3/2/2005Laser-Based Speed Cameras Fail Accuracy Test
Laser expert questions the accuracy of popular mobile speed measuring device.
BBC News hired laser expert Dr. Michael Clark to verify the accuracy of the latest mobile speed camera devices following several high-profile court cases on the subject. Clark demonstrated that reflections and movement of the laser gun when clocking a vehicle can produce erroneous results.
If, while measuring a car's speed, the aiming point moves across the side of the car, that extra distance is added to the speed calculation. Because the gun takes measurements every one-third second, a few additional feet in the distance equation can add between 1 and 30 MPH to the measured speed.
Clark demonstrated the effect with a Tele-Traffic LTI 20.20 mobile speed camera, which is popular among UK police. He clocked a stationary wall at 58 MPH and a stationary car at 4 MPH.
Tele-Traffic maintains that its lidar speed gun accurately measures the speed of moving objects and will show an error if the camera is panned. The device uses a proprietary algorithm to predict where the vehicle should be, throwing an error code if the measurement deviates from this prediction.
Clark counters with actual police video evidence showing speed gun inaccuracy in the field. He has used such evidence to win 4 out of 5 cases in UK courts. If just one percent of UK tickets issued last year were inaccurate, that would mean 20,000 drivers were improperly charged £60 fines and faced higher insurance premiums as a result.
"What's actually happening is the device is sending out a laser beam that is hitting the wing mirror on the car, then it is being reflected onto the [roadside] sign … it's then coming back off the sign, back onto the wing mirror again and back into the receiver."Source: MOBILE SPEED CAMERAS (BBC News (UK), 2/28/2005)
As the devices use a distance measurements to work out the speed of a car, Dr Clark believes that such reflections could cause erroneous speeds readings.
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