could defeat them.

One more thing to note: this study was conducted in the United Kingdom, where license plates are typically yellow. The exact speed required to create enough of a doppler shift to fool the cameras will vary slightly with different plates.

D. Worthy, R. Garner, J. Gregory, & J. Taylor-Ashley (2013). P3 10 Red-shifted Speed Cameras Journal of Physics Special Topics, 1229 (11).