There's many a slip twixt camera and pic



This was published 17 years ago

There's many a slip twixt camera and pic

So how do speed cameras get it wrong?

According to one expert, who spoke on condition of anonymity, cameras themselves were unlikely to be at fault. The less visible parts of the system were the likely culprits.

The Poltech cameras on the Western Ring Road have four main components:

  - Sensors in the road that vehicles trigger.

  - A black box that amplifies the signal.


  - A box containing the speed calculator.

  - The camera, taking panoramic and close-up images of the vehicle.

The sensors are pressure-sensitive (piezo) strips cut into the asphalt, which convert the pressure of a car into an electrical signal.

On the Western Ring Road set-up, the three strips are cut into the road surface at intervals of 1.51 metres - the extra centimetre allows for a tolerance in favour of the driver of about 1 km/h at 100 km/h.

This establishes a benchmark distance over which the time a vehicle takes to pass can be measured, and therefore its speed.

The electrical impulse from the piezo sensor is amplified, then the time the car took to go over the first two sensors is checked against that taken over the second and third sensors, and the speed calculated. With an inbuilt program to round down fractions, the motorist has a benefit of a 2 km/h tolerance.

If the speed, taking into account the tolerance, exceeds the speed limit, the box sends a signal to the camera to take an image of the vehicle.

But camera installations require regular maintenance to keep them accurate.

The most likely cause of false speeds were false "inputs" - right from the start. Trucks can set off false readings through "seismic vibrations" that can mistakenly set off the piezo sensors. Badly worn car suspension can cause wheels to bounce and momentarily lose contact with the road surface, confusing the readings. Loose cabling connections can produce noise and false camera signals.

Faulty amplifiers and noise filters can allow stray signals to pass.

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