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Are Police LIDARs Always Correct?


The most common method used by police to detect peoples speed is to use what is called a LIDAR, commonly referred to as a radar gun.

After being caught speeding by police, many people question whether the LIDAR is 100% accurate, as they maintain they were not exceeding the speed limit. Police will often show them their reading, and for most this is the end of the matter. However, it is important to be aware that the LIDAR is not completely reliable and errors can occur.

The most common operator error is known as β€œthe sweep effect.”

The sweep effect is a condition that occurs when the sequence of range measurements obtained by the LIDAR are not measured from the same spot on the target.

This condition often occurs when a police officer aims the LIDAR at the front or rear of a passenger vehicle. If the aim of the LIDAR device wanders between the licence plate and the top of the passenger compartment, an inconsistency of about 1 metre in the range data can occur. Depending on the sequence of events, this can cause the vehicle to appear to have travelled either 1 metre further or 1 metre less than the true distance.

If this sweep effect goes undetected, it could cause the calculated vehicle speed to be as much as 13 km/h higher or lower than the true speed.

Other possible sweep effect conditions are due to two or more targets intercepting the laser beam during one measurement. This can occur because of intervening objects interrupting the beam, or because of poor aiming allowing the beam to sweep between two side-by-side targets.

If you were caught speeding and believe that there was an error in the speed recorded, contact Armstrong Legal and speak with one of our traffic lawyers.

Written by John Sutton on April 13, 2018

John is partner of Armstrong Legal and head of the Criminal Law Division. The experience John possesses, being a high quality mix of defence and prosecution skills, together with his team at Armstrong Legal, mean you can be certain of accurate, dependable and practical advice on how your matter can dealt with. View John's profile


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