The Stationary Brick Wall That Is Moving At 58 MPH

from the pretty-fast-for-a-wall dept

Bob Dole writes “Cameras never lie, right? Well, if they don’t then someone needs to explain how brick walls can move at 58 MPH and stationary cars can be clocked at 4 MPH. A British laser expert explains that if a laser speed gun operator’s hand slips while measuring speed, it can add (or subtract) significantly to the speed measurement. The accuracy of speed cameras has long been an issue in Europe and Australia. Now there’s some science to back up the skepticism.” Reminds me of the old joke that people say whenever they get into an accident with stationary objects: “it hit me.” Well, if a stationary wall can go 58 MPH, I guess it can jump out and hit a car as well…


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Comments on “The Stationary Brick Wall That Is Moving At 58 MPH”

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8 Comments
Hi Speed says:

Re: No Subject Given

Fixed Road-Side speeding cameras are subject to the vibration from their mounts which can cause up to a two inch variation from starting to finish of the radar calculation, caused by something as simple as a semi passing by it. Car mounted are even worse as they are subject to vibration from the engine, road conditions and even wind sheer (on the vehicle not the mount). There are too many variables on a moving target to get a accurate reading 100% of the time.

The Arbiter says:

Re: Re: No Subject Given

So sorry, not true. The vibrational characteristics of the RADAR mount are irrelevant. Traffic RADAR measures the frequency shift between two objects where there is relative motion between them. The vibration is not a factor.

Also, RADAR, being a high frequency radio wave, operates at the speed of light. The “calculation” is completed almost instantaneously. RADAR is not affected by the “vibration” of the mount….no matter what it is mounted on.

The Arbiter says:

Re: I read somewhere

In a LASER Speed Enforcement device, the typical beam spread is approx 3 feet at 1000 feet distance. Still not big enough to cause problems. Additionally, the officer will only target vehicles he can clearly identify. There would be no point in him targeting a vehicle that he can not clearly see and identify.

Anonymous Coward says:

It seems like the truth is that the reliability and accuracy of these devices is completely dependent on whether or not you have some sort of vested interest in their accuracy.

As to the person saying that the movement of the device has no effect, Um, wrong.
Yes you are right that the measurement takes place very quickly, so quickly in fact that even a minuscule vibration in the proper direction will get added to the reading (or subtracted from if in that direction).

The problem stems from the fact that the system has no way to tell if the doppler change is coming from the movement of the object being watched or from the movement of the transceiver or laser/detector pair because there is no mechanism to detect movement of the device and compensate for it.

The faster and more sensitive the device the more prone to that sort of error it becomes. Averaging over many readings will help with the problem, but then the device would have to track the object for a longer period over a larger distance. That would usually work Ok for a Cop but it’s not possible for a mounted camera the way they are built now.

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