Any car moving at highway speeds will make noise; even with the engine off, the doppler shift of the noise of the tires will be measurable!

Frequency detected = [(Vsound +/- Vobserver) / (Vsound -/+ Vsource)] * Frequency source.

In this particular example we have a moving source but stationary observer. So, the equation is something like this:

Frequency detected = [Vsound / (Vsound -/+ Vsource)] * Frequency source

This machine works something like this:

It detects the Frequency of the sound produced from the car (any source is equally acceptable and proper) as it changes so, if we know the Vsound we can calculate the Vsource, which means we can calculate the speed of the car.

The ONLY problem here is this. In the previous equation we considered that there is no wind, so the velocities are absolute and no relative ones. If we want to be accurate all the time, we MUST calculate the velocity of the wind in real time. The differences are huge if we don’t calculate the wind velocity. Think that if the wind is to the detector lets say at 15mi/h then the speed measured will be 15mi/h higher than the real speed. If we consider that the wind is not the same even in a few meters distance the measurement MUST be done at very close distances (lets say 10-15m from the detector). Apart from this, the system has to be built with a machine that distinguishes the different cars. Lets say we have 5 cars very close together. How do we know which one speeds by only calculating a doppler shift (red or blue) ? It’s easy to have an officer read the results and using a radio inform the other officers that stop the offender. Another solution is to video record all the time and when a speeder is caught to see the video which one is going faster = he is the “criminal”.

BUT, all this stuff is a little too risky to be used in a court room. So, for the moment we have to wait until they come up with an idea that distinguishes the real speeder from the other drivers. When they do it, it will be the dooms day. The police can indentify ALL the speeders even in a highly used motorway.

Thnx for your time. Have a nice day from Athens, Greece.

And yes, fixed location sonar speed detectors would be a pretty inescapable detection – but what’s the max range?

Speed cameras can be bypassed if you have a car that can drive past them at 184+mph :p

(or speed REALLY fast so they won’t bother chasing you down.)

Lærdi háskúlin í Tennessee hevur mennt eitt nýtt slag av fartfellu, sum mátar ferðina á bilum út í frá ljóðinum tá teir koyra framvið. Mikrofonir taka upp ljóðið av bilunum, eitt telduforrit eyðmerkir hvønn einstaka bil; og teir máta so ferðina út í fr…

Let’s hope no one in government figures out how to mount radar, laser and doppler shift radar with cameras in fixed locations as they do in Europe.

I’d like some train-based public transit upon release of this technology please, Mr. President, so I can choose the days I risk getting ticketed driving my above average vehicle on subpar roads designed for 1980′s safety standards.

Thanks,
Joe

Keep your eyes closed and have someone ride by on a bicycle and see if you can guess the speed from the sound of it going by. It has nothing to do with trans or windspeed or anything like that.

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1. As someone mentioned before, if you see the measuring equipment it and reduce your throttle as you pass it could measure a much greater doppler shift than it would if you had kept the same throttle setting. The error could be a much faster speed reading than you were actually traveling.

2. Winds will affect the reading. If you are traveling into a headwind the reading will be increased. Tailwind and the reading will be decreased. This may be solved by requiring that a wind reading be taken at the same time as your speed reading.

MM

For what it’s worth, this is the same principal that astronomers use to measure the speed at which the universe is expanding, as red and blue shifts in light (due to objects traveling away and toward us, respectively) emitted by stars are also due to the doppler effect.

I wouldn’t worry about this being implemented in practice though, knowing your speed is worthless if they can’t ID you, and that simply couldn’t be done based on sound alone. Some other information would be necessary to achieve that.

The only thing that this system provides beyond what is possible using current radar systems is an accurate way to detect spead while itself would be undetectable. It would also make it easier to detect the speeds of multiple vehicles at once.

Jon.

So why not just get one of those license plate sprays/covers that defeat cameras? Not sure if those products actually work, but if they do… their fancy machine wouldn’t be able to fine you.

Don’t they have a better way of spending our tax dollars?

This is incorrect, if you read it again, it talks about the exhaust sound’s Doppler Shift, not the volume of the exhaust itself. The Doppler Shift will allow the radar to calculate the speed it is traveling.

Only stupid greedy ludite nazi bureaucrats would be naive enough to think that ingenuity won’t be used to defeat this too.

Civil disobedience..Right-on!!