driving offence, page-9

This is from the Autospeed web site:

Lidar operates in the way that many people believe that radar does - the operation is pulsed, and the speed is determined by timing the time difference between returning pulses.It must be stated that Lidar is far more likely to have a speed that is correctly attributed to a certain vehicle (even if the displayed speed is wrong). Target misidentification would be extremely rare, as the beam is coherent and has a very narrow beamwidth. If the laser beam is aimed and held on a certain part of the vehicle, then the speed-reading would be very accurate.

There are, however, panning errors that can give incorrect readings. Take a case of a car doing 60 km/h (16.7m/s). Assume that the Lidar takes a reading in 1 second. Now, what happens if the police officer starts the speed reading with the laser shining at the base of the windscreen, and at the end of the interrogation one second later, the beam is aimed at the front number plate, 2m ahead of the base of the windscreen? Then, even if the car was completely stationary, because the beam has been panned in this way, a reading of 2 m/s will be displayed (which is 7.2 km/h). In the case of that 60 km/h car, the displayed speed will be 67 km/h, which might result in being booked. If the time used for the speed-reading is less, the situation is worse. For example, take the same scenario, but with the time 0.5s. In that case, the additional speed will be 2/0.5, which is 4 m/s, or 14 km/h. This type of situation was graphically illustrated a few years ago when a defence attorney panned a Lidar unit on the back wall of the courtroom and obtained a speed reading! It must be stated that the error can, of course,also occur the other way, to the advantage of the driver.

Other Considerations...Police are tending to use smaller and smaller tolerances for booking speeding motorists (for example, in Western Australia it is a blanket 8km/h). The problem is that this is grossly unfair to a driver who may very well think he/she is driving at the speed limit.The first issue is that of actual tolerances allowed by Australian Standards (AS2898.2), where, depending on the radar, the tolerance is either 2 km/h up to 100 km/h, and then 2%, or 3km/h and 3%. What this means is that these speeds or percentages must be taken off the detected speed.Secondly, there is the issue of speedometer inaccuracy. Australian Design Regulations (ADR-18) allows for the speedo to be ±10%inaccurate. Think about it, the car you buy brand-new can have a speedometer that is 10% inaccurate, and it complies with standards! Consider now what can occur with your brand-new car that under-reads by 10%. There you are, thinking that you are doing 100 km/h, but are in fact doing 110. The police detect you, and should allow 3 km/h, which means that you really should not be fined until they detect you doing at least 113 km/h. But in WA, they will book you for a DETECTED speed of 108 km/h -so much for fairness or claiming that the issue is road safety! So, next time you are booked for speeding, don't just pay up. Consider whether the infringement you have been ticketed for has been correctly applied; if not, contest it.

AutoSpeed - The Problem with Radar - Part 25

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