How Does Laser Speed Measurement Work?

ProLaserIIILidar has always been my favourite speed measuring device. I could target individual vehicles with accuracy and rapidly measure their speed, even on a busy highway. Radar could not do the job nearly as well as it could not be easily relied on for a specific vehicle's speed in a busy environment.

An officer starts their shift by checking the instrument to insure that it is operating as intended by the manufacturer. An automatic power on self test must produce the expected responses. Next a series of known fixed distances were tested by operating the laser are receiving the correct measurements. Finally the aim point was verified by passing the sighting dot over a distant object and listening to the change in pitch of a generated tone. Once this was done the instrument was ready for use.

A safe site with a good view of traffic was selected and the lidar was put to use. The aiming dot was placed on the vehicle to be measured and the trigger pulled. A train of laser pulses was emitted, received and analyzed by the device which calculated the change in distance from the vehicle to itself over time and displayed the speed for me to see and decide whether to take enforcement action or not.

The nature of the very narrow laser beam made it precise to aim at individual vehicles. At 300 meters a spot roughly the size of an orange was reflected from the vehicle. As long as there was a clear line of sight between the vehicle and the lidar there was no doubt about who the measured speed applied to.

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Moving vehicles

Does the lidar require that the measuring take place from a stationary point?

I was stopped recently and the officer insisted that his device (not sure if it was lidar or not) indicated a speed faster than my odometer was reading. The officer was on the opposite side of the road travelling toward me. He was kind enough to show me the reading, so I haven't got any doubt about the measurement.

Mathematically, it is feasible for one vehicle to calculate the distance traveled based on stationary points, astronomers do this all the time with asteroids that move against a starry background, but a laser or radar is measuring the change in distance over time. If the measuring vehicle is moving toward the target then the speed of the measuring vehicle must be taken into account because there is less distance between the two vehicles than if one vehicle is stationary so the change in distance is greater. I think this is what caused the discrepancy.

Here lies the dilemma: the posted speed is 90, target vehicle's odometer reads 120, clearly speeding. Officer's vehicle is moving toward the target at 90 when the measurement is taken (assuming the officer is doing the posted limit). The measurement is 130. The province has an excessive speed of 40 kph over the posted limit in which the vehicle can be impounded for 7 days. Would impoundment for excessive speed be appropriate?


Laser is used from a stationary position. If the device was moving inside a police vehicle then it has to be radar and there are circumstances where radar will read high. The most common is where the patrol vehicle speed is not correct. Since it is used to subtract from the closing rate of speed to determine the violator's speed, if it is low not enough will be subtracted and the violator's speed will be higher than the true speed.

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