Speed Cameras

[Mowbray]

How do speed cameras know when to take your picture when you're speeding?

Thanks

[Loosehead]

Assuming you are talking about the standard Gatso. They have some form of speed detection device, probably Radar. When an object is detected going faster than the speed the camera is set to detect, 2 pictures are taken half a second apart.

[mattie]

If you notice there are painted white lines infront of a fixed speed camera.

They use an equation speed = distance/time, so if you cross the lines too fast then the camera is triggered.

[Mowbray]

yes but do the camera's have a laser of some sort to detect the car thats speeding?

[Loosehead]

I pretty sure it's radar, a beam is projected and when it bounces back the Doppler shift is measured and the speed calculated.

[fo3nix]

yea it takes advantage of the doppler effect in order to determine speed. interestingly, they don't need to paint the lines on the floor to determine the speed. i think they either put them there to be able to do a rough manual check of the speed, or just as another warning sign that the camera is there.

[brachiopod]

I think that the white lines are there for some sort of legal 'proof' that a speeding offence took place.

The speed detection using radar is just the triggering mechanism, and not proof of speeding per se. But with the distance markers, and the time between the two photpgraphs being a known quantity, evidence of the offence in the form of a photographic record can be used.

There was a case only a month or two back, where the defendant got off the charge because he proved to the court that the white markers were not painted at the correct intervals.

[Tim Baxter]

This thread has been answered before. The radar detects the speed and logs it.  At the same time it starts the camera mechanism to take two photos.  As said before the distance and time travelled can be used to calculate the speed between the lines.  Now you have two proofs of speeding which historically is what the law demands.  You have the radar evidence corroborated by the photograph.  The same system is applied to a plod with a gun.  In his 'professional' opinion a car is exceeding the speed limit, this is corroborated by the evidence of his gun.

[kempie]

Don't think much of this two proofs system vis-�-vis radar-guns.

If the PC's professional opinion of a speeding vehicle is reliable (in a legal proof sense) should it not become unreliable the moment he targets a non-speeding vehicle and thus voiding all previous infractions caught by that PC in that shift and beyond?

[Tim Baxter]

'Professionals' are only right 'x' % of the time.  They are allowed the odd off-day, minute or whatever.  One imagines that this is why corroborative evidence is so important.