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Uncertainty in Laser Speed Enforcement

If you ever got a speeding ticket, you probably wondered how they caught you. How did they measure your speed and was the measurement accurate?

Police in BC use various methods to determine a target vehicle’s speed for speed enforcement. In most cases, when the officer is stationary at the roadside, the officer uses Laser, also known as LIDAR (Light Detection and Ranging). This involves projecting a laser beam to measure the speed of a vehicle. While LIDAR can be an effective tool for law enforcement, there are factors that can introduce uncertainty in the readings obtained.

The accuracy of the LIDAR reading depends on the distance between the device and the target vehicle. If the distance is too great or too small, the readings may be less accurate. For example, if the LIDAR device is too far away from the vehicle, the beam may not be strong enough to reflect off the vehicle, resulting in an inaccurate reading. If the LIDAR device is too close to the vehicle, the readings may be affected by the angle at which the beam strikes the vehicle, which can also result in an inaccurate reading.

Speed and direction may also impact the accuracy of the reading obtained. When the target vehicle is accelerating or decelerating, or if it is moving in a curved path, the reading may be less accurate than if it is driving in a straight line at a constant speed.

Surprisingly, the condition of the vehicle can also impact the reliability of a LIDAR reading. A tinted windshield (which is illegal in BC) or a dirty or damaged surface on the vehicle can affect the LIDAR accuracy. Rain and snow can refract and absorb the beam which is why it is rare to see police officers standing in the rain or snow for speed enforcement. Other reflective surfaces in the vicinity of the targeted vehicle are also known causes of beam interference.

One of the more commonly known problems is LIDAR slip, which is when the vehicle movement itself causes inaccuracies in the speed measurement. When a vehicle is in motion, it is constantly changing its position, while at the same time, the laser beam emitted by the LIDAR travels to the vehicle and back. If the vehicle is moving towards or away from the LIDAR gun, the speed measurement is affected by the Doppler effect, which is a change in the frequency of the laser beam due to the motion of the vehicle.

When the LIDAR gun is not held steady, or when the operator is not aiming the gun accurately at the vehicle, the beam may hit a different part of the vehicle, such as the wheels or the road surface, causing the speed measurement to be inaccurate.

One of the more significant concerns with LIDAR is targeting. The operator of the LIDAR gun operates on the assumption that the target is in the same location as where the beam is pointed. The difficulty arises because the target is merely a dot in a scope and the officer cannot see the beam. If the beam reflects from a point that is different from the dot in the scope, the officer will obtain a speed reading from another location on the target vehicle or a different vehicle altogether.

Although LIDAR is a generally accurate technology for measuring vehicle speed, there are sources of uncertainty that can affect the accuracy of the readings obtained by the police.

Training, proper use and proper functioning of the device are all issues that can be challenged at trial. Many officers are unaware or simply cannot remember the potential sources of uncertainty.

We defend speeding tickets all around BC and we purchase and test police LIDAR devices which are why we say, if you got caught, call us. We’re the BC Driving Lawyers, the original driving lawyers defending all traffic cases everywhere in British Columbia.